For the Use of the Ladies.

In Six Dialogues

From the Italian of Sig. Algarotti.


Quæ legat ipsa Lycoris. Virg. Ec. x.


Printed for E. Cave, at St John's Gate,


The Fourth Dialogue.

Encomium on Experimental Philosophy, and an Exposition of the Newtonian System of Optics.

THE next Day when we had disengaged ourselves from Poetry, which had at first given Birth to our Discourses on Light and Colour, and afterwards endeavoured to disturb and interrupt it, I began after the following Manner. It is now Time, Madam, to lead you into the most retired Sanctuary of Philosophy, from whence the Profane and those who are filled with Vortices, Globules, Atoms, subtile Matter, and like Imaginations are entirely excluded. This Philosophy which I am now conducting you to, is less pompous than that we have al <4> ready discoursed on, but in Return, performs all its Promises; a Philosophy, which is contented with giving a History of Physics, and leaves to others the Province of making them the Subject of a Romance. You have had an Instance of the one in its Method of explaining the Nature of Vision, and the System of Vortices has given you an Example of the latter more pompous and splended Philosophy, which boldly goes to the first Causes of Things, and laying down certain arbitrary Principles, upon these builds the World, and explains all the Phænomena of it according to its own Fancy.

As the Eye is now discovered to have an entire Resemblance to the artificial Camera Obscura, all Philosophers will hereafter explain the Nature of Vision after the same Manner. But there will not be that conformity in their Opinions concerning the Solidity of Bodies, Gravity, Light, and colours, since the Cause of these Qualities can be known only by Conjecture. And how very hazardous a Thing this is, you have already <5> seen in the Cartesian System of Globules, to say nothing of mallebranche's Reformation, which, notwithstanding all the Applauses at first given it, is now exploded, and shares the same Fate as the Globules. And this you may believe has been the Case with all the general Systems that have hitherto appeared, concerning the Causes of Things. They resemble vast Empires, which totter and fall by their own Weight and Greatness. I find then, said the Marchioness, that the wherefore, which so greatly excites our Curiosity, will be for ever hid from us, and the Pleasure of conjecturing, which is generally so adapted to the Taste of Men, will have no Charms for Philosophers. This really is a Circumstance, which will make the Condition of those Gentlemen not greatly to be envied by the Vulgar.

Conjecture (answer'd I) according to the Assertion of one of the most ingenious Authors in the World, is not to be allowed in any Thing but Geometry. In this Science, if the Certainty of <6> Principles does not directly guide us to what we seek, it never leads us however to any Thing contrary, and always rewards our Searches with something equivalent. But what Uncertainty and Inconstancy is there in Natural Philosophy? Some affirm that there is a Vacuum, or Space void of all Body, and others will have every Thing to be Body. This Diversity of Opinions with regard to Principles, must unavoidably produce very great and innumerable Disputes as they advance further, which extend so far as the fixing the Essence or Nature of Body, a Thing which one would imagine should of all others be the most certain in Physics, since Boyd and the Properties which depend on its Essence are the perpetual Subject of Philosophical Inquiries. I cannot help comparing these Philosophers to those Critical Writers who correct some corrupted and defective Passage of an ancient Author. One of these Gentlemen gives one Reading, a second another, each supported with the finest <7> and most elaborate Arguments, and the Encomiums of the Journalists of the Learned. Some old Manuscript of the Author himself at length appears drawn out from the Dust and Obscurity of a Library, and then all the fine Readings of these profound Critics, and the Time they spent in inventing them, are sent into the Regions of Ariosto's Moon, and are there treasured up among other lost Things. Observations and Experiments are the authentic and original Manuscripts of Nature; and these, by overthrowing so many fine Systems, instruct us every Day, to be more cautious in puzzling ourselves to form Hypotheses. I look upon this Advice as a very great Benefit to Mankind, since it exempts them from no small Trouble and Fatigue. But the Misfortune is, that Men persist in a Refusal of this friendly Admonition, and are obstinately bent to spend their Time to no Purpose.

This is indeed a Practice that redounds greatly to the Honour of these Observations of yours (said the Mar <8> chioness.) A System need only be ingenious, simple and elegant, to give them a Right to declare open War against it, they may be termed the Erostratuses of Natural Philosophy, which found their Glory in the Destruction of all that is grand and splendid. I must confess that I cannot be pleased with so malicious a Character. To what a Height would your Displeasure rise, answered I, if I had told you all that these Observations are capable of effecting. There is perhaps no System better grounded than this, that Animals have Wings given them in order to fly, and Legs to walk. And yet Observation has discovered to us certain Insects who have large Wings without ever making Use of them to fly with; and in the same Manner, there is one Animal, which, notwithstanding it has Legs disposed like those of others, formed in the same Manner and of the like Proportions, yet almost always walks upon its Back with its Legs upwards, just as if the first were not sensible they had Wings, nor the <9> last, Legs. It is true, however, that if Observations had done no greater Service to the World than to destroy, we should not be much indebted to them. But by overthrowing perplexed and useless, and sometimes very troublesome Systems, how many good and useful Discoveries have these Observations furnished us with, in the Room of those Hypotheses they have overthrown?

Some melancholy Philosophers have imagined the Rays of the Moon were cold and humid, and therefore greatly to be feared, since there must be expected very dangerous Effects from their Influence. Ad in fact you see many Persons, even now, who believing the Effects of this Planet from the Tradition of their Fore-fathers, retire as soon as ever the Moon begins to rise, and (as they express it) gets Strength; and vast Numbers of People are perswaded that they have the Head-ach, if by walking in the Evening they have unhappily been obliged to receive the Infection of its Light.

The Experiments which have been <10> made upon this, give us full Liberty to walk at what Hour we please, without apprehending any Danger from that Quarter. The Rays of this Planet collected in the Focus of a burning Glass or a Lens, do not cause any sensible Effect in the Bodies upon which they fall, notwithstanding they are sometimes two thousand Times denser when thus collected, than they ordinarily are. A Thermometre, which is an Instrument containing a Liquor that contracts with the least Cold, and dilates with the least Heat, does not suffer any Alteration in the Focus of these Glasses when they are exposed to the Rays of the Moon; whereas if they are exposed to those of the Sun, the Vehemence of their Heat surpasses that of the hottest Furnaces, so that the Amiantus which defended the precious Ashes of Antiquity from the devouring Flames of the funeral Pile, cannot defend itself from the violent Heat of these Glasses. The Rays of the Moon do not appear to have any other Quality than that of supplying the Absence of the Sun, and <11> inspiring a soft and pleasing Melancholy in the Hearts of Lovers.

These Sorts of Observations, said the Marchioness, I have no Objection to, which let the fine Systems alone, and deliver Mankind from ill-grounded Fears. To this Spirit of Observation, answered I, we are indebted for our Deliverance from other much more important Fears, Comets, Pillars of Fire, Showers of Blood, and Ignes fatui, anciently Marks of Divine Wrath, do not at present disturb so much as one half Hour's Sleep, except it be in those Persons who will always be the Vulgar, and always of a Temper fitted to receive these Impressions of Terror from others.

But how much are we indebted to the Study of Experiments! Astronomy, Natural History, and Anatomy, seem by the Assistance of this to be rather new Sciences born among the Moderns, than transmitted from the Ancients to us. To this Anatomy owes the Circulation of the Blood, and all the animal OEconomy, whose very Simplicity was <12> the Cause of its being so little known among the Ancients. To this, Chemistry is obliged for its Phosphorus: Astronomy, for its exact Predictions: Hydrostatics, for an easy Method of Living, and carrying fresh Air for Respiration, in an Element denied to Men: for Acoustics, and their speaking Trumpet; their Projects, for rendering the Perfection of Hearing equal to that of Sight, and innumerable musical Instruments, the Offspring of Harmony. To this, Optics owe their Telescopes, Microscopes, Magic Lanthorns, and an infinite Number of other Wonders that perfect or flatter the Sense of Seeing.

