ac may be equal to AC, and dh to DH. The equal spectra may coincide in particular points, that is, individual lines. in the one, indicating particular colours, may coincide with individual lines marking the same colours in the other spectrum, and yet other lines may not coincide, indicating different colours. When a ray of white light, therefore, is incident on the separating surface of the two media which give these two spectra, a very large portion, or rather the whole of the colours, indicated by the coincident lines, will be transmitted, while a very small portion of the colours indicated by the non-coincident lines will be reflected, the greatest quantity of the colours being reflected where the non-coincidence is greatest, and the greatest quantity being transmitted at the points of coincidence. Where there are many separating surfaces, and many elements in the body, the spectrum obtained by the prismatic analysis of the transmitted light will be cut up by obscure portions exactly as it is found to be in all coloured media.

When the constitution of any coloured body is altered by heat or pressure, the refractive and dispersive power of its elements are changed, and the resulting colour altered, according to the ratio in which the refracting forces are changed in the elementary molecules. Changes of this kind are finely exhibited in the growth of certain coloured crystals. In the tourmaline, for example, we have sometimes a red nucleus which absorbs one of the doubly refracted pencils, namely, the green one, and transmits only the red. When this nucleus was completed, some change had taken place in the circumstances under which the crystallization was proceeding, and the molecules, though still combining as tourmaline, combine in such a manner as to produce no colour-no difference in the tint

At a

of the pencils-and no absorption of one of them. subsequent stage, the structure which produces the red colour again appears and disappears, forming in succession coloured and colourless laminæ round the original nucleus !

Another example of great interest is afforded by certain specimens of fluor spar, in which the colours of the fourth class are produced.1 The structure which produces a white phosphorescence, is succeeded by one which produces a coloured phosphorescence, and this again by a structure which produces no phosphorescence at all. The changes of structure to which these different effects are owing, arise, in all probability, from a change in the arrangement of the atoms in the molecular groups of which the body is composed.

See Edinburgh Transactions, vol. xvi, p. 112,

CHAPTER IX.

NEWTON'S DISCOVERIES ON THE INFLEXION OF LIGHT-PREVIOUS RESEARCHES OF HOOKE-NEWTON'S ANIMADVERSIONS ON THEM OFFENSIVE TO HOOKE-NEWTON'S THEORY OF INFLEXION AS DESCRIBED BY GRIMALDI, HAVING MADE NO EXPERIMENTS OF HIS OWN--DISCOVERIES OF GRIMALDI, WHICH ANTICIPATE THOSE OF HOOKE-HOOKE SUGGESTS THE DOCTRINE OF INTERFERENCE-NEWTON'S EXPERIMENTS ON INFLEXION-HIS VIEWS UPON THE SUBJECT UNSETTLED-MODERN RESEARCHES-DR. YOUNG DISCOVERS THE LAW OF INTERFERENCE-DISCOVERIES OF FRESNEL AND ARAGO-FRAUNHOFER'S EXPERIMENTS— DIFFRACTION BY GROOVED SURFACES—DIFFRACTION BY TRANSPARENT LINES PHENOMENA OF NEGATIVE DIFFRACTION-EXPERIMENTS AND DISCOVERIES OF LORD BROUGHAM-EXPLANATION OF DIFFRACTION BY

THE UNDULATORY THEORY.

AMONG the optical discoveries of Newton, those which he made on the inflexion of light hold a high place. They were first published in his Treatise on Optics in 1704, but we have not been able to ascertain at what period they were made. In the preface to this work, Sir Isaac informs us, that the third book, which contains his experiments on inflexion," was put together out of scattered papers ;" and he adds, at the end of his observations, that "he designed to repeat most of them with more care and exactness, and to make some new ones for determining the manner how the rays of light are bent in their passage by bodies for making the fringes of colours with the dark lines between them. But we were then interrupted, and cannot now think of taking these things into consideration.' The earliest notice of the inflexion of light by English

VOL. I.

N

philosophers was taken by Dr. Hooke in a discourse read to the Royal Society on the 27th November 1672, "containing diverse optical trials made by himself, which seemed to discover some new properties of light, and to exhibit several phenomena in his opinion not ascribable to reflexion or refraction, or any other till then known properties of light." The Society desired him to pursue these experiments, and to register some account of them, in order to preserve his discoveries from being usurped." After an interval of more than two years, he communicated to the Society a second discourse "on the nature and properties of light, in which were contained several new properties of light, not observed that he knew of by optical writers." These properties were,

"1. That there is an inflexion of light differing both from refraction and reflexion, and seeming to depend upon the unequal density of the constituent parts of the ray, whereby the light is dispersed from the place of condensation, and rarefied, or gradually diverged into a quadrant. 2. That this deflexion is made towards the superficies of the opaque body perpendicularly.

66

3. That in this deflexion of the rays, those parts of diverged radiation that are deflected by the greatest angle from the strait or direct radiations are faintest ; those that are deflected by the least are the strongest.

"4. That rays cutting each other in one common foramen, do not make the angles ad verticem equal.

“5. That colours may be made without refraction. “6. That the true bigness of the sun's diameter cannot be taken with common sights.

"7. That the same rays of light falling upon the same point of the object will turn into all sorts of colours, only by the various inclination of the object.

“8. That colours begin to appear when two pulses of light are blended so very well and near together, that the sense takes them for one."1

66

These observations of Hooke on the Inflexion of Light, were referred to, not very courteously, by Sir Isaac Newton, at the close of the celebrated Discourse on Colours, of which we have already given an account. After treating of the colours of natural bodies, he says, "that there is another strange phenomenon of colours which may deserve to be taken notice of. Mr. Hooke," he adds, you may remember, was speaking of an odd straying of light, caused in its passage near the edge of a razor, knife, or other opaque body, in a dark room; the rays which pass very near the edge being thereby made to stray at all angles into the shadow of the knife. To this Sir William Petty, then President, returned a very pertinent query, Whether that straying was in curve lines? and that made me (having heard Mr. Hooke, some days before, compare it to the straying of sound into the quiescent medium) say, that I took it to be only a new kind of refraction, caused perhaps by the external ether's beginning to grow rarer a little before it came at the opaque body, than it was in free spaces, the denser ether without the body, and the rarer within it, being terminated not in a mathematical superficies, but passing into one another through all intermediate degrees of density; whence the rays that pass so near the body, as to come within that compass where the outward ether begins to grow rarer, must be refracted by the uneven denseness thereof, and bended inwards towards the rarer medium of the body. To this Mr. Hooke was then pleased to answer,

1 See Birch's Hist. Royal Society, vol. iii. pp. 63, 194, and Hooke's Posthumous Works, pp. 186-190.