The Fifth Dialogue.

THE next Morning, as soon as the Marchioness was up, she sent for me into her Closet, not concealing that agreeable Disorder which she knew her Beauty could receive no Injury from. I find indeed, said she, that your Philosophy begins to grow a very serious Thing. I can assure you, I have slept much less this Night than before. I cannot tell whether this be the Reason of it, but I find that Philosophy and sound Sleep do not agree very well together. My interrupted Dreams have transported me into the Region of Optics, where I saw nothing but Prisms, Lenses, Rays differently refracted, coloured Images, in short, all those Experiments, and all that philosophical Apparatus which you described, arose successively in my Imagination <84> like Visions and Fantomes. Whatever Charms these Things may have in themselves, I could never have imagined they would employ my Thoughts so strongly at a Time when it is not very customary to think of Philosophy. No really, answered I, it was a Time more proper to think on the Philosopher. You are not to suppose, replied the Marchioness, but he had his Share, and has no Reason to complain of me. If this be the Case, Madam, answered I, there is not great Mischief done. I rather advise you to recall these Dreams to your Memory as often as you can, and you will be still the more sensible that they deserve it. How do you suppose it possible, continued she smiling, that I could think on these Experiments, without admiring the Penetration and Ingenuity of the Philosopher who invented them, a Man who seemed directed by Nature herself, in the Method of tracing her Laws? I perceive, said I, that you take the Thing too seriously. In the present Case I think you might be contented with the Expositor. <85> How can I be too serious? We are examining whether Colours be immutable or not, whether the Rays of Light be differently refrangible, we are building up and throwing down Systems, and in short, our Inquiries aim at nothing less than Truth itself. Can you reasonably accuse me then of being too serious in Matters of such Importance? But these Systems, answered I, and this Truth, however pompous they may sound to the Ear, ought not, however, to disturb our more agreeable Dreams. It would really gain me a fine Reputation in the World, if it were publickly known that I had made you dream of Lenses and Prisms. We should act in Things of this Nature, as those who study to make the best Use of their Passions, do with Regard to Love. These Persons (who do not appear to be the most injudicious) never devote themselves so intirely to it, as to interrupt their more important Concerns but only as much as is sufficient to make them pass away two or three Hours <86> agreeably with that Sex with whom it is hardly possible to converse upon good Terms, without at least a Pretence of being in Love.

You at once give me Lessons in Love and Philosophy, replied the Marchioness. But you know when People first fall in Love, they have little Leisure for these grave Reflections, but suffer themselves to be transported farther than they ought. This is exactly my Case with regard to Philosophy, in which I have hardly yet advanced one Step. I am so little Mistress of myself, that I have gone so far as to seek Methods to confirm Sir Isaac Newton's System. After all you have told me, judge whether my Transports were not very strong.

Let us see, Madam, answered I, what they have produced, for we often owe the finest Things to strong Passions. The Illiad, Æneis, the Poems of Dante and Milton were produced in their most vigorous Time of Life. To these may be added (at least for that Veneration with which they are regarded by their <87> Countrymen) the Lusiade of Camoens, composed in a Time of the greatest Revolutions and Conquests in Portugal, and the Araucana of the Spaniards, where the Poet himself is the Hero. Perhaps between the broken Slumbers of last Night, your Imagination produced something greater than all I have been mentioning.

I am afraid, answered the Marchioness, that it will amount to very little. I was thinking, that if Light is composed o Rays of different Colours, which blended together, produce white, we ought to examine whether these Colours, after being separated by the Prism, might not be mixt together again. I for a long Time considered but with little Success, in what Manner this could be effected.

Sir Isaac Newton himself, said I, has delivered you from this Trouble; for that Method of confirming his System which you mention is so good, or rather so evident and immediate a Consequence of Order, that he has made several Experiments to that Purpose. <88> I am now going to shew you one of the most celebrated, and at the same Time the most simple, to which this great Philosopher was conducted by that Spirit of Order which you possess in common with him.

The Image of the Sun made by a Prism in the dark Chamber, is received upon a Convex Lens, that the coloured Ray, which at going out of the Prism, diverged, may by the Lens be made to converge and unite, and by this Means be again blended together. I am quite ashamed both of my Stupidity and myself, interrupted the Marchioness. I had all the Materials necessary for the Execution of my Idea, ready prepared to my Hand; they only wanted putting together, and I had not the Sense to do it. You were greatly in the Right to be unwilling that a Person should hear the Voice of Philosophy, who is incapable of making it an Answer. I might rather, answered I, apply to you that famous Saying of Antiquity, I wish with all these Qualifications, you were one of us. You will find in Optics them <89> selves, some Consolation for what you call your Stupidity. Men, those reasonable and curious Beings, lived above three hundred Years before they found out that a Convex and Concave-Glass put together, would make a Tellescope; and tho' they had the Materials every Day at Hand, were at last indebted to mere Chance for this useful Instrument. It was a greater Disgrace to Mankind not to discover it before, than Honour to them that they found it at last. So that this useful and fine Invention is of the Number of those which are standing Monuments of the Weakness of human Nature. You comfort me, replied the Marchioness, at the Expence of Mankind. --- But to return to our Subject. I suppose that Point beyond the Lens, where the coloured Rays are united, will appear totally white.

The Rays, answered I, have no sooner passed through the Lens, but they begin to mix and efface each other; and they lose the fine harmonious Proportion established betwixt them in the Spaces of the coloured Image, the Original of <90> the Music of the Eyes. When they are all confined at length, and incorporated together in the Focus of the Lens, they form a little circular Image totally white, a Republic (if I may use the Expression) of Colours, where they are all equally tempered together: The red can no longer display its lively Flame, the green boasts no more the smiling Livery of the Spring, nor the blue the lucid Robe of Heaven, but all blended together, restore the Whiteness of the Sun from whence they came, in such a Manner, however, that as they recede from the Focus, they again renew their Splendor and Colour, but in a contrary Order, and the ravished Eye again wanders from one Pleasure to another.

You will easily understand how they come to be inverted, if you remember the two sticks crossed together, mentioned by Des Cartes, which you once believed to have a greater Efficacy in explaining the Phænomena of Optics than they really have. The Appearance that the Colours again make be <91> yond the Place where they were blended, demonstrates that the Rays lose neither their Colour nor Quality, as some might perhaps believe, but that the Whiteness which appears in the Focus of the Lens, is produced only by their Mixture.

