[10] Newtoni Opera, tom. iv. p. 205, Letter to Oldenburg.
[11] M. Biot, in his Life of Newton, has stated that Newton was preceded in the invention of the reflecting telescope by Gregory, but probably without knowing it. It is quite certain, however, that Newton was acquainted with Gregory’s invention, as appears from the following avowal of it. “When I first applied myself to try the effects of reflection, Mr. Gregory’s Optica Promota (printed in the year 1663) having fallen into my hands, where there is an instrument described with a hole in the midst of the object-glass, to transmit the light to an eye-glass placed behind it, I had thence an occasion of considering that sort of construction, and found their disadvantages so great, that I saw it necessary before I attempted any thing in the practice to alter the design of them, and place the eye-glass at the side of the tube rather than at the middle.”—Letter to Oldenburg, May 4th, 1672.
[12] Letter to Oldenburg, February 10, 1671.
[13] This gentleman was the author of a paper in the Philosophical Transactions, entitled “Optical Assertions concerning the Rainbow.” How such a paper could be published by so learned a body seems in the present day utterly incomprehensible. The dials which Linus erected at Liege, and which were the originals of those formerly in the Priory Gardens in London, are noticed in the Philosophical Transactions for 1703. In one of them the hours were distinguished by touch.
[14] Newton speaks with singular positiveness on this subject. “For I know,” says he, “that Mr. Lucas’s observations cannot hold where the refracting angle of the prism is full 60°, and the day is clear, and the full length of the colours is measured, and the breadth of the image answers to the sun’s diameter; and seeing I am well assured of the truth and exactness of my own observations, I shall be unwilling to be diverted by any other experiments from having a fair end made of this in the first place.” On the supposition that his prism was one of very low dispersive power, Mr. Lucas might, with perfect truth, have used the very same language towards Newton.
[15] Letter to Oldenburg in 1672, containing his first reply to Huygens.
[16] In an experiment made by Newton, he had occasion to counteract the refraction of a prism of glass by another prism of water; and had he completed the experiment, and studied the result of it, he could not have failed to observe a quantity of uncorrected colour, which would have led him to the discovery of the different dispersive powers of bodies. But in order to increase the refractive power of the water, he mixed with it a little sugar of lead, the high dispersive power of which seems to have rendered the dispersive power of the water equal to that of the glass, and thus to have corrected the uncompensated colour of the glass prism.
[17] See the article Optics in the Edinburgh Encyclopædia, vol xv. p. 479, note.
[18] “This result was obtained,” as Newton says, “by an assistant whose eyes were more critical than mine, and who, by right lines drawn across the spectrum, noted the confines of the colours. And this operation being divers times repeated both on the same and on several papers, I found that the observations agreed well enough with one another.”—Optics, Part II. Book III.
[19] Optics, Book ii. Prop. iv.