Superstition, Credulity, and a greater Love for the Marvellouse, than Truth, Negligence, and a Want of proper Means for Observation, have for a long time been insuperable Obstacles to Knowledge. How great a Treasure of Wonders has Natural History, after having rejected the Absurdities of the Ancients, demonstrated to us! New Modes of Production, Breathing, Seeing and Living, new Conformation of Parts, <13> new Societies, new Modes of Being, unheard and unknown to past Ages.

The Reason of Mankind has been polished in Proportion as that of Brutes has been more considered; and the Arts themselves have been brought to Perfection by Observation upon certain Animals, commonly regarded either with Horror, or as the Refuse of Nature. Spiders have furnished our Manufactures with new Species of Silks; and the Eggs of a Fish, tho' yet unknown, may, with a little Apparatus, give us a fine purple Colour not inferior to that so much celebrated by the Ancients. Shall I inform you of the Experiments made concerning the Weight of Air, and the Power it has of dilating itself when compressed; Experiments once termed the Wonders of Magdebourg? Shall I give you an Account of the Equilibrium of Fluids, the Vegetation and Culture of Plants, which have afforded so great an Advantage and Ornament to Society? By these Experiments your Garden if embellished with the playing and agreeable Murmur of <14> artificial Fountains; and these supply the Northern Tables in great Plenty with those delicious Fruits that Nature had confined to a warmer Hemisphere. The Orange by these transported from the China to the Portuguese Soil, sweetly allays the burning Heat of the Summer; and these Experiments have extracted from the Vines of the Rhine transplanted upon the burning Rocks of Canary, that agreeable Juice so pleasing to the Taste of Ladies. Still better and better, said the Marchioness. Is not this the Cluster of the promised Land by which you hope to allure me? and as we are in the Country, would intice me by the Hopes and Advantages which accrue from Observation to Agriculture and OEconomy? Without Dispute you have those ancient Consuls in your Head, who from the Triumph returned to the Culture of their Farms.

If I had a Design of inticing you, answered I, Madam, I should rather entertain you with talking of the polite Arts of which you are so fond, and which owe both their Original and Progress to <15> Observation and Imitation. Gratitude requires you to think yourself obliged to these for the Pleasure you receive from the fine Lineaments and elegant Air of Countenance in the Medusa of Strozzi, the just Gradation of the Anger of Achilles, the Variety and Strength of Passions in Cassandra, that Masterpiece of the Timotheus of our Age, the majestic Solidity of the Portico of the Rotunda, the elegant Strokes of Guido, and the magic Colouring of Rubens. What Riches has this industrious Philosophy brought into the Treasuries of Painting, by the Observation of so many new Animals and Plants? What an additional Beauty has it given by the Imitation of the Eastern Varnish, to certain Works designed for the Use of those People to whom Superfluity is necessary? What a fruitful Source of Similies and Descriptions has it by its new Discoveries opened to Poetry? The Sun, Stars, Shepherds, the Hyrcanian Tiger, and the like common place Comparisons, are delivered from the Weight which they <16> alone have long borne in Poems, and we from the Fatigue of hearing them so often repeated. How great a Progress has the Elegance of Dress, which so greatly heightens the Beauties of Nature; and in short, the most polite of all the Arts made in our Age, by the Assistance of a curious Observation made upon pleasing Objects, Beauty itself, the most valuable Treasure with which Nature can enrich and adorn Youth, would often be a vain and useless Gift, if Observation had not consulted its Safety, by bringing in the strange, yet salutary Art of furnishing ourselves with a particular Distemper whenever we please? How many fair Circassians, who formed for the Pleasure of the Universe, are confined to a Seraglio, and subjected to the Caprices of one single Tyrant, and how many Beauties in England, the Mistresses of free born Hearts, owe their Empires and their Arms to an early Inoculation of the Small Pox?

But not to insist on what you may possibly think I might acquire too great <17> an Advantage from, nor to say any Thing further of Natural Philosophy, which seems a Province the most adapted to the Discoveries of observations, is not Policy indebted to these for that wife and real Government, which renders the Southern Suns less pleasing than the Cloudy Regions of the North, where the Liberty of the People is made compatible with the Superiority of the Nobles, and the Authority of the Sovereign? Metaphysics, that Labyrinth of Reason, are obliged to these for a certain System of the Origin and Progress of our Ideas, and we for the Knowledge of our selves. The Chaos of Chronology and History has from these Observations received its Light and Order. Sir Isaac Newton, that divine Philosopher, who may be regarded as the Founder of human Knowledge, has from Observations drawn chiefly from the ordinary Course of Nature, ranged historical Facts in a certain Series, by joining Epochas, which the Ignorance or Pride of Mankind had set at a great Distance <18> from each other, in the same Manner as a judicious Observation had united the Boundaries of the Earth in Geography.

Conducted by this infallible Guide he, according to the Expression of one of his Countrymen,

Display'd the lucid Robe of Day,

and thence extracted the true Properties of Light and Colours (which had till that Time lain concealed and involved) without forming, like Des Cartes and his Followers, any imaginary System to explain the Nature of them.

This is a World entirely new, enriched with the most shining Truths, and discover'd by Newton alone, for there are not the least Traces of any Philosopher who ever appear'd there before him. His Treatise of Optics, the Production of thirty Years Study and Search, is an excellent Model of true Philosophy. One single Experiment of his has advance our Discoveries more than all the ingenious and magnificent <19> Systems put together, that had ever gone before him.

An antique Column of the most worthless Stone has more Beauties in the Eyes of a Connoisseur, and will conduce more to the Perfection of Architecture than all the Galleries of Emerald and Adamant, invented by the Poets to adorn their inchanted Palaces.

But your Connoisseur, said the Marchioness, cannot admire or taste the Beauties of a Column, however antique and well-proportion'd it may be, unless he first knows what a Column is. And how it is possible that we should understand the Properties of Light and Colour, as you tell me Newton does, without first fixing what the Colours themselves are, and explaining the Nature of them? Des Cartes, whose System you Moderns explode, tells me, that if a Ray of Light meets with the solid Parts of a Body, it is repelled and reflected, and I understand this very well, because he had before told me that a Ray of Light is nothing but a String of little Globules. But how can I ever understand <20> the new Discoveries concerning the Qualities of Light, unless I am first informed what Light itself is?

Is there any Thing, answered I, more inexplicable than the Nature and Causes of muscular Motion? All the Dissertations of Philosophers upon it have been in vain. (And indeed the more these dispute upon the Causes of Things, the more inexplicable do they generally make them.) And yet an excellent Painter, a Michael Angelo, by laying down certain Rules drawn from repeated Observations, would have informed you, that when the Body makes such a particular Motion or Effort, certain Muscles must be elevated, and others lowered and depressed. So that there is no Posture so strange but he could have foretold the various and almost infinite Motions they would produce. The famous Piece of the last Judgment, in the Vatican, is an evident Proof of his Knowledge in this point.

The Loadstone is a Secret of the same Kind. Its Nature and the Cause <21> of its surprizing Effects are, and perhaps always will be as entirely lost to Philosophers as the Punic Language is to Critics.

Our Ignorance of its Cause, however, does not hinder us from making very many Discoveries of tis Qualities. We know, for Instance, that if it be armed with Steel it can attract a much greater Quantity of Iron than it can unarmed. We have discover'd that on one Side it attracts another Loadstone, and repels it on the other, and constantly directs itself to the Poles of the World. To this, Natural Philosophy owes the Discovery of a great Number of Truths, to this Navigation its Compass, and Commerce and Mankind very many Advantages.

If it is ever possible to arrive at a Knowledge of the Nature of Things, this is the only Method, by observing and investigating, with the greatest Attention, their most secret and hidden Properties, their primitive and elementary Qualities, on which all the rest depend.


Hitherto you have seen only the different Modes of the human Imagination in those Systems, continually changing and succeeding each other, to gratify the Pride of Man, that always deluded yet still credulous Being. But at present Light and Truth itself address you in the Language of Sir >Isaac Newton. Let us hearken to it attentively, said the Marchioness. Let it dissipate any Remains of Darkness, that may still obscure my Mind, and conduct me into this new World of Philosophy.