I now understand, said she, what you meant Yesterday by saying, that the Immutability of Colours still remains, even when Rays of a different Colour intersect and cross each other; for if it were otherways, we should not see the prismatic Colours re-appear beyond the Place where the Rays united.

This is the very Experiment, answered I, upon which the Immutability of Colours was grounded; and all the Experiments of Sir Isaac Newton have this peculiar and admirable Quality, that each single one is not contented with demonstrating only one Thing, as is the Case of most others, but demonstrates many more at the same Time. This arises from the close Union, and almost Geometrical Con <92> nexion which the Properties of Light have between themselves.

Sir Isaac Newton's Experiment, said the Marchioness, seems to resemble the Battles of the Ancients, which often gained the Victor a great Number of Provinces at a Time. And those of most other Philosophers, answered I, the Battles of the Moderns, the most noisy Preparations, the most consummate Contrivance, and the Blood of Millions, amount to no more than the taking a Place, which perhaps might have surrendered in two Months by a Treaty.

But to return to your Experiment. I call it your's, because tho' you did not invent it, you saw the Necessity there was for its Invention in order to compleat the System. Our Philosopher never quitted it till he had varied it a thousand Ways.

It was necessary to prevent some one of the coloured Rays from passing through the Lens, in order to see whether the Whiteness of the little circular Image would by that means be changed.


This Sir Isaac Newton did, and intercepted the Passage sometimes of one Colour and sometimes of another; and the Whiteness ceased, and degenerated into that Colour which arises from the Composition of those Colours which were not intercepted, but suffered to pass through the Lens.

The Want of any Colour in the circular Image, which is not suffered to pass, will be very agreeably discovered by looking thro' a Prism, which resolves the Image into its component Colours, by giving them a different Refraction.

If all the Rays be transmitted thro' the Lens, the Image will be white; but viewed through a Prism, it will appear of all Colours like the Rainbow: If any Ray be intercepted, this by the Prism was discover'd to be wanting in the Image, till at last suffering only one Colour to pass thro' the Lens, this one Colour too was seen by the Prism.

If all the Colours were successively intercepted by the Teeth of a Comb made to pass very swiftly over the Lens, the circular Image would remain white, <94> and the different Colours not be distinguished, by Reason of the quick Impression which they all successively make upon the Eye.

You may perhaps have observed, that when a lighted Torch is moved nimbly round in a Circle, the whole Circle which it forms in the Air, appears on Fire; and the Reason of this is, because the Sensation (which in the several Places of that Circle it raises in the Eye) remains impressed till the Torch returns again to the same Place.

In the same Manner (when the Colours follow one another with an extreme Rapidity) the Impression of each of them remains in the Eye, till a Revolution of all the Colours be compleated. The Impressions therefore of all the successive Colours are at once in the same Part of the Eye, and jointly excite the Sensation of Whiteness there.

The same was afterwards proved by a Wheel, whose Circle, painted with the several Colours of the Prism, appeared white, when turned very rapidly round itself.


I must confess, said the Marchioness, that even if I could have found the Experiment I fought for, I should never have been able to diversify it so many different Ways, tho' the Inconstancy which you, Gentlemen, lay to our Charge, might perhaps have given me no small Assistance.

This Inconstancy, answered I, which perhaps is not so great a Defect as some believe, was not wanting in Sir Isaac Newton. With the most fertile, and almost poetical Imagination, he continually invented new Experiments, which tho' different from each other, all concurred to prove the same Thing. They seemed to rise under his Hands as the Poets make Flowers spring under the Feet of their Beauties.

The coloured Image formed by the Prism becomes white, when viewed thro' another Prism, in such a Manner, that it contracts the Image and mingles the Colours. I have observed the same Phænomenon in the Rainbow, which is the Effect of a Separation of the solar Rays in the Drops of Rain <96> opposite to the Sun. It appears white when viewed thro' a Prism, turned in such a Manner, as to contract it and blend the Colours together.

Those who live near the Cataracts of Rivers, see a Rainbow every Day, when the Sky is clear, formed by the Sun in the dashing of the Water against the Rocks; making People have the Opportunity of seeing this Experiment much oftener than we can.

I would, if possible, replied the Marchioness, have nothing to envy others. Tho' we have no Cataracts, a Fountain will imitate the Rain, present us with the Colours of the Rainbow, and we may make our Observations at Pleasure. We will call this Fountain, if you think proper, The Fountain of Optics.

Till your Garden furnishes you with Proofs of the Newtonian System, answered I, as the Gallery did with Objections against the Cartesian, you may return to the dark Room: There you will see that the Whiteness of a Paper placed over against the coloured Image <97> of the Sun, does not suffer any Alteration, if it be placed in such a Manner as to partake equally of all the Colours: Whereas, if it approaches nearer to one than another, its Whiteness will be tinged with that Colour which is next it. Is it possible now for Truth to descend from Heaven with a more shining Train of Proofs?

I was very confident, said the Marchioness, to venture my Thoughts upon a Thing which cost Sir Isaac Newton so much Study. How is it possible I could ever have found out the very least of these Experiments as easy and simple as they now appear? But in Exchange, answered I, you can easily invent Things that perhaps would have puzzled the Philosopher. It is much more proper for you to know in what Proportion to mix Hope and Fear, Smiles and Frowns, to prevent a Lover's Passion from growing languid, than to be exactly acquainted what Quantity of different coloured Powders is necessary to be mixed together to form a white. This is another Experiment <98> which our Philosopher tried, that there might be nothing wanting to confirm his System. It is true, the white which arises from this Composition is dusky, grey, and obscure, like the Colour of Ashes: And the Reason is, because the Colours of these Powders are too imperfect and faint, compared with those of the Prism, to make a lively clear white.

However, if this Mixture be exposed to the Sun, in order to increase the Force of the Light, that dusky obscure white will become bright and clear, tho' it will never equal the Strength of the Paper exposed to the same Light.

For this Reason in coloured Prints, one of the finest Inventions of our Time, which, with only three Colours, perfectly imitate all the Variety of Painting, the Paper is left uncovered for strong Clears and Whites.

Water thickened with Soap, and agitated so as to raise a Froth, is more proper to demonstrate that a Composition of Colours produces white. After the Froth is a little settled, there will appear upon the Surfaces of the Bubbles <99> of which it is composed, different Colours, which when viewed at a Distance cannot be distinguished from each other, and the whole Froth will appear perfectly white. This Experiment has, besides its being an agreeable one, this further Advantage, that it is very easy to make.

