[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. [20] In the same paragraph, when speaking of black bodies becoming hot, and burning sooner than others, he says that their “effect may proceed partly from the _multitude of refractions_ in a little room and partly from the easy commotion of so very small corpuscles.”—Optics, Part iii. Prop. vii. p. 235. [21] See page 354. [22] When Newton speaks of bodies losing their reflecting power from their thinness he means the reflecting power of their second surfaces, as is evident from the reason he assigns.—See Optics, Part iii. Prop. xiii. p. 257. [23] _Edinburgh Journal of Science_, No. 1. p. 108. [24] See the _Phil. Trans._ 1829, Part I. p. 189. [25] _Idem._ [26] _Phil. Trans._ 1819, p. 11. [27] If this view of the matter be just, we should expect that the specific gravity of the black would exceed that of the yellow phosphorus. [28] Since the two preceding chapters were written, I have had occasion to confirm and extend the views which they contain by many new experiments. [29] _Physico-Mathesis de Lumine coloribus et iride aliisque annexis._ Bonon. 1665. [30] This doctrine is thus announced. 1. That the same rays of light falling upon the same point of an object will turn into all sorts of colours by the various inclination of the object. 2. That colours begin to appear when two pulses of light are blended so well and so near together that the sense takes them for one. [31] This effect is so great, that at the distance of _four_ inches from the point of divergence, the angular inflexion of the _red_ rays of the first fringe is 12′ 6″, while at the distance of about twenty feet, it is only 3′ 55″. [32] See the twenty-ninth query at the end of his Optics, where the sides of a ray are compared with the poles of a magnet.