I observ'd to you the other Day, answer'd I, that every Ray of Light, however slender, is nothing but a Collection of innumerable other Rays, which are not all of the same Colour, notwithstanding the whole Ray appears white: But some of these Rays are red, orange, others yellow, green, blue, indigo, or violet, besides innumerable Degrees of intermediate Colours between each of these seven principal Ones. These Rays of different Colours, which are called pri <23> mary or homogeneal, blended together form a heterogeneous Compound Ray of a white or golden Colour, such as a Ray of the Sun appears, just in the same Manner as different Colours mix'd together upon a Painter's Pallet, compose a new One, which has something of all the others in general, but is different from each of them in particular. These are the Reasons why that Poet (whom you discovered rather from his Esteem for you than by his Stile) in speaking of Light, uses the Expression golden and sevenfold, an Epithet anciently appropriated to the Nile and the Shields of Heroes. This sevenfold Light is the inexhaustible Treasury of those innumerable Colours, which form the gay Picture of the Universe, and its Rays are not tinged with the Purple or Sapphire, either when they are refracted through a Prism, or reflected from a Surface, but derive their Colour from the Sun himself with that Heat and Lustre, which they receive from him, tho' not discovered by Vulgar Eyes.


In short, every Ray may be consider'd as a Fibre composed of innumerable other Fibres or Filaments, each of which has a peculiar, proper, and unchangeable Colour, which it would infallibly discover to our Eyes, if it could be seen, separated from the other Colours, which concur with it in forming the white or yellowish Colour of Light. But how indefatigable must be the Industry of that Philosopher, who could divide and resolve the whole Ray into its primary and elementary Rays, so that every one of them should exhibit its own Colour? It is certain this Division could never happen if these primary and homogeneal Rays were not of such a Nature that some are refracted more than others in passing all with the same Inclination out of one Medium into another, (from Air into Glass, for example) and by this Means are disunited and separated from each other.

This Proposition, that Rays which differ in Colour differ also in Degrees of Refrangibility, is the fundamental <25> Discovery, upon which the Newtonian System of Optics is built. Agreeably to this it is demonstrated that the violet Rays are the most refrangible of all. Next follow the indigo, then the blue, green, yellow, orange, and lastly the red, which are the least broken of any by Refraction. Do I explain myself clearly enough, Madam, for you to conceive my Meaning?

Extremely well, answered she: I understand very easily, that Nature, by making Rays which are different in Colour, different likewise in Refrangibility, has furnished Philosophers with a Means of separating them, which otherwise would be impossible. The Properties of this Light which you have been describing, are indeed very surprizing. A Philosopher must have a very great and enterprizing Genius that could make such Discoveries; and his Credit ought to be supported by very weighty Arguments drawn from frequent Observations, which I must confess I am extremely impatient to be acquainted with. I who was at first a <26> Cartesian, afterwards a Disciple of Mallebranche, now find myself without any System at all. This is a Vacuum that does not greatly please me, therefore I am in Haste for it to be replenished with other Observations.

These will very soon amply repair the Losses you have sustained by them, answered I. It would be well if every thing else in the World that does us an Injury, would make as good a Compensation for it.

Imagine yourself to be in a Place of Milton's visible Darkness, or rather still darker a Place, if you will be absolutely deprived of all Light; and this shall be our Theatre of Reasoning and Observations.

Let a Ray of the Sun enter in at a Hole made in the Window-Shutter, and let a Glass Prism be placed horizontally at this Hole, to refract the Ray in such a Manner as to throw it upon a Wall opposite to the Window, so that as it is going out of the Prism, it is almost horizontal and parallel to the Pavement of the Chamber; whereas if it <27> had not been refracted, but direct, it would fall upon the Pavement itself, and there form a white Image of the Sun of a Figure almost round. That Spectrum, or Image of the Sun, which the refracted Ray forms upon the wall, is very different from that which the direct Ray formed upon the Pavement; for as the Image of the direct Ray was almost round and entirely white, that of the refracted Ray is of a Figure nearly resembling a Fish at Cards, and varied with an infinite Number of Colours, among which the seven primary ones are distinguished and placed in a shining Order, one after another.

The jolly Peacock spreads not half so fair

The eyed Feathers of his pompous Train;

Nor golden Iris so bends in the Air,

Her twenty-colour'd Bow through Clouds of Rain,             Fairfax.

I am very glad to find that Tasso, said the Marchioness, who had before a little transgressed against the Laws of Refraction for the sake of his Armida, has amended his Fault, and is at present reconciled to Optics.


These Colours, answered I, with which the Image is painted, are disposed in such a Manner, that the red is in its lower Extremity; above this is placed the orange, afterwards the yellow, then the green and blue indigo, and lastly the violet, which is placed in the upper Part of the Image. Innumerable Degrees of intermediate Colours insensibly connect and unite the seven primary ones. Neither Correggio, Titian, nor his Rival Rosalba did ever unite and shade their Mezzo Tinto's with so much Exactness to form the Oval of a Face.

In order to explain this great Change, we must suppose one of these two Cases, either that Light is composed of Rays differently coloured and differently refrangible, so that the Prism does nothing but separate them from each other when they are transmitted through it; and by this means the different Colours are formed, and the Image which would otherwise be round, is of an oblong Figure. This is one manner of explaining this Phænomenon. The other is, that the Light in passing <29> through a Prism, acquires colours which it had not before, and moreover, that every Ray is shattered, dilated, and split into many other diverging Rays painted of a different Colour; and this is the Reason why the Image is coloured, and of an oblong Figure.

This last is the Supposition of Grimaldo, a Philosopher, who preceded Sir Isaac Newton; and this System is called the Dispersion of Light. You see it is necessary, unless we admit the different Refrangibility, to suppose this Dispersion, in order to explain why the coloured Image of the Sun should have a Length much greater than its Breadth, after being refracted by the Prism.

This Experiment, said the Marchioness, which has cost me so much Attention to understand, and this Oblongitude of the solar Image, are not sufficient to prove the different Refrangibility, because all this Phænomemon may be as well explained by Grimaldo's Dispersion of the Light, which is a System very different from Sir Isaac Newton's. I want to see some Experiment, which <30> cannot possibly be explained by any other System than the Newtonian, and that I believe would satisfy me. This, answered I, is the very thing necessary to prove not only the different Refrangibility, but every other Principle in Philosophy; and this Sir Isaac Newton has done (without knowing, perhaps that it would one Day give Pleasure to a fine Lady) whatever a certain Author may say to the contrary, who accuses him with having drawn more Consequences from his Observations than he ought to have done, one of the greatest Faults that a Mathematician can be charged with. This Author reproaches Sir Isaac with having inferred the different Refrangibility of the solar Rays from the preceding Observations; whereas Sir Isaac declares in express Terms, that the Observation is not sufficient for that Purpose, because this strange Appearance of the Image may proceed from the Shattering of the Rays, as Grimaldo supposes; or from an Inequality of Refractions, not constant, but only casual, and therefore can have nothing deduced <31> from it. The more scrupulous this great Philosopher appears in his Reasons, the more licentious does his Adversary seem in his Accusation. In order to remove the Dispersion of Grimaldo, and the fortuitous Inequality of the Refractions, he invented the following Experiment, which is as it were the Arbitrator and Judge of the Controversy.

He received the coloured Image formed by the Prism, and cast upon the Wall, upon the Face of another Prism placed upright, in such a Manner, that the red of the Image should fall upon the lower, and the violet in the superior Part of this Face, and the other intermediate Colours should fall respectively in the intermediate Spaces betwixt the red and the violet.