Philosophy, said the Marchioness, from what I can discover of it, is like a Game of Chess, which upon any other Occasion I should make no Scruple to call an ingenious Pretext to waste Time. The very least Piece in the one, and the least Experiment in the other, is often of the highest Importance. A Pawn in the Hand of a skilful Player may give Check Mate, and a little Froth will furnish Sir Isaac Newton with a whole Magazine of Observations and Discoveries. Half the World that preceded him had the same Bubbles and Froth before their Eyes, without so much as seeing them. The Ancients themselves would a thousand times have observed and neglected them.

The Eyes of the Ancients, answered <100> I, were much better able to judge of the Elegance of a Statue or a Temple, than the Importance of an Experiment. Seneca was acquainted with a Sort of Prism, which receiving the Light of the Sun on one Side, displayed to the Eye the Colours of the Rainbow. All the Explication which he gives of this Phænomenon is, that there is really no Colour, but only the Appearance of a false Colour like that on the Neck of a Dove, which appears and dis-appears according to the Motion of the Eye and the Change of its Situation. This fine Explication sufficiently shews how very little the Ancients studied Nature and pursued her Steps; for if Seneca had taken the least Trouble to examine his Prism, he must have seen the Difference between the Colours produced by that, and those on the Neck of a Dove.

A particular Sort of Microscope known to this Philosopher, and which perhaps the ancient Artificers made Use of in the curious ingraving of their Intaglia's and Cameo's, the Enigma and Admiration of our Time, this Mi <101> croscope I say, composed of a Ball of Glass filled with Water, had no better Fortune in his Hands than in the Prism. He ascribed the magnifying of the Objects viewed thro' this Glass to a Quality of the Water, and not to the Figure of the Glass which contained it.

The Weight and other Properties of Air were known to the Ancients; and in order to explain how Water ascends in a lifting Pump (which is occasioned by this Weight of the Air) they had Recourse to a certain imaginary Horror that Nature has for a Vacuum: So that rather than leave the smallest Space empty, she chose to violate her own Laws of Gravity, and make the Water ascend. And as one Folly produces a thousand more, the great Horror that Nature testified for a Vacuum in this Pump, extended only to a certain Height, and beyond this perhaps was changed into Love, for there she admits as great a Vacuum in the Pump as you could desire.

But to quote still more Examples --- <102> Nero in his golden Palace (one of the most magnificent Effects of despotic Power in the Universe) erected a Temple of Stone so very transparent, that Day-Light entered into it even when the Doors were shut. Pliny who gives us this Relation, instead of contenting himself with affirming that it was more transparent than Alabaster, says, that it did not transmit the Light like other pellucid Substances, but included it within itself: If this had been the Case, the Temple must have appeared must more luminous by Night than Day.

The Ancients were fonder of what raised their Admiration than increased their Knowledge; and perhaps thought Experiments (which are the only Method by which we can arrive to a true Admiration of Nature) were too material to imploy the Attention of a Philosopher, who ought to consult nothing but Reason. They never imagined these Experiments would one Day lead industrious Posterity to such an Exactness as to subject Flame to the Exami <103> nation of a Balance, which was anciently regarded as a light Substance, for whose Sake they invented a particular Sphere of Fire where it was affirmed to ascend.

Nor could they forsee that these Experiments could help us to calculate how much we daily lose by a continual insensible Perspiration, how many million Tons of Water the Mediterranean Sea perspires in a Summer's Day, how much a Man decreases by the Weakness of the Muscles from Morning to Night? In short, that they should inable us to counterfeit Nature herself; and by certain chemical Mixtures emulate her Etna's and Vesuvio's, and imitate her Thunder much better than their own rash Salmoneus.

If an Antient had been asked whether the Phosphorus of Bologna, for Example, shines by its own Light, or by a borrowed one, it is impossible to know how many ridiculous Fancies they would have uttered by consulting Reason; whereas a Modern with a single Experiment has put the Matter out of all <104> Question. Pray what is this Phosphorus, said the Marchioness, this Subject of the Absurdities of the Ancients and Experiment of the Moderns? It is a certain Stone, answered I, found in a Mountain near Bologna, which calcin'd with Fire, acquires the Property of shining in the dark like a Coal, if it has been exposed a little while to the Light of the Sun, or only to the open Air. From this Quality it has obtained the fine Greek Name of Phosphorus, which signifies a lucid Body. This is an Honour given almost to every Thing appropriated to learned Uses. An Etymologist could not perhaps give a better Name to this Place than that of Phoslophus (which in our Language signifies the Lucid Hill) and would thus consecrate it for ever to Philosophy and Learning. We are much obliged to your Scholar, said she, for conferring so fine a Name on this Place which certainly deserves it.

The Question, continued I, is at present reduced to this, whether the Phosphorus only takes in and imbibes the <105> Light to which it is exposed, and from thence carried into the dark, shines with a borrowed Light, or whether the external Light puts its Parts into such an Agitation as to kindle a Light which it contains within itself, which by this means loosed from its Prison, if I may use that Expression, emits itself from the Phosphorus, which in this Manner shines by its own Light. This last Supposition is more honourable for it, and better agrees with the fine Name it bears.

In order to decide this Point, a modern Philosopher chose a Sort of Light which would shew whether the Phosphorus, when exposed to it, imbibed any of its Rays, and by that means reveal the Theft of this new Prometheus. I already see, interrupted the Marchioness, how this Modern proceeded. He placed the Phosphorus in one of the Colours of the prismatic Image, in order to see whether it would acquire the Colour as well as the Light: If it did actually take the Colour, it is evident that it imbibes the external Light, and shines with a Splendor not its own: <106> But if it does not take the Colour of the Image (since Colours are immutable and suffer no Alteration) the Light only agitates its Parts, and as you express it, loses its Splendor out of Prison; and thus the Phosphorus shines not by a borrowed Lustre but its own, and ought rather to be compared with the Sun himself, than Prometheus.

It is too true, answered I, that fine Ladies can be just what they please. You would be extremely in the wrong, if you hereafter complain of any Want of Sagacity in Physics. What you have been describing is the very Method which the Bolognese Philosopher took; and by this Experiment he confirmed to his Fellow-Citizen the Phosphorus, the Honour of shining by its own Light. It is probably that the innumerable other Phosphori of the same Nature, lately discovered in France, owed their Lustre to the same Cause: These, by inriching Philosophy with new Wonders, have deprived the Bolognese of the Honour of Singularity, which is partook with only one other in the whole philosophical <107> World. Do not Diamonds, the most valuable Phosphorus of Nature, shine in the dark, because the external Light kindles, and revives the internal Light, of which they are a rich and inexhaustible Treasure?

