Catalogue Entry: OTHE00098

^[1] Dioptrice, cap. viii. ix., 1629.

^[2] Optica Promota: Definitiones, 3. Lond. 1663.

^[3]

Isaaci Vossii De Lucis Natura et Proprietate, Amstel. 1662. As the opinions of Vossius have not been referred to by any of our historians of science, the following passages may be interesting.

"Primus itaque color, si tamen color dicendus sit, is est albus, pelluciditatem proxime hic accedit. Insunt itaque et lumini omnes colores, licet non semper visibiliter; nempe ut flamma intensa alba et unicolor apparet, eadem si per nebula aut aliud densius corpus spectetur, varios induit colores. Pari quoque ratione, Lux, licet invisibilis ant alba ut sic dicam, si per prisma vitreum, aut aerem roridum transeat, similiter varios colores induit." — P. 6.

"Omnem tamen lucem secum colores deferre et eo colligi potest quod si per lentem vitream, aut etiam per foramen, lumen in obscurum admittatur cubiculum in muro aut linteo remotiore manifeste omnes videantur colores, cum tamen in punctis decussationis radiorum et locis minimum lenti vicinis, nullus color sed purum tantum compareat lumen." — P. 64.

"Quapropter non recte ii sentiunt qui colorem vocant Lumen modificatum." — P. 59.

^[4] Phil. Trans., vol. vii., No. 80. Feb. 19, 1672

^[5] Phil. Trans., vol. vii., No. 81, p. 4004. March 25, 1672.

^[6] See Journal des Savans , 1672, pp. 80 and 121; and Phil. Trans., No. 83, p. 4056, May 20, 1672.

^[7] Gregory's Catoptrics, App. 261. In this controversy, Newton never claimed any credit for the invention of a new form of the reflecting telescope, and was certainly surprised at the notice it excited among persons that either were, or ought to have been, acquainted with the previous invention of Gregory. In his letter to Mr. Collins, he speaks in the kindest manner of Gregory. "I doubt not that when Mr. Gregory wrote his Optica Promota, he could have described more fashions than one of these telescopes, and perhaps have run through all the possible cases of them, if he had thought it worth his pains. Because Mr. Cassegrain propounded his supposed invention pompously, as if the main business was the contrivance of these instruments, I thought fit to signify that that was none of his contrivance, nor so advantageous as he imagined. And I have now sent you these farther considerations on Mr. Gregory's answer, only to let you see that I chose the most easy and practicable way to make the first trials. Others may try other ways, nor do I think it material which way these instruments are perfected, so they be perfected. — Dec. 10, 1672." See the Macclesfield Collections, vol. ii. pp. 346, 347, or Newtoni Opera by Horsley, vol. iv. p. 288.

^[8] July 13, 1672, in the Macclesfield Collections, vol. ii. p. 333.

^[9] Sir Isaac seems to have been the first person who suggested the idea that vision might be rendered indistinct by the collision of the rays when they cross one another at the focus of mirrors or lenses. In speaking of the use of more than one <51> eye-glass in the Gregorian telescope, he states, that "by the iterated decussations of the rays, objects will be rendered less distinct , as is manifest in dioptric telescopes, where two or three eye-glasses are applied to erect the object." — Letter to Collins, Dec. 10, 1672; Macclesfield Collections, vol. ii. p. 344. In the course or some experiments on this subject, I found that the sections of the cone of rays, are never so distinct and well-defined after the rays have crossed as before. — (Treatise on New Phil. Inst., pp. 44 & 193). And Captain Kater, in comparing two equal telescopes, the one Gregorian and the other Cassegrainian, found that the intensity of the light within the focus was nearly double of what it was without the focus. In other experiments, he found the ratio as 1000 to 788 — Phil. Trans., pp. 13, 14. Mr. Tulley, however, in making similar experiments, did not confirm the results obtained by Captain Kater. I have found, in confirmation of these facts, that the negative diffractive fringes produced by rays which do not cross one another before they enter the eye, are more distinct than the positive ones which do cross. — Treatise on Optics, Edit. of 1853, p. 117.

^[10] Dr. Hook made several experiments with the speculum executed by Mr. Reeves, and did not find it so bad as Gregory thought. See Newton's letter above referred to.

^[11] Letter from Gregory to Collins and Newton, Sept. 26. 1672.

^[12] Biog. Brit., Art. Newton, p. 3217.

^[13] Smith's Optics, vol. ii. Remarks, p. 80.

^[14] Caleb Smith proposed to correct the colour produced by the two refractions, by a concave lens placed between the speculum and the small receiver, or by making the surface of a rectangular glass prism concave. — Phil. Trans. 1739, p. 326.

^[15] See Prof. Rigaud's Biographical Account of John Hadley, Esq., pp. 7-11.

^[16] Phil. Trans., vol. xxxii. No. 376, March and April, 1723, p. 303.

^[17] Phil. Trans., July and August 1723, p. 382.

^[18] Gregory's Catoptrics, pp. 250, 285.

^[19] Ibid., p. 385

^[20] The Hon. Samuel Molyneux and Hadley in Smith's Optics, vol. ii. p. 302, § 782.

^[21] Ibid., p. 363, § 913.

^[22] This telescope, according to Dr. Smith, was so excellent that it was scarcely inferior to Hadley's of 5 feet $2\frac{1}{2}$ inches in length. It bore a power of 226, as determined by Mr. Hawksbee, Mr. Folkes, and Dr. Jurin. See Smith's Optics; Remarks, p. 79.

^[23] This process, drawn up partly by Molyneux, and partly by Hadley, is printed in Dr. Smith's Optics, vol. ii. p. 301.

^[24] Maclaurin in Smith's Optics, vol. ii., Remarks, p. 81.

^[25] This telescope was removed from the Observatory upon the establishment of the Astronomical Institution, and is, we believe, now lying dismantled in some garret of the city.

^[26] For an account of the Decline of Science in England, here alluded to, we refer the reader to Sir John Herschel's Treatise on Sound, to Mr. Airy's Report on Astronomy, in the Report of the British Association for 1833, and to Mr. Babbage's interesting volume, On the Decline of Science. See also Quarterly Review, October 1830, and North British Review, vol. xiv. p. 235.

^[27] See Transactions of the Astronomical Society, vol. ii. p. 413.

^[28] A fine reflecting telescope, with a speculum two feet in diameter, and a focal length of twenty feet, has been recently constructed by Mr. Lassels, who has made with it several important discoveries within the limits of our own system.

^[29] A box containing a second speculum is shewn at Y.

^[30] This disc of flint-glass was executed by Messrs. Chance, Brothers, and Company, of the Smethwick Glass-works, and was rewarded with a council medal of the Great Exhibition. — See Reports of the Juries, p. 529.

^[31] This proposal, which was first made by the author in September 1844, is likely to be now carried into effect. A committee of the British Association, and of the Royal Society, have, after a careful consideration of the subject, applied to Government for the necessary funds.