If the first Prism, which was placed horizontally, refracted the Rays upwards, this second, placed upright, must refract them sideways, either on the right or the left; so that if they were at first thrown almost directly upon the Wall opposite to the Window, they must now strike it obliquely, and <32> with some Inclination. The Refraction then which the Colours are to suffer in passing sideways thro' this second upright Prism, is the Thing which must determine the Question, either in Favour of Sir Isaac Newton's Supposition of the different Refrangibility, or for Grimaldo's Notion of the Shattering of the Rays, or lastly, must give the Preference to a fortuitous and casual Inequality of the Refractions, which cannot agree with any System at all. For, if the solar Image, formed by the first Prism which refracted the Rays upwards, received its Colours and oblong Figure from a Dispersion or Dilation of every incident Ray, a second transverse and sideways Refraction caused by the second Prism, must after the same manner shatter and dilate the Rays of this Image sideways, and render it of the same Oblongitude in Breadth as it had before been in Length; so that a new Image would be painted upon the Wall of the Chamber which is behind the second Prism; and this Image would be coloured differently from what it <33> was before, and be changed from an oblong Figure, to one almost square.

Lastly, if the Colours and oblong Figure of the Image formed by the first Prism, were occasioned by a fortuitous and accidental Inequality of Refractions, who can tell what Variations Chance might have produced in the Combination of the second Prism, and in the new Refraction which that Prism gave to the Light?

But whatever Effect Chance might produce in this Case, it is certain it could never agree with what the Newtonian Systems aims at. According to this System, if the colouring and oblong Figure of the Image formed by the first Prism, were occasioned by the Separation of Rays differently coloured and differently refrangible, a second Refraction made sideways can only incline this Image, and must leave it just the same as it was before with regard to its Colours and oblong Figure.

How will it incline the Image, said the Marchioness? I do not understand the Reason of this. You will soon <34> understand it, answered I, when you reflect, that if the second Prism was removed, the Rays would all strike the Wall almost in a direct Line. Now if the second Prism refracts the violet Rays, that is, turns them sideways and transversely out of their Path, more than it does the red, those must strike the Wall more obliquely than these, or in other Words, the violet must fall at a greater Distance from the Prism than the red. The intermediate Colours too, between the red and the violet, will fall upon intermediate Places of the Wall: Thus the Image will appear inclined, and as it were learning with its violet Extremity farther from the Prism than the red is. These Effects must happen according to Sir Isaac Newton's System; and these in reality do happen, as I myself have often had the Pleasure of seeing.

If after the second Prism there be placed a third and fourth, in order that the Image may be successively refracted sideways through them all. Those Rays which were refracted more <35> than the rest in the first Prism, will be more refracted too in the following Prisms. But the Image will not be dilated sideways, nor coloured differently from what it was at first.

Nature, said the Marchioness, has pronounced the grand Judgment, and of three Systems that contended for it, the Newtonian has carried the golden Apple. I must confess this Decision does not displease me; for to say nothing of the accidental Inequality of Refractions which does not deserve the Prize, Grimaldo's Supposition of the Shattering and Dilatation of every particular Ray, had something too perplexed and imbarrassing in it. If you think the Judgment which Nature has pronounc'd in favour of our Philosopher, answer'd I, to be so just, that of his Adversary, whom I lately mentioned, will appear extremely capricious, who asserts, that Sir Isaac Newton has, by agreeable Experiments, confirmed the Observations of Grimaldo. I am not so greatly surprized at this Adversary, replied the Marchioness, who does not appear to <36> have any great Knowledge of the Matter, as at Grimaldo himself, who neglected to prove the Truth of his Dispersion of the Rays, by so easy and simple a Experiment as this, which requires nothing more than a second Prism after the first. One would imagine that it should have been very obvious to a Person bent upon making a System. Say rather, answered I, to a Person long exercised in the Arts of Observation, for a Love of building Systems and making Experiments, are two Things that seldom go together. But it generally happens that the most simple Things are the most difficult, and consequently the longest before they are found out. The Circulation of the Blood, for Example, appears to be a very easy Discovery, and one would imagine should have been very anciently made. When an Orifice is opened in the Arm the Arteries swell from the Heart towards the Extremities of the Body, and the Veins, on the contrary, from the Extremities of the Body towards the Heart. This evidently shews that certain Vessels, that is, the Arteries, <37> are designed to convey the Blood from the Heart to the extreme Parts, and other Vessels, namely the Veins, to carry it from the Extremities to the Heart. Besides, the Death of Seneca might have furnished the Ancients with a physical Experiment as well as a moral Precept. It was impossible that all the Blood should be emitted through the opening of the Veins, unless those of the superior Parts had a Communication with those of the inferior; or in other Words, unless it circulated through the whole Body. It appears then that it was much easier for the Ancients to discover the Circulation of the Blood, who had so many Experiments ready prepared to their Hands, than for Grimaldo to find the Falsehood of his Dispersion, because his Experiments must have been the Effect of his own Labour and Invention.

It is true, some Bigots for Antiquity pretend to find this Discovery in Hippocrates, so that according to them, all the Inventions of the Moderns, and all our Distempers were known to the An <38> cients. But this Notion is the same as if a Vellutellus, or some other zealous Admirer of Petrarch should discover the Newtonian System of Optics in these Verses,

While the great Author of his Frame expires,

The conscious Sun withdraws his active fires;

Pale sickly Shades ov'rspread his languid Ray,

And ev'ry beauteous Colour fades away.

The most simple Things are generally discovered the latest, and with the greatest Difficulty. This Aphorism, said the Marchioness, is too well verifyed even in the Toilet, where an elegant, but simple Disposition of our Hair or our Patches, often costs great Trouble, and the utmost Anxiety of Mind.

According to this Principle, answered I, Sir Isaac Newton's Experiments must have cost him an infinite deal of Labour. For if the preceding Experi <39> ment, to prove the different Refrangibility of the Rays, is at once simple, agreeable, and conclusive, all the others which he invented for the same Purpose are no less perfect, and yet appear so very obvious, that every one would imagine he himself might have as easily found out.

How, said the Marchioness, is not this Experiment sufficient to prove the different Refrangibility without seeking for any more? Have I acted wrong in suffering myself to be too easily convinced? No, Madam, answered I, a Lady cannot err in this Point. But Sir Isaac Newton himself does not desire you should assent to his System so soon. This Experiment, without Dispute, is sufficient to demonstrate the different Refrangibility, but not to satisfy a Philosopher resolved to try Nature a thousand Ways, and put her to a thousand Proofs, in order to establish his Belief on a sure Foundation. You seem, said the Marchioness, to represent Nature as a Coquet, and Sir Isaac Newton as a jealous Lover, who never thinks he has <40> Proof enough of the Fidelity of his Mistress. This, however, answered I, was the only Object of his Love. I am very sorry that I cannot shew you all the Experiments which he invented for this Purpose, in order to present you with the most finished and beautiful Piece that philosophical Jealousy ever composed. But the Parts I shall shew you will assist you to form an Idea of the whole, just as the Obelisks and Amphitheatres discover the Grandeur of ancient Rome. Let me intreat you, said the Marchioness, to make me a compleat Newtonian. I plainly see that by my Conversion I shall acquire the Knowledge of Truth without losing that Pleasure which I found in being deceived.

In the dark Chamber which we have prepared for our Experiment, continued I, let a white Thread be extended horizontally against the Window, but at some little Distance from it. Let two Rays of the Sun enter at two Holes made in the Window-Shutter, which refracted by the two Prisms, may paint <41> two coloured Images upon the opposite Wall. When this is done, we must recommend ourselves to the Genius which presides over Optics, and then patiently wait till the Half of this Thread be illuminated by the red Rays of one Image, and Half of it by the violet Rays of the other. Let the Wall opposite to the Window be covered with a black Cloth, that the Colours which would otherwise be reflected by the Wall, may not disturb the Experiment; for at present we want no other Colours but those of the Thread, which must be alone distinguished. This Thread must be observed through a Prism placed before the Eyes in such a Position, that all the Objects seen through it appear higher than they really are. The Thread too will appear to be transported higher by the Refraction; but because the violet Half should suffer a greater Refraction than the red, it will be much more transposed than the red, so that the Thread will appear divided into two Parts, the one illuminated with violet, the other with red, <42> and the red will appear lower than the violet.

This Experiment, if we should exclude every other, is intirely agreeable in all its Parts to the Newtonian System. If the violet Part of the Thread be illuminated with indigo, the Thread will appear less divided than at first, the indigo Half approaching nearer to the red than the violet did, which must necessarily happen, because the Difference of Refrangibility between the indigo Rays and the red, is less than that between the red and the violet.