You may judge, replied the Marchioness, of my very great Sagacity in philosophical Affairs, when I never observed a Phænomenon which I every Day carry about me. Perhaps, Madam, answered I, you were never in a Place dark enough to make this Observation. Signore Boccari one Day visited a Lady in some Indisposition, who was resting behind a Screen far from the Air, and in a Place absolutely dark. The Lady asked him whether he had not a Light in his Hand; which he denied; but she constantly insisting she saw something glitter, the Doctor at last suspected it to be his Ring which shone in that profound Darkness, and perceived he had a long Time, without knowing it, carried a Phosphorus upon his Finger. You may judge of the Value he afterwards set upon this Ring. He made <108> innumerable Experiments with it about the Time that Mr. Du Fay, Father of so many Phosphori, found that Diamonds had the same Property. How empty and barren must the Philosophy of the Ancients have been, said the Marchioness, and how copious, how charming is ours, whose Observations enhance even the Value of Diamonds?

I will give you still greater Evidence, answered I, how much the Ancients were to blame in their Neglect of Experiments, and you will be fully convinced that there is none even of those which seem of least Importance, but what is of great Service to Natural Philosophy. That Froth we lately mentioned is an Instance of this, for as little philosophical as it may seem to vulgar Eyes, it was the principal Thing by which Sir Isaac Newton found out the Cause of those various and almost innumerable Colours which we see in Bodies.

He had discovered in general that certain Bodies appear of a certain Colour, because they reflect one <109> Sort of Rays more copiously than the rest, and other Bodies of different Colour, because they reflect a different Sort; so that if Light consisted of only one Sort of Rays, there could be only one Colour in the World, nor would it be possible by Refractions and Reflections to produce any new one.

This Discovery, which would perhaps have satisfied any other Philosopher, served only to excite the Curiosity of ours, and was but a Prelude to innumerable others.

Why should this Silk reflect the blue more freely than any other Sort of Rays? The Reason is this.

If one of those Bubbles which are formed by blowing Water a little thickened with Soap, be covered with Glass in order to prevent it being agitated by the Air, it will appear tinged with a great Variety of Colours, which spread themselves like so many rings one within another, and incompass the Top of the Bubble. And in Proportion as the Bubble grows thinner by the continual subsiding of the Water, these Rings <110> slowly dilate and overspread the whole, descending in order to the Bottom of it, where they all afterwards successively vanish.

The Variety of these Colours depends on the unequal Thickness of the Bubble of Water in different Parts. But how to determine these Inequalities is not so easy, and perhaps would have been impossible to any but Sir Isaac Newton, who examined these Rings a thousand ways, guided by his constant Tutor Geometry, and a Spirit of Observation, whose Fertility seemed to increase in Proportion to the Difficulties it encountered.

He discovered that a certain determinate Thickness is necessary in a Plate of Water, (for Example) in order for it to reflect a particular Colour, and a different Thickness to make it reflect any other Colour, and in general, that a less Thickness is necessary to reflect the most refrangible Rays, as violet and indigo, than those which are least refrangible, as red and orange; all this is to be considered with Regard to Sub <111> stance of equal Density. But when the Density in one Substance is less than in another, as the Density of Air compared with that of Water, a greater Thickness will be necessary in the first than in the last, to reflect the same Colour.

After the same Manner he defined the Thickness necessary for the Transmission of Colours. From the Analogy or Similitude between the Plates of Substances which he considered, and the Particles of which Bodies are composed, he demonstrated that their Colours depend on nothing but the different Thicknesses and Density in their Particles, whence it follows that some are more proper to reflect or transmit the Rays of one Colour, and others those of a different one.

This Analogy holds good in many Instances. Thus the one and the other are transparent: The Leaves of Gold and the Particles of many other Bodies transmit one Colour and reflect another, just as the Bubbles of Water which we have been mentioning: These Rings <112> appear of a various Colour when viewed in different Situations; the same happens in certain Silks, in the fine Web of the industrious Spider, and as Tasso sweetly sings.

The Feathers so, that tender soft and plain

About the Dove's smooth Neck close counched been,

Do in one Colour never long remain,

But change their Hew 'gainst Glimpse of Phœtus Sheen.

And now of Rubies bright a vermeil Chain,

Now make a Carknet rich of Emeralds green,

Now mingle both, now alter, turn, and change

To thousand Colours rich, pure, fair, and strange.

Fairfax's Tasso.

This appears very evident in those Powders that Painters make Use of, for when they are groun'd, (that is when their Parts are subtilised) their Colours <113> change a little. Bodies may in some Measure be regarded as Stuffs, for their Threads, every one in particular reflecting a particular Sort of Rays, the whole Stuff appears of that Colour which is the Result of all the Rays reflected from the several Threads of which the Stuff is composed.

But what becomes of those Rays which are not reflected? said the Marchioness. Can you give an Account of them? They are either transmitted, answered I, or stifled and extinguished, and are thus lost among the Particles of Bodies. A Leaf of Gold placed between Light and the Eye is transparent, and appears of a greenish blue; but a Collection of these Leaves placed one upon another loses both Colour and Transparency, the Rays which pass thro' the first Leaf being absolutely extinguished in passing successively thro' the rest.

White Bodies (to continue our Similitude) are Stuffs composed of Threads which reflect all Colours; black, on the contrary, absorb and extinguish every <114> Sort of Ray within themselves. For this Reason black Bodies heat much more easily than any other; and a black Cap, such as the English Ladies wear in St. James's Park, would not at all agree with your walks in the Italian Sun.

White Bodies as they reflect and drive every Sort of Ray from them, heat with more Difficulty than the others which imbibe and absorb the Rays, and neither transmit or reflect them.

From the same Causes arise the various Colours which we discover in the Air. The different Density and Thickness of the Exhalations and Vapours that rise from the Sea and Earth, paints the Heaven with varying Splendors, when rosy-fingered Aurora opens the Portals of the Morning, and calls back Mortals to their several Labours, or when Vesper invites them to an agreeable Repose; it is difficult however, to trace a Cause by which the Colours at the Rising and Setting of the Sun are almost all the same, and succeed each other in a certain Order. But this we certainly know, <115> that the several Colours of different Persons Eyes proceeds from the Difference of Texture in the Iris, which is that Coat in the Eye that incompasses the Pupil.

The Variety of Fibres in the Iris kindles in some the imperious Look of a black Eye, in others the insidious Sweetness of the blue. But it is not easy to assign a constant Cause why the Northern Nations should generally have white Hair and blue or grey Eyes, and we of a warmer Imagination, not in a warmer Climate, should have our Eyes black as our Hair.

This System however, will give us the Explication of a Phænomenon which is perhaps inexplicable in any other, and will thus make us a Compensation for not being able to give a Reason for every thing in particular.

The Phænomenon is this: Two transparent Liquors, the one red and the other blue, cease to be transparent when we look thro' both at the same Time. This Phænomenon which so greatly surprised the Person who first observed <116> it, is only a Consequence of the Newtonian Doctrine. The one of these Liquors transmits only red Rays, and the other only blue. Hence it follows that the Rays transmitted by the one will be extinguished and absorbed by the other, and the Eye which looks through each of them receive none. And this Phænomenon is one of those whose Explication becomes a Demonstration of the System which is capable of explaining it.

That Opinion, said the Marchioness, that blind People can distinguish Colours by their Feeling, now begins to appear credible to me: Or rather, is it not another Proof of our System? If our Feeling was finer than it is at present, as fine as that of blind Persons perhaps is, could not we discover the Colour of a Body by feeling the different Thickness of its Particles? And thus we should effect by our immediate Sense, what a Newtonian would by his Calculations, if any one should acquaint him with the hidden Texture of Bodies.

Your blind Observators, answered I, <117> might distinguish Colours by their Feeling, even in the Cartesian System which you have now given up. For according to that, there must be a Difference in the Particles of Bodies of different Colours in order that the Rays of Light may be differently modified. A Proof like this you perceive is too ambiguous to deserve a Place among those we have already mentioned; and there is the same Defect in what is said of a singular Kind of Barometer, which the Chinese make Use of to form a Judgment of the Weather by. This Barometer is a Statue placed upon a Mountain which foretels the Changes of the Sky and Air, by varying its own Colour.

But had we not better seek for a Phænomenon nearer ourselves in the Nation of Gallantry and Politeness, which can be explained by the English System alone? Why should the Belles of that happy Country lay on more red when they appear in the Boxes at an Opera, than when they embellish with their Charms the agreeable Walks of the Thuilleries?


You lead Sir Isaac Newton's System, said the Marchioness, into Places where you would have found it very difficult to lead its Author. Not if you, Madam, answered I, had been there.

The Light of a Candle is not so white as that of the Day; it has a yellowish Cast, and viewed thro' a Prism, yellow appears the most shining Colour in it. The less then the Spanish Wool is charged, or in other Words, the more it reflects other Rays besides the red, the stronger will the yellow appear, which is the predominant Colour in this Sort of Light; just as in a Chamber where the Light enters thro' coloured Curtains, the Objects in the Chamber will the more easily take the Colour of the Curtains in Proportion as their own Colour is weak and faint. This is evidently a Reason why the Quality of Spanish Wool should be augmented for the Opera, that neither the Cheeks of the Ladies, nor the Eyes of their Admirers may lose any thing, but find the same Advantages by the Light of the Evening as by that of the Day.


In the Cartesian System this wise Precaution would be entirely useless; for if the Spanish Wool can modify the Light of the Day, it would in the same manner modify that of the Evening, be it of what Colour it will.

Must it not be a Mortification, said the Marchioness, for the Ladies of this happy Climate (if they ever have Time to consider it) not to have a System in their own Country able to explain this Phænomenon in their Vermilion, but be obliged to call a foreign one from beyond Sea to their Assistance? So much the more glorious is it for this to extend its Empire over all Nations, even so far as to give them Lessons for the Toilet. This is not the only one, answered I. If you would not have a blue appear green by Candle-Light (which might disconcert the Harmony of a Suit of Cloaths, and cause innumerable Chagrins) you must be careful to chuse it very clear, otherwise the blue Rays mixed with the yellow, which the Silk would reflect in greater Quantities by <120> Candle-Light, might perhaps make it appear green.

These are the Gordian Knots of the Optic Science, which this System dissolves without eluding the Oracle. These Phænomena inexplicable to every other System, are without Difficulty explained in this. Every ambiguous Explication, every Proof that has not the Force of Demonstration, is intirely rejected by the Newtonian Philosophy.

An Analogy, for Instance, discovered between the Production of Colours and that of other Things, which would supply another System with a Proof, can serve only for Ornament and Luxury to this.

It has been lately discovered that Insects, Men, Animals and Plants, instead of being continually re-produced by Nature, only unfold themselves from their respective Plants or Seeds (in which they are really contained) whenever they find a proper Disposition for it; that is, both Animals and Plants wait for a proper Repository, certain Juices, Degrees of Heat, and other Things requisite to un <121> fold them. In the like Manner Colours are not, as was once believed, produced at every Refraction, or Reflection, or some other similar Cause, but unfold themselves, if I may use the Expression from the Bosom of Light, which contains them within itself, whenever it is refracted by a Prism, or reflected from the Particles of Bodies. And this Method of their Production seems much more agreeable to the universal Laws and established Order of Nature.

After the same Manner appears the Colour of the Rainbow, the coloured Circles sometimes seen round the Sun and Moon, and those of a certain Light, which is often seen towards the Northern Parts of Heaven, and called Aurora Borealis.

Whatever Riches and Magnificence, replied the Marchioness, Nature has displayed in so great a Variety of Colours, she seems to have observed some Frugality in their Production. At least the Newtonian Nature appears to me a better OEconomist than the Cartesian. The first has made Light a Magazine <122> and Reservoir of Colours, which she produced once for all insusceptible of any Alteration, only with a certain Disposition that renders them capable of being separated from each other, and shewing us by this Separation a Colour, which all united and mixed together, they could not do; whereas the Cartesian Nature is every Moment obliged to give new Rotations to her Globules, and at every Refraction and every little Circumstance, consider in what Manner to vary them. This must be an infinite Fatigue to her, and a very great Application of Thought.

But these Dispositions, answered I, which the Colours have to separate themselves, and which you so greatly admire, however convenient they may be to Nature by sparing her so much Trouble and Thought, are sometimes very inconvenient to us.

Inconvenient? replied the Marchioness. Is it not to these we are indebted for the beautiful Variety of the World? How tiresome and unpleasant would it be for us to see in all Objects a Repe <123> tition of the same Colour! You apprehend it, answered I, to be a great Evil always to see the World in Chiaro oscuro, if I may use that Expression, and to be obliged to be dressed always in the same Colour, and what is worse, in a Colour like that of your own Complexion. You may add, replied she, the Dread of losing a Topic of Discourse so very pleasing to Ladies.