If from Indigo this Half be illuminated with blue, the other Half still remaining red, the Thread will for the same Reason as I before mentioned, appear less divided than at first, and still successively less divided than at first, and still successively less if it be illuminated with the other colours in order, green, yellow, and orange, till at length becoming red like the other Half, the Thread will no longer appear broken nor divided in two as at first, but whole and continued, because now the Colours of each <43> Half have no Difference in their Refrangibility.

A like Experiment may be made with a Paper, the one Half coloured with red, and the other blue; placed afterwards upon a black Cloth, and looked at through a Prism, it will appear broken and divided into two Parts. A Paper illuminated with four Colours, (which I have myself seen demonstrated) that is red, yellow, green and blue, ranged one after another in the order I have named them, appeared through a Prism divided into four Pieces, like the Steps of a Ladder. The blue is sometimes the highest of all, and sometimes lowest, as the Position of the Prism requires. This Experiment varied in as many Ways as the fruitful Imagination of Paul Veronese would vary the Subject of a Picture, always succeeded so well, as to have greatly confirmed this System, if its Author had suffered it to need any Confirmation.

I must ingenuously confess, said the Marchioness, that tho' I have always regarded the Mathematicians with a sin <44> gular Veneration, I do not yet understand what their Demonstrations are. However familiar they may at present be rendered, I do not comprehend them enough to find the Solution of a Problem among the Patch-Boxes and Perfumes on my Toilette. I now begin to fear that my Ignorance of the Deity I worshipped greatly increased my Veneration for it. Their Evidence ceases so great a Noise in the World, that I was perswaded every thing else, however well it might be proved, had only a small Degree of Probability when compared with these. At present, I cannot conceive it possible for any mathematical Demonstration to be more certain than Sir Isaac Newton's different Refrangibility, and yet this is a thing merely physical. But you are to consider, Madam that the Person who has treated upon this physical Subject, was the greatest Mathematician that ever appeared in the World. We may affirm then, replied the Marchioness, that as every thing which Midas touched was transformed to Gold, so every thing <45> that Sir Isaac Newton handled became Demonstration.

If ever Physics could hope to vye in Certainty with Geometry, answered I, they might with some reason expect it when treated by Sir Isaac Newton, tho' there is a very great Difference in the Nature of their Proofs.

Physics can only consider a vast Number of Particulars, make Observations upon them, and thence deduce general Propositions, whereas Geometry by a more expeditious and certain Way, abstracts particular Cases, and founds its Demonstrations upon Nature; and the Idea of the Thing itself, whereon it treats.

All that a Mathematician demonstrates to you concerning one Triangle, will be true in all, be they of what Species they will, because he considers nothing but what is necessarily included in the Nature of a Figure terminated by three right Lines; and as this is found in all Triangles that can possibly be either made or imagined, his Proposition will hold true in all.


On the other Hand, a Naturalist will tell you, that all Bodies here below gravitate, and if left to themselves descend; but he does not, like the Mathematician, deduce this Proposition from the Nature of Body, (for that is unknown to him) but from a daily Observation that Gold, Silver, Gems, Water, Air, and a thousand other Bodies gravitate, and do it constantly by Day and Night, in Summer and Winter, fair and cloudy Weather; from whence it may reasonably be inferred by Induction, that every other Body gravitates at all Times, and in all Places.

But however reasonable this Method may seem, so great a demand for Proofs implys a Want of Demonstration, just as a too great Attention to Dress, argues some Defect of natural Beauty in a Face. Who can tell, but notwithstanding such a Multiplicity of Observations, some may question whether there be not a certain Body with which we are yet unacquainted, that has no Gravitation? Or if there be not some Country in the unknown Tracts of the southern Pole, where Bodies have <47> not that Quality of Gravitation which we discover them to be indued with in all the known World? Or lastly, whether in past Ages there may not have existed a certain Body that did not gravitate? You will grant, however, said the Marchioness, that where the Number of Observations is so great as that from whence the Gravitation of Bodies, and the different Refrangibility of the Rays of Light are deduced, that Person must be inexcusable who doubts their Evidence, unless it was prescribed him by a Physician for his Health.

If there are some, answered I, too excessive in their Doubts, there are many others too bold in their Assertions. All do not imitate the prudent and necessary Reserve of our Judicious Philosopher: Some are contented with one single particular Case, from which they hastily deduce a general Conclusion, like those who form a Judgment of the Temper and general Character of a whole Nation from the particular Humour of a single Person, whom they have seen perhaps once or twice at a Coffee-House.


Sir Isaac Newton's Antagonist whom I lately mentioned, imagining he had overthrown the System, and principally the different Refrangibility, in order to prove himself the Opponent of this great Author, even in the Method of philosophising, has put together a general System (hinted at by others, but not followed) formed upon particular Cases, which well examined, are nothing but Consequences of that which he imagined himself to have overthrown. He supposes certain Grounds, and a Mixture of Light and Shade; and the different Combinations of these are according to his Opinion, the Cause of different Colours --- Can a Combination of Light and Shade, interrupted the Marchioness, ever produce red or yellow? A Phænomenon must be very unfortunate, whose Explication depends on this System. Perhaps, answered I, those Phænomena which contradict general Laws, those Monsters of Optics, if there are any such, are sent by Nature to this System for their Explication; and do not these fine Colours <49> of yours to, deserve a little Punishment for all the Mischiefs they have caused? But see the unhappy Condition of the poor prismatic Colours, which certainly do not merit the Correction that yours do; and from hence you may form an Idea of the Value of this System.

It affirms, that when a Ray of the Sun is refracted by a Prism, these colours are produced by means of two sorts of Images; the one formed by the Dispersion of the solar Rays, and the other by that of the Rays of Heaven, which are contiguous to those of the Sun. What, said the Marchioness, does this Philosopher attempt to bring this Dispersion again upon the Stage? Had he never seen the Experiment of the second upright Prism, which has for ever banished this Supposition from the Province of Optics?

Authors, answered I, have their Eyes formed differently from those of other Men. The Sun is light, and the Sky comparatively dark. This was sufficient to furnish the Philosopher we are speaking of, with Abundance of Rela <50> tions betwixt Light and Shade (the Veils, as he expresses it, formed by these two Images) from whence he draws and Explication of the Difference of Colours in a Prism. I fancy, said the Marchioness, this Explication will not be very simple; it appears to me sufficiently perplexed. Not to insist upon this and many other Difficulties which this System is chargeable with, we will confine ourselves to the following Objection, answered I.

If it be true that this Diversity of Colours depends on a Mixture of the Rays of these two Images of the Sun and Sky, and from their shadowing each other, it is certain that if there can be found a Method to prevent the celestial Rays from coming to the Prism, and consequently from being refracted and mixt with those of the Sun, the Colours will vanish with all that fine Theory which arises from the Mixture of those two sorts of Rays.

Now this may easily be effected, if before the Ray of the Sun (which enters at the Window of the dark Room) <51> be refracted by the Prism, the Middle of the Ray be transmitted through another Hole made in a Table, or a Pastboard placed at some Distance from the Window: In this Case so far is the Prism from receiving the celestial Rays contiguous to those of the Sun, that it receives no Rays from the Sun himself, but what flow from his Disk, and is not at all affected by those which proceed from his Edge. It is evident then that if this System be true, the Colours of the Images in this Case could not appear, which is absolutely contrary to Experience, a Disgrace pretty familiar to this System.

You are to suppose, answered I, that an Author is often as strongly desirous of giving his Name to a new System, as a French Lady can be of giving her's <52> to a new Fashion. How happy would they be, if, like that Chinese Emperor, who burnt all the Books of History preceding his Reign in order to make his Name the first Epocha of it, they could destroy all preceding Systems to make their own the Epocha of human Knowledge.

Besides, Sir Isaac Newton's System came from a Country too far beyond the Alps, to be favourably received among Italians. It would be very surprizing, if a System, produced in England, had not been treated with Aversion by some Persons in a Country so near the Sun as ours. I do not see, replied the Marchioness, why a System should meet with the worse Usage for being a Native of England. For my Part, as much an Italian as I am, I do not believe that I should be prejudiced against a well grounded one, even if it had been produced in Ireland or Nova Zembla.