All these Misfortunes, answered I, and this terrible Addition of yours would happen, if the Rays had no Disposition to separate from each other, or if they were all of the same Colour. The Cameleon and wrinkled Faces would be considerable Losers too. Some of these, without the Trouble of changing their Skin, are discovered in the Space of twelve, or at most twenty four Hours to change their Colours.

Astronomers on the other Hand would gain no small Advantage if the Rays were inseparable and of the same Colour. And what would not an Astronomer sacrifice to determine the exact Time of an Eclipse of Jupiter's Satel <124> lites, or to get a distinct Sight of the Occultation of a Star by the Moon? These are a Set of People whose Looks are always directed to Heaven, and have very little Regard for our Earth any farther than as she is a Planet, and upon that Account enters into the cælestial System. This is all they regard in her, and therefore would feel very little Concern if the Ladies of this Planet were under an Impossibility of changing every Day the Colour of their Cloaths, or subject to many other the like Inconveniences.

But what Opposition, replied the Marchioness, can there be betwixt the Separability of the coloured Rays and the Observations of these Gentlemen, to make them quarrel with us for the Pleasure we receive from this Variety? There is a very great Opposition betwixt them, answered I, and you will be soon convinced of it, when I tell you that the Separability of the Rays is prejudicial to the Perfection of Telescopes, which Astronomers regard as their very Eyes. I before positively affirmed that <125> Convex-Glasses, of which Telescopes are composed, unite the Rays, which proceeding from a Point, fall upon them into another Point. But the Truth is, I spoke rather with Regard to what they should do than what they really do. You seem, said the Marchioness, to have represented these Glasses in the same Light as tragic Writers do their Heroes; for these generally chuse to feign them what they ought to be, rather than imitate their Characters such as they really are.

It must be confessed, Madam, that I used a Sort of poetic Licence in telling you Convex-Glasses united the Rays into a Point, for it is not so truly a Point as a little Circle. This Circle, which is called the Abberration of the Light, proceeds from two Causes, the Figure commonly given to the Lens, and that Disposition which the Rays of Light have to separate themselves when they are refracted. But indeed the Figure of the Lens is but a very small Obstacle when compared to that of the different Refrangibility. All attempts therefore <126> to render Telescopes more perfect by giving a new Figure to the Lens, so as to make it unite the Rays truly in a point, would be in vain.

In the Golden Age, of which the Poets give us so many fine Descriptions, when the Rivers flowed with Milk, and the Oaks distilled the fragrant Treasure of the Bee, when the Rams appeared in the Meadows arrayed in native Purple, and the Lamb displayed its vivid Scarlet to the Sun, before the Hand of Art had taught the Fleeces to imitate different Dies: In these happy Times, tis probable, the Telescope would have given a more distinct View of the Objects of Nature herself painted in their genuine and beauteous Colours, when the Heart of Man, animated with purer Passions, appeared without Disguise, and Love had not yet learned to sigh by Custom or Art, nor ever wept from any Motive but Joy. But in our Iron Age, in which both the Passions and Colours have degenerated from their original Purity, whatever be the Figure of the <127> Lens, the Point where the blue or green Rays unite, will always be different from that of the yellow and red; there must always be an Abberration of the Light; and that little Circle will never become a Point. This is a Circumstance very inconvenient for Astronomers, whose Affairs require that every thing should be seen extremely distinct; but this Circle where the Rays unite instead of the Point where they ought to unite, or the different Refrangibility, which is the Cause of their uniting thus irregularly, is a great Obstacle to the obtaining the Exactness which Astronomers desire.

These accurate Gentlemen, said she, must have Patience, and pray for the Return of the Golden Age. In the mean Time they must limit their Desires and Wants as other reasonable Men do, and be contented with the different Refrangibility of the Rays instead of that very great Distinctness in Objects which they desire. It is not possible to have all Things at one Time in this World. Is the Knowledge of so <128> many fine and surprising Properties of Light so very small an Acquirement, that they cannot be satisfied without something more? Their Desires and their Wants however are so reasonable, said I, and have so great a Connexion with those of other People, that Sir Isaac Newton has thought upon a Method of satisfying them. With his own Hand he ground Glasses for Telescopes of a new Figure, in order to correct the Deficiency of ordinary Lenses. Then was the Time to have All, or hereafter hope for Nothing.

While his Mind was imployed in this Thought a new Scene of Optics opened to his View, he discovered the different Refrangibility of the Rays, laid aside the Work he had begun, and invented a new Telescope, in which a polished Minor supplies the Place of what in ordinary Telescopes is called the Object-Glass, and which had the greatest Share in the Abberration of Light.

I myself have seen the first Telescope of this Sort, worked by those Hands which had pointed to the Planets their <129> Road in the vast Desarts of a Vacuum, and opened to Geometry the immense Carier of Infinity. This Instrument is preserved in a City of England where Philosophy and Politeness hold a mutual Empire; with this are treasured up those Prisms which the first Time differently refracted the Rays of Light in the Hands of our Great Philosopher, separated its Emeralds, Hyacinths and Rubies, and unfolded to human Eyes the celestial Riches of the lucid Robe of Day.

In the Reflection from a Mirror the Rays are not separated as they are in being refracted thro' a Lens, consequently the Objects may be much more distinctly seen.

It has been tried in Italy (for even among us Truth and Sir Isaac Newton have their Admirers) that if a distant Object, the one Half red and the other blue, be viewed with an ordinary Telescope, this Instrument must be considerably shortened to give a distinct Sight of the blue Half, and on the contrary must be lengthened to shew the <130> red distinctly; whereas in Sir Isaac Newton's Telescope they appear equally distinct with the same Length.

This reflecting Telescope has besides another Advantage over the ordinary ones, for one of Sir Isaac's Invention one Foot long is equivalent to one of twelve in the common Sort; and one of forty Foot, to a hundred. And this is another Convenience to Astronomers who find great Difficulty in managing long Telescopes.

It is well for us, replied the Marchioness, that these Astronomers will be now contented, for it seemed before a pretty difficult Matter to please them. How is it possible, answered I, for them not to be satisfied with Sir Isaac Newton, who seems in every Thing to have studied their Advantage?

His System of Optics (besides procuring them a more convenient and perfect Telescope) has vindicated the Honour of Astronomy from an Aspersion, which seemed in some Measure to disgrace it in the Eyes of the World. You are not ignorant that the Honour <131> of this Science among Men consists principally in its exact Prediction of Eclipses and Events, no less observable to philosophical than to vulgar Eyes. Thales of Miletus was reverence among the Greeks like a God for having predicted the Year in which an Eclipse of the Sun was to happen, that is, when the Moon would be interposed betwixt us and the Sun, and by that means deprive us of his Light.