You are not to imagine yourself, answered I, to be ranked with the Generality of Mankind. a Sea, a River, or a Chain of Mountains placed between <53> some People and a certain Truth, are insuperable Objections to their Reception of it. Perhaps as the Romans found a certain Je ne sçais quoi, in the Stile of Livy, which discovered the Paduan, so our Italians find a Je ne sçais quoi, in every Truth that comes from beyond the Alps, which does not agree with their Taste. These Gentlemen, said the Marchioness, must have a very distinguishing Judgment to discover such Differences as these, or it rather argues, they have no Taste for Truth, who find any thing foreign in the Proofs of a different Refrangibility. In this I may venture to affirm myself a better Italian than they, since the least Difference in this Case must turn to our Disadvantage.

You, Madam, said I, are a Citizen of the World, and your Senses formed for Truth are Proof against the Objections of those who only appear to be zealous for it. You will find a new Demonstration of the different Refrangibilities drawn from the Difference in the Focus of a Lens, through which the different Colours are viewed. <54> The Image of the Letters of a Book, formed by a Convex Glass, illuminated by the red Rays of a Prism appears distinct at a certain Distance from the Glass: The Image of the same Letters, when illuminated by the blue Rays, is not distinct, but at a less Distance. In the like Manner, the four Colours, red, yellow, green, and blue, of the Paper we before mentioned, are not equally distinct when placed at an equal Distance from the Lens. The blue is nearest, next follows the green, afterwards the yellow, and last of all the red, whose Rays being less refrangible than the rest, must be collected and united at a greater Distance from the Lens.

Might not some wise Caviller, said the Marchioness smiling, gravely object, that the Book which first received the red, and afterwards the blue Rays, was wrote in English; and that in order to prove the different Refrangibility, it was necessary for it to be Italian? But after all, it is an infamous Thing to be so hardened against Truth. Are not these <55> Experiments sufficiently decisive? And can there in any Country of the World be assigned a Cause, why the Image of one Colour should be nearer to a Lens than that of another, unless it be the different Refraction which they suffer in passing through the Lens. Do not put yourself in a Passion, Madam, said I, the different Refrangibility will be true, notwithstanding these Objections. You may still safely believe it, as many other judicious Persons do, in Defiance of that obstinate War which the Antagonist of our Philosopher declared against it. The Newtonian System had the same Fortune as that Field where Hannibal encamped when he besieged Rome, which did not fell at all the worse upon that Account: Or rather, you may look upon these Objects as the satyrical Verse, those miserable Invectives which the Licentiousness and Malignity of the common Soldiers, mixt with the Acclamations and Glory of a Roman Triumph. The Beauty and Simplicity of this System did not deserve to pass unmolested by Envy and <56> Censure, that Tax which Merit is bound to pay to the Public.

A celebrated Minister[1] at once capable of the sublimest Projects and the lowest Employments, and a whole College, joined their Forces against the Applauses paid to the Cid. For a like Reason Moliere's Misanthrope was recited to the same Audience that listened to the Sermons of Cotin. How often have the celebrated Caracci had the Mortification of seeing their fine Pieces sold in their Life-time by the Ell, (if I may use that Expression) which now are the Ornaments of the best chosen Galleries, and paid by the Admiration of Connoisseurs, better than by the Gold of the Rich!

It was necessary for the Honour of Sir Isaac Newton's System, that it should be attacked on all Sides, and that some should dispute the different Refrangibility of the Rays, while others wrangle about the Immutability of Colours, which is another of their Properties discovered by our penetrating Philosopher.


The Experiment upon which this new Quality of Colours is principally founded, was renewed in France by Monsieur Mariotte, a Man exceedingly well versed in the Arts of Observation. This Experiment in his Hands produced an Effect contrary to what Sir Isaac Newton had given Reason to hope for. Thus a System, the slow and considerate Offspring of Reasoning and Experience was esteemed imaginary and vain; and a grave Philosopher, who spent his whole Life in the Study of truth, passed for a Reveur, or an Impostor.

What is this you tell me, said the Marchioness, of an Experiment in France contrary to that of Sir Isaac Newton? Is it possible that two Men equally attentive and accustomed to Observation, should need a third to decide a Matter of Fact? It is not surprizing that two People should reason differently upon the Circumstances of a Fact according to their different Principles, as in the Case of a Person who shifted his Linnen three times a Day, one asserted that he must be extremely neat <58> and the other that he was a very Sloven. But to dispute and mutually to deny the Fact itself, I always thought reserved for silly Women and Enthusiasts.

It is certainly, answered I, a great Reflection upon Philosophers, when we find them dissent in Matters like these; it at least demonstrates that one of them must have been inattentive in observing the Laws of Nature. Those rational Horses so far superior to us Men, which Gulliver discovered in the Island of the Houyhuynms, the last Place where he arrived at in his allegorical Voyages, were extremely surprized to find such Contradictions among our Philosophers, that is, those of our Species who take the most Pains to cultivate their Reason. These happy Animals know not the Meaning of Uncertainty and Doubt in Matters of Fact. The Dishonour that Philosophers receive upon this account is very great even among us, and there are too many Examples of it.

Two famous academics which have equally Truth for their End, and Emulation for their Companion and Guide, <59> disputed upon a Fact, by which is demonstrated the Refraction which Light suffers in passing from a Vacuum into Air. That academy which maintained the Refraction was at length victorious; and it was necessary that even this Truth should be contested in order to be admitted. Some from Experience will tell you, that in breathing, the Air passes from the Lungs to the Heart, while others alledge the same Experiment to prove that it does not. Many discover certain little Machines and Organizations in the Glands of a Body, which others affirm cannot be seen. Fancy and Prepossession have the same Influence here as in all other Things, and make us think we discover in Objects, without us, what runs strongest in our own Mind. Thus a few irregular Strokes will appear to the Eyes of a Painter the Contour of a Leg or a Face, Wind-mills become Giants to Don Quixot, and Fires and Beeches seem transformed to a fine Lady in the Eyes of a Lover.

An Observator must not search into <60> Experiments in order to find his own Opinions there, like that Person who looked for his Pedigree in Homer, because both the one and the other will every where meer their own Imaginations. Natural Philosophy, like Poetry, requires a Man formed expressly for it, a Malpighi, a Reanmur, a Boyle, not moved by Authority, seduced by Fancy, nor terrified by Difficulties, a Man, (if we will believe a celebrated Writer) dexterous, active, and curious as the French and English, and reserved, cautious and circumspect as the Italians and Spaniards. Why not the Patience of some other Nations, said the Marchioness, instead of the Caution of ours, which is too like Diffidence to redound greatly to our Honour. This Author, answered I, considers only the good Qualities of different Nations. But you would be better pleased, perhaps, if the Part we contribute to the Formation of a perfect Philosopher, were the religious Attention of our passionate Lovers. I am acquainted with one of <61> these, said the Marchioness, who would be a Newtonian, if Philosophy was his Mistress.

This Attention, answered I, might carry these Gentlemen to a great Height of Superstition, as it does a Naturalist, who prescribes it as a Rule of Art, whenever an Experiment is made, to mark exactly the Country, Hour of the Day in which it was effected, what Wind blew at that Time, the Degree of Heat, and Drought of the Air, with many other Circumstances of this Nature, which in some Cases may be proper, and even absolutely necessary, but in others I really do not see of what Service they are, for in looking upon a Paper of two Colours with a Prism, it is of no Sort of Importance whether the Wind blows East or West, whether the Time of the Year be Autumn or Spring, the seventh or twentieth Day of the Month. These circumstantial Naturalists resemble an Antiquarian, who should copy the Cornice of an Inscription with as much Exactness as the Inscription itself.


Physic, replied the Marchioness, has almost diverted itself of the Prejudice of observing certain Seasons of the Moon, as a proper Time to apply its Medicines; and perhaps Natural Philosophy has assumed it in her Stead, that these superstitious Notions may not be destroyed, but there may always be pretty near an equal Dose of them in the World.