As Astronomy successively became more perfect, what would have caused a Temple to be erected to a Thales, would only disgrace a Hally, Cassini, or a Manfredi.

The Observatory is required to shew the precise Minute in which the Eclipse will happen, and its exact Quantity, or in other Words, whether the Moon will observe the whole Sun or only a Part, and how much precisely the obscured Part will be.

Not long since all the Calculations of the most celebrated Astronomers had predicted two total Eclipses, whose principal Merit consists in not being <132> very common, and in introducing a sudden and unseasonable Night. An Event which tho' foretold and expected, never fails to terrify that whimsical Species of Animals called Man, that Azylum of the strongest Contradictions, nourished by long Hopes, impetuous Passions, the most evident Truth and the most palpable Error, capable of making Attempts beyond the Powers of his Nature, and subject to Fears which his Reason contradicts.

All the Philosophers got up very early on the Days appointed for this Spectacle, in order to prepare themselves for Observation. They all waited in the Midst of the Eclipse to see the Light of the Sun entirely extinguished, and a most gloomy and profound Night emerge from the Splendor of a fine Day. But the Event was contrary to their Expectations. There remained round the Moon a luminous Ring which made some falsely take them for annular Eclipses. When the Sun is nearest to the Earth, and the Moon at the greatest possible Distance, if there <133> happens in these Circumstances a Central Eclipse, as they call it, the Moon cannot hide all the Sun, but there remains all round its Edge a luminous Border resembling a Ring. But Astronomers gained nothing by this Explication, which was of no Signification in the present Case, and the World was but ill satisfied with Astronomy which is now believed an Impostor. The one murmured and were discontented, while the other racked their Brains to find the Reason of this Ring, which had the Confidence to appear in Spite of all their Calculations.

Some laid the Fault upon a luminous Atmosphere which incompasses the Sun, as our Air does the Earth, and became visible when the greater Light was obscured; others accused the Atmosphere of the Moon, which being illuminated at the Time of the Eclipse, appeared like a lucid Ring. But unhappily for them the first was innocent, and the second seemed so doubtful, that it might appear <134> rather a Mark of their Consternation, than an Explication of the Phænomenon.

I find myself greatly affected with Concern, said the Marchioness, for these unfortunate Astronomers abandoned by Heaven and Earth, and in the utmost Danger of losing their Reputation. It is however the Office of Humanity to have Compassion on the Afflicted. They must have Recourse, answered I, to the Newtonian Oracles if they would silence the Voice of Calumny. These were the Anchor of Hope in the present calamitous State of Affairs.

When the Rays of Light pass near the Extremity of a Body, they bend and incline towards the Body itself, and are thrown into its Shadow. If the Edge of a Knife be placed in a Ray of Light in a dark Chamber, the Rays which pass at some Distance from it, appear to bend and approach nearer its Back. Grimaldo was the first who observed this Quality which is called the Diffraction or Inflection of Light, and our Philosopher afterwards illustrated <135> it with many new Experiments; and tho' he did a great deal, desired to do still more.

The solar Rays which pass near the Edges of the Moon must bend and cast themselves into the Shadow of the Moon herself. Observers then, who in the Time of an Eclipse are immersed in this Shadow, must receive these Rays bent from the Edges of the Moon, and see a luminous Ring round her, a Sort of Twilight resembling that which we see in the Evening and sometimes in the Morning, at the Horizon. The only Difference between our Twilight and this Ring is, that the first is occasioned by the Refraction which the Light suffers in coming from the Celestial Spaces into our Air, and the second by the Diffraction which it undergoes in passing near the Moon: But both are composed of Rays which do not seem destined by Nature for that Purpose.

To give a stronger Confirmation that this is the true Reason of this Ring, there have been made several Globes of artificial Moons, which exposed to the <136> Sun and full Moon, have shown upon Earth the Effects of that Diffraction which had appeared so fatal to Astronomy in Heaven.

Astronomers, replied the Marchioness, have certainly great Reason to be satisfied with Sir Isaac Newton and his Diffraction, which has delivered them from such eminent Danger. But I must confess myself to be yet throughly discontented with it. Will you permit me to ask why those Rays which pass at some Distance from Bodies, must be inflected and bent? The Idea which this new Property of Light present me with, is so very strange that I cannot conceive it. I find, answered I, you are more difficult to satisfy than the Astronomers themselves. You desire to know the Cause of this Diffraction. Well, I'll inform you, but you must promise not to draw back and frown when I tell you it is the Attraction which Bodies exercise upon Light. The Attraction! said the Marchioness with an Air of Surprize. You say this either to laugh at my Credulity, or punish my <137> too presumptuous Curiosity. What, do Bodies attract the Light as a Loadstone does Iron? What ill Consequences would arise supposing it were so? answered I, Or rather how great an Advantage has the Science of Optics in particular gained from this Attraction between Light and Bodies, as all Natural Philosophy in general has from the universal Attraction of Matter, of which that between Bodies and Light is a Consequence?

Attraction is the Key of all Philosophy, the great Spring that actuates the Frame of Nature; the universal and mysterious Force discovered and calculated by Sir Isaac Newton, proposed to the Examination of Philosophers by the great Lord Bacon, and obscurely sung by the British Homer.

The Marchioness recollect herself, and looked very attentively in my Face to see whether I spoke in Earnest. Do you tell me seriously, she replied, that all Bodies are attracted? This is quite a new World to me when I am an utter Stranger and Foreigner. Do not be <138> discouraged, said I, for this has happened to many professed Philosophers who have exclaimed against this Attraction, affirming, that to admit it is introducing into Philosophy certain occult Qualities which the Ancients supposed in Bodies, such as Sympathy, Antipathy and the like, whose Numbers multiplied with the Phænomena themselves, by the Assistance of which they in an Instant explained, or rather perplexed every Thing.

They farther add, that it is recalling those occult Qualities from those Colleges of Europe, where Ignorance still affords them an Asylum to introduce them into true Philosophy, from whence they for the Happiness of Mankind had been banished by the Authority of Reason.

But all these Objections signify nothing, for Attraction is so far from being an occult, that it is an extremely evident Quality of Matter, on which clearly depends the Explication of Diffraction, Refraction itself, and innumerable other Things. It is not a Name without Reality, invented to explain <139> two or three Appearances, but a general Principle diffused thro' all Nature, and extends from the smallest Grain of Sand to the greatest Planet. The Peripatetics resembled those Ancients, who for every little River or Tree, nay even for the Fever or the Cholic, created a new Deity. But Sir Isaac Newton appears as a great Philosopher, who by the Help of Attraction establishes the Existence of an all-powerful, infinite and only God, the Supreme Director of the whole Frame of Nature.

Sir Isaac Newton (when he affirms that Light in passing near the Extremity of Bodies is attracted by them, does not pretend by that to give a compleat Explication of Diffraction; all he undertakes is only to point out that Property of Matter on which the Explication of Diffraction depends, but the Cause of this Property is to seek. This he commits to those Philosophers who have superfluous Time enough to throw away in Search of a Thing which appears above the Reach of human Faculties. In short, all that Sir <140> Isaac Newton attempts is to establish Facts and the general Properties of Matter, and from these geometrically to deduce Phænomena and Effects; and this is the Order we have hitherto observed in our Discourse upon Light and Colours.

This new Property, replied the Marchioness, is of a Nature which my Thoughts cannot so easily come into. This is one of those historical Facts of which it is impossible to get a perfect Intelligence without entering into the Cabinet. I understand, or think I understand, how the Rays of Light are differently refrangible. This, among many other Things, is very intelligible. But that Bodies should attract the Light, and that at a certain Distance too, and in general that every thing should be mutually attracted, seems to me very difficult to conceive.

Some Remains of Cartesianism, answered I, which you are not yet entirely free from, deceives you in this Point. Perhaps you have hitherto flattered <141> yourself that Refraction arises from some one of those Causes which in treating upon the Cartesian System were rendered so familiar to you, and upon this Account you think Refraction more intelligible than Diffraction. Sir Isaac Newton himself appears in some Places to have indulged the Prejudices of this Sect. In order to speak the Language then current in Philosophy, he said, that Attraction perhaps might be the Impulse of a subtile Matter proceeding from Bodies. But in Fact after ha had proved the Heavens to be empty, and the Celestial Bodies mutually attracted in those vast Spaces, what Room did there remain for the Impulse and subtile Matter? HE seems to resemble certain Authors, who, to make their History agreeable to any particular Nation, are sometimes obliged to intersperse it with fabulous Episodes, and give it the Air of a Romance. Is it not a great Reproach upon Mankind that even the Truths of Sir Isaac Newton were obliged to use some little Artifice <142> in order to meet with a Reception among them?

Is not this rather an Artifice of yours, interrupted the Marchioness, to surprize me by a Motive of Honour, and in this Manner make me believe that I have no better Conception of the subtile Matter than of Attractions or how Bodies should be indued with Motion, for Example, than with that which you rightly term a mysterious Force?

Your Illusion, answered I, proceeds from hence, that you are familiarized to one Idea and not to the other. You every Day see Bodies move and mutually communicate their Motion, but you have never seen them attract each other. You are surprized at Attraction while Motion appears to have no Difficulty. In this you differ from Philosophers who like Poets for the Solution of any difficult Point are obliged to have Recourse to the Divinity for the Explications both of Motion and its Communication.

A Portuguese accustomed to reverence Spectacles upon the Nose of the <143> gravest Persons as a Mark of the greatest Dignity, would probably be surprized to see a Chinese Mandarin let his Nails grow to an enormous Length for the very same Reason. And the Cause of his being surprized in one Case and not in the other is, that a long Habit has connected in his Mind the two Ideas of Honour and Spectacles which have nothing in common with each other, but has not connected those of Honour and long Nails. I shall at least deserve more Compassion than the Portuguese, replied the Marchioness, because the Surprise of seeing Matter united to Attraction instead of Motion will I believe be common to all Countries.

As universal and excusable as this Surprise may be, I answered, it must however at last submit to Reason. If you had never seen Bodies move it would have been impossible for you to guess how Motion would be joined with Extension and mutual Impenetrability, which would be all the Qualities you would then have been acquainted with. It is Observation which has made you <144> admit this Property in Matter, and Observation must make you allow Attraction. We are as yet but Children in this vast Universe, and are very far from having a compleat Idea of Matter; we are utterly unable to pronounce what Properties are agreeable to it, and what are not. We see Bodies in very near the same Manner as a Person who should receive his Senses by little and little. It would certainly be great Temerity in him to affirm that there cannot be a Property in Matter capable of moving the Eye, and should assign as a Reason for this Assertion, that he had never been able to observe any such Property.

This Person would not act like the Cartesians who form a World and a Man just as their own Caprice directed them; he would grow more cautious in limiting the Power of Nature and pronouncing the Qualities of Bodies, in Proportion as he should acquire new Senses, which would every Day furnish him with fresh Discoveries of these Things.


Philosophers may be said in some Degree to acquire new Senses, or rather those they before had become every Day more refined, and are by that Means in a Capacity of perceiving what perhaps they had once no Idea of. Hence it evidently follows, how very cautiously they ought to proceed in assigning the Number of Properties in Matter. It signifies nothing to say that some are more intelligible than others, for to speak ingenuously, they are all equally mysterious to us.

Will you any longer make a Difficulty, Madam, to admit Attraction as a Property of Matter when it is proved so many Ways, and principally by the heavenly Phænomena, which give the most shining Testimony to a Thing that you yourself so evidently demonstrate the Truth of? I assure you for my Part I shall never look for any farther Demonstration of it than what you afford.

But I, replied the Marchioness, shall not be so easily satisfied, for I have need of the daily Attestation of all Heaven <146> itself to convince me of a Thing which yet appears so strange and surprising. It is absolutely needful then, continued I, to give you a full Conviction of it. And indeed it would be doing a great Injury both to Sir Isaac Newton and yourself, to endeavour to make you believe a Thing without giving you good Reasons. I am sorry this System cannot be explained to you with all the Force of those Demonstrations and calculations which accompany it; without these you must certainly lose a great deal. I will have Patience, said she; if I cannot see it in all that Lustre in which it would appear to a Mathematician, and will act like those Virtuosi, who when they cannot get a Picture content themselves with a Copy of it. I flatter myself you will render it as like the Original as possible. It is at present too late, I replied, for this Expedition. To-morrow we will ascend into Heaven, and from thence bring Attraction down in Triumph to Earth. Some Astronomical Facts and Propositions in Geometry, which you may <147> safely believe upon the Word of Sir Isaac Newton, shall serve for a Hippogryph, or flying Car, to transport us to the Skies.

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