It must be confessed, however, answered I, that there may always be hoped some good Effects from this diligent Disposition, even tho' it be carried to such a Degree as to become ridiculous: But we can never expect any Good from Negligence. You will see an evidence Proof of this, in that celebrated Experiment of Sir Isaac Newton, before which all the ancient Idols of Optics fall to the Ground; those imaginary Systems which supposed Colour might be changed by Refraction, Reflection, the Confinity to Shadow; and in short, that Colour is nothing but a certain Modification, as they express <63> it, of Light, which may easily be changed by these Circumstances.

But Sir Isaac Newton has shewn the Falshood of this Opinion, and demonstrated that a Ray, for Example, red, well separated from all the rest, will constantly keep its Colour in Spite of any Refraction or Reflection it is made to suffer, or any other Method that the fruitful Invention of a Naturalist can contrive to torment it. The same Constancy will hold good in all the other Colours, if they are well separated. The grand Experiment which furnishes us with these agreeable and surprizing Truths, is this which follows.

The Image of the Sun formed by the Conjunction of a Prism and Lens is received upon a Paper; by this Conjunction the Colours are much more unmixed and better separated than they would otherways be.

Having thus made a more perfect Separation, the Rays of different Colours must pass successively through a Hole in the Paper, that they may be <64> refracted by a second Prism, in order to prove whether this new Refraction can produce any new Colour. If it should, we must confess that Colour is nothing more than a certain Modification which the Light acquires in passing through the Prism; and then Philosophers may set their Imaginations to work, in order to find out what Notions, Figures, and the like, are necessary to produce this Modification.

But on the other Hand, if the Ray constantly preserves its own Colour without the least Alteration, we must agree that Refraction has no Share in the Production of Colours, and abandon the ancient System of Modification; and all its ingenious delusive Dreams will vanish at the rising Morn of the Newtonian Truth.

Now the Experiment we are upon, demonstrates that a homogeneal Ray, red, yellow, blue, or of any other primary Colour, is not in the least altered, by a new Refraction, nor even by a great Number of Refractions which it is successively made to suffer. It is not <65> changed either in its Colour or Degree of Refrangibility, which remains constantly the same. Thus if two Rays, one red, and the other violet, be made to fall one after another upon the second Prism, with the same Incidence (that is, if both the Rays coming from the same Point, fall likewise upon the same Point of the Prism.) If the Rays, I say, fall upon the second Prism in this manner, the violet after the second Refraction, will strike the opposite Wall in a Situation higher than the red, and the intermediate Colours in intermediate Situations, those which had suffered the greater Refraction in the first Prism, suffering the greater int eh second likewise; all these Colours will pain upon a white Paper directly opposite to them, a little Circle perfectly round, and not oblong like the Image made by the first Prism, and this Circle will be of the same Colours as the Rays, without any Addition or Mixture.

I beseech you to stop and take Breath, said the Marchioness, you were ingaged in such a long Period, that I began to <66> think you would never get to the End of it. I should be sorry, Madam, answered I, if the Length of the Period has rendered me obscure, and made you lose so fine an Experiment. No, no, said the Marchioness, I understand it very well. Does not all your Meaning amount to this, that the homogeneal Rays of Light are immutable both with Respect to Colour and Degree of Refrangibility? I am extremely rejoiced, said I, to find I may be as prolix in my Periods as I please, without Apprehension of Obscurity, and may employ one as long as those of the Azolain Dialoguists, to tell you that this is the Experiment which Mr. Mariotte endeavoured to make, and by some Misfortune found that after the second Refraction certain new Colours were united to the red and blue. I know not how this happened, but probably it was from some Defect in the Prism which he employed.

The ill Success of this Experiment would have done the Immutability of Colour no small Injury in the learned <67> World beyond Sea, if it had not been repeated in England before some learned Frenchmen, the Motive of whose Voyage was purely philosophical. This Repetition of the Experiment clearly demonstrated, that Mr. Mariotte, tho' otherways an accurate Observer, has failed in some of the Circumstances necessary to bring it to good Effect. Thus were these two Nations more divided by their different Ways of thinking, than by a little Arm of the Sea, reconciled upon this Point.

This Law of Nature common to all Nations acquainted with Light, met with a less favourable Reception in Italy than in any other. The most obstinate Antagonists of the Newtonian system have risen among us; there seems to be some Fatality in this, that a Nation which the Italians once found so difficult to subdue by Force, should in its Turn find us the most difficult to be subdued by Reason.

In order to contribute my Share to the Establishment of this Law even among us, I caused the same Experi <68> ment to be repeated at a Place in Italy, celebrated for the learned Men it has always produced, and neuter enough not to be suspected of Partiality. No Minister of State, said the Marchioness, could act more politickly in the Choice of a proper Place for the holding a Congress. Within a very little, answered I, all my Politics would have signified nothing: For tho' we made use of Sir Isaac Newton's own Method in separating the Colours, and the Chamber was extremely dark, yet a certain Light of a blueish Cast always mixt itself with the Colours refracted by the second Prism. 'Tis true, this Light was irregular and inconstant, but however, it was sufficient to serve for an Excuse to the Incredulous.

This Appearance gave us a real Inquietude; and we would never have slept in Peace if we had not, after much Study, found out the Cause. We observed that the Borders of the coloured Image were not so well terminated as they ought to have been, provided the Prism, by which it was coloured, had <69> been good. We discovered a Light round the Edges of the same Nature with that which was observed to be mixed with the Colours refracted the second time, and we perceived that several Streaks of this Light crossed the Image from one End to the other. From all these Circumstances it appeared, there must be several Irregularities in the Prism, as Bubbles of Air inclosed in the Glass, Unevenness in the Surface, and other Things of the like Nature, which were probably the Reason why the Light was irregularly refracted, and by this means rendered it impossible for the Colours upon the Image to be perfectly separated.

Various and repeated Experiments made it evident that the Fault of this apparent Alteration of Colours, which we observed in the Image, must be imputed intirely to that irregular Light which we had before suspected to be the Cause, if we may call that an Alteration which was nothing but the Addition of one Colour to another.


I congratulate you, interrupted the Marchioness, that after such a Discovery nothing will hereafter disturb the Tranquility of your Sleep. Heaven preserve me, answered I, from that cold tedious Tranquility, which is however the Object of our Desires. 'Tis in Philosophy as in all other human Affairs, where the Accomplishment of one Desire often gives Birth to another.

When we had discovered the Cause of this Defect in our Experiments, our next Business was to find some Remedy for it. The Difficulty of acquiring it was a new Motive for our Labour and Inquietude.

Our Prisms in Italy are of no other Use than to amuse Children, or hang up as a fine Show in some Window in the Country, and not for the Service of Philosophers, who are often too exact for Artificers, and require a greater accuracy than is in the Power of Art to bestow.

We resolved to write to England, which has its Fawkeners for Lapidaries, and its Grahams for Clock-making, <71> where, in short, every Thing seems to be contrived for the Service of the most curious and importunate Naturalists, if Fortune and our good Genius had not unexpectedly furnished us with some which were just arrived from that Country. These we esteemed as sacred as the Romans did the Article or Shield which fell from Heaven in the Reign of Numa; and we could have wished for a Mamurius to make as many Copies of these as he had formerly done of that.

With one of these Prisms we renewed the Experiment, and the coloured Image painted by it, emerged so beautiful, so well terminated, and so lively, that the other seemed no more, when compared to this, than a rough Draught to a finished Picture. The Colours refracted by a second Prism remained so immutable, that neither the most penetrating Eye, nor even the Zoilus of the Newtonian System would have discovered the least Alteration.

Perhaps, said the Marchioness, Nature has reserved the Merit of demonstrating Truth to the English Prisms, <72> that is, to those by whose Means she at first discovered herself. It would be a very curious Phænomenon, answered I, to observe such a Partiality in Nature, as for her to prefer such a Prism made in London, to one produced in Murano. But the Truth is, that if we consult her as we ought, she always answers the same, whether the Prism be English or Italian, provided it be good and well worked; and the Chamber, where the Experiment is made, extremely dark.

If all these Circumstances are rightly prepared, the Colours, tho' refracted three or four times, will remain unchangeable, neither more nor less with Regard either to Colour or Figure, than an Object exposed to a homogeneal Light, and viewed through a Prism. The Variety of Colours, the Change of Figure, and the Confusion discovered in Objects looked upon in this Manner, proceeds from nothing but their reflecting more or less all Sorts of Rays, which being afterwards differently refracted, produce all these Irregularities.


If upon a Circle of Paper there fall at the same time the red of one Image and the blue of another, so that the purple Colour, composed of both these, may appear, it will, when examined through a Prism, seem divided into two separate Circles, the one red and the other blue; and the Reason of this Appearance is the different Refraction of these two Colours. If the yellow and green of two other Images should fall at the same Time upon the Paper, so that it would be illuminated with four Colours at once, it will appear oblong and divided by Refraction into as many Circles as there are Colours, and these Circles will seem placed upon one another. I know you are going to add, said the Marchioness, interrupting me, that if this Paper be exposed to the Light of the Sun, who contains every Sort of Ray in himself, it will appear of a Figure still more oblong, and painted with all the Colours of the Rainbow; whereas, if it be illuminated by only one homogeneal Light, it will alter <74> neither its Figure nor Colour when observed through a Prism.

You must forgive the weakness of human Nature, Madam, answered I, if we generally finish the Sentence we have begun. It was reserved for Sir Isaac Newton and you, to understand Nature by half a Word, and find out Natural Philosophy, notwithstanding its Uncertainty. It is superfluous to tell you that Flies and other little Objects, placed in a homogeneal Light, appear very distinctly to the Eye with a Prism; and the very smallest Elziver Print may be read without any Difficulty. But the same Things will not have this Effect when placed in the heterogeneous Light of the Sun, because of the Confusion and Quantities of Colours that arise.

In this Case I abandon the Prism to the Disposal of Poetry, to make Use of it in Comparisons, which do it no great Honour. That celebrated Author, whose fine Ode you so much admired, compares it to false Eloquence which obscures the Face of Truth, lavishes its <75> Ornaments without Distinction, and diffuses its glaring Colour over every Thing. It is certain that the Poet in this Comparison, means that Prism which transmits the Rays of every Sort. And when it transmits only the homogeneal Rays, said the Marchioness, may we not compare it to true Eloquence and true Spirit? For as it shews us the Objects removed out of their Place without any Alteration, so true Wit often surprizes us only by varying common Subjects in a new Manner.

You are so well acquainted with the Prism, answered I, that you may safely compare it with your own Wit. But I know not what Comparison you will find for the Immutability of Colours, unless you seek for it perhaps in your own Heart; and then knowing that Reflection has no more Influence on that Immutability than Refraction has, you will be better acquainted with it than you at present are.

If the Colours with which Bodies are variegated and painted were only a Modification, which the Rays of Light <76> acquire by being reflected from their various Surfaces, as was once believed, a Body which appears red in the Light of the Sun, would still remain red when placed in any other Colour of the painted Image, for Instance, blue, because it could modify this blue Colour refracted and modified already by the Prism, as well as it does the Light which comes directly from the Sun.

But Sir Isaac Newton has proved by Experiments, that every Body placed in homogeneal Rays is of the same Colour as the Rays themselves, without allowing, however, the Supposition that Bodies by Reflection modify the Light in such a Manner, as to make it receive this or that particular Colour. Thus all white, red, yellow, blue, green Bodies, as Paper, Scarlet, Gold, Ultra-marine, and Grass, in red Rays appear totally green, in blue, totally blue, and so of the rest. The only Difference consists in this, that all these various Bodies placed in the same homogeneal Light, are not all equally luminous, but every Body appears the most lively when <77> placed in a Light of its own Colour, except Paper, and all other white Bodies which equally receive any Colour, and may be regarded as the Cameleon and Proteus of Optics.

Would this Diamond then, interrupted the Marchioness, if placed in the Rays of our coloured Image, take indifferently any Colour, and at one Time be transformed to a Ruby, at another to a Topaz, an Emerald or a Sapphire? So much the better, answered I, as it would dart only one unmixed Colour, and all the varying Slendors with which it sparkles in the direct Light of the Sun, would vanish in this homogeneal Light.

It is very pleasant to observe how the fine Dust or Atoms in the Air change Colour as they pass from one Ray to another, just as a River varies its Colour according to the various Qualities of its Bottom.

Other Bodies, as I before mentioned, are not equally lively when placed in different coloured Lights. The Varnish, for Instance, with which Martino, <78> the Emulator of the Chinese Art, makes so many fine Curiosities, is extremely luminous in the red Light, less in the green, and still fainter in the blue. On the contrary, the Lapis Lazzuli, which has the Honour to serve for a Repository to your Snuff, is extremely lively in the blue, less so in the green, still fainter in the yellow, and almost dark in the red.

The same Inequality holds good in Bodies seen by a transmitted Light, as is proved by Glasses of different Colours; so that every Body reflects or transmits in great Abundance, those Rays which are of its own Colour, and reflects or transmits the other Rays, in Proportion as they are nearer or farther off its own Colour in the Order of Refrangibility.

It follows from hence, said the Marchioness, that no Colour even that seems the most perfect, can be absolutely unmixed. And perhaps it will never be in the Power of Art to dye a Silk in such a Manner, as for it to reflect only one Sort of Rays.

It would be more difficulty, answered I, to sort the various Colours together <79> and flatter our Sight by the Concords of Harmony, if they were all homogeneal and unmixt. In this Case, all the Delicacy of Nature would be requisite to find out the innumerable intermediate Shades betwixt any two Colours. But now as every Colour is more or less blended with some other, we are by that Means furnished with a much more easy and expeditious Way.

Hence it is that the Gradation from one Shade to another (tho' perhaps there may be several intermediate ones wanting) does not appear defective to our Eye, which discovers in each the same Basis of all the Colours that softens them, and serves for a Support to the Harmony.

There would arise other Incoveniencies besides these, if coloured Mediums did not transmit every Sort of Ray; for this would aggravate a Distemper bad enough in itself, which infects and tinges the whole Body, and the Eye itself with an unpleasing yellow.

Persons under such Circumstances as <80> these, when it is very improper to pay Visits to Ladies, would become absolutely blind, unless to yellow Objects, and could see no Faces but of those who are troubled with the same Disorder.

A perfectly complaisant Lover, replied the Marchioness, if his Mistress should happen to be ill of this Distemper, must cloath himself in yellow, (a Colour which tho' regarded with great Veneration in China, is not looked upon as a good Omen for Lovers among us,) and perhaps get the Jaundice in order to be seen by his Charmer, and give her a Proof of his Affection.

So perfect a Lover, answered I, would not be an improper Person to prove this Immutability of Colours, if we had no more Proofs to demonstrate it than what you have already seen. What remained to be shewn was, whether the Confines of Shadow, with which the Light is bounded, are capable of making an;y Alteration in the Colours? This would have been fine Sport for those <81> Naturalists who lay hold on every thing, in order to build a System.

Our Philosopher put the Colours to the Test of this new Experiment, and found nothing wanting in their Constancy which maintains its Ground, even when it happens that Rays of different Colours cross and intersect each other; it appears, in short, to defy every thing judged capable of bringing any Alteration upon them.

We must have Recourse to Romance, said the Marchioness, to find some equal to these Colours, which do not give Place even to the Ephesian Anthia, a Model of the firmest and most obstinate Constancy, in Spite of all that a Romancer can produce to make her at length ---- What? answered I, resemble the Matron her Fellow-Citizen, reply'd the Marchioness. The Constancy of these Colours must certainly appear very surprizing to Ladies, answered I. I do not doubt but the greater Part of them would much sooner admit the System of Lucretius, who, without so many <82> Experiments, affirmed not only that they are mutable, but that any one Colour may be changed into all the rest.

[1] Cardinal Richelieu.

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Professor Rob Iliffe
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