principal experiments. He alludes to his result, as already obtained, without distinctly stating what it was, in a letter, dated February, 1669.

The steps by which his investigation proceeded are clearly traced. At the same time that he was trying the phenomenon of colours, he was also attempting to grind lenses of those particular forms to which we have alluded, as recommended by Des Cartes, for converging light accurately to a single focal point. Besides the difficulties attending this process, Newton was soon led to perceive that, even supposing the form of the glass ever so accurate, there is an inherent defect arising from the production of colour. This, indeed, in spherical lenses, had already been noticed: but its laws had never been investigated; and it seems probable that, in the case of the Cartesian lenses, the mathematical condition of accurate convergence which they ought to fulfil, was what led Newton to the probable connection between refraction and the production of colours, and suggested the hint of differently coloured primary rays, each undergoing a different degree of refraction.

and

The phenomenon then appeared in lenses; but in the prism it would be found in a state of greater developement: this, then, was the "instantia ostensiva" of Bacon; and this was the case which Newton accordingly selected to reason upon. His whole series of experiments is characterised by the most admirable exemplifiation of philosophical caution. A narrow beam of the sun's light admitted into a darkened room, passed through a prism, exhibited on a screen opposite, not a circular image of the sun, as it ought to do, if the rays all obeyed the same law of refraction, but an image elongated to nearly five times its breadth, and coloured from one end to the other by a succession of vivid tints passing indistinguishably one into another, but of which Newton marked seven principal divisions: in this particular point he was guided by some distinctions, which probably appeared well marked to his eye, but which certainly would not occur to an indifferent observer, unless

prepossessed with that idea. Most individuals, probably, seeing the spectrum for the first time, would rather distinguish it into three principal tints, red, yellow, and blue, with intermediate shades. To return, however, to Newton: the first question as to the cause which suggested itself was, whether the difference of colour might not be owing to the difference of thickness of the glass through which the different rays had to pass. This was accordingly put to the test, by causing two rays to pass, one through the thick part of the prism, another through a part near the edge; but each ray produced a complete spectrum of its own: this, therefore, could not be the real cause. The next suggestion was, that the effect might be due to irregularities in the glass; but this must take place equally in all positions of the prism, and, in fact, be doubled, if the light passed through two prisms; accordingly, two prisms of exactly the same angle were placed together, one being inverted, so that they together formed a solid with parallel surfaces, the light passing through both, came out unaltered, uncoloured, and giving a perfect undistorted image of the sun: this supposition was, therefore, rejected. But the rays from the different parts of the sun's disk form a small angle with each other, and their incidence on the prism is, therefore, slightly different; might not this be magnified in the course of two refractions, and so account for the effect? This was simply matter of calculation and measurement, and was shown to be utterly insufficient.

Newton next enquired whether the rays might not be bent into curve lines, after passing through the prism, and so, in proportion to the degree of inflection, might fall on different parts of the screen, with different degrees of obliquity. This was refuted by measuring the length of the spectrum at different distances from the prism: it was always in exact proportion to the distance, and the rays consequently strictly rectilinear.

Having disposed of all these suppositions, the question was reduced to narrower limits; a perfect exemplification of the process recommended by Bacon. Newton now

thought of trying the actual properties possessed by each ray separately. Through a hole in the screen, any one ray could be transmitted, while the rest were stopped. The transmitted ray was subjected to further experiments by being again refracted through a second prism. The rays were found to be refrangible by this second prism, in different degrees; the violet most, the red least; precisely, that is, in the same order as oy the first prism in forming the elongated spectrum. This was the "experimentum crucis ;" and Newton came to the conclusion that the sun's light is not homogeneous, but is a compound of a number of primary rays, which (distinguished according to the order of spaces, reckoning from one end of the spectrum to the other) have each a different degree of refrangibility corresponding to the difference of colour, and that the same power of refrangibility is inherent in the same ray, or part of the spectrum, whatever subseqent modifications it may be made to undergo.

Reflecting Telescope.

Having thus investigated the main fact, Newton immediately recurred to the practical applications, with reference to which he had originally undertaken the enquiry. An accurate refraction in lenses appeared now not only practically difficult, but demonstrably impossible; and having completely satified himself, that in all the primary rays, the simple law of reflection holds good with perfect accuracy, he devoted his attention to the improvement of reflecting telescopes, which he saw were thus theoretically unlimited in the perfection which might be given them, if only perfect figures could be practically given to the specula. In considering Gregory's construction, it occurred to him, that instead of reflecting the image back again through a hole in the large mirror, it would be more convenient to place a small plane reflector diagonally within the tube, and so throw the rays out through a hole in the side, where the

eye glass might be fixed. He accordingly constructed such an instrument with his own hands: and though it was only six inches long, it bore a magnifying power of about 40 times; and this, he remarked, was more than any refracting telescope, as then made, of six feet could do: he represented this as an epitome of what might be done." And it is in the same letter (before referred to) in which he describes this telescope to his friend, Mr. Ent, that he alludes to his discovery of the composition of light, observing that a refracting telescope, if made according to the most perfect theory of those curves for lenses which Des Cartes had devised, would still scarcely perform better than a common telescope. This, he adds, may seem a paradoxical assertion, "yet it is the necessary consequence of some experiments which I have made concerning the nature of light.” Still the telescope appeared to be the subject predominant in his mind. And it seems probable, that although Gregory's construction was proposed earlier, Newton's was the first actually made. He soon completed another of somewhat larger dimensions, which being, by request of the Royal Society, sent up to them for examination, was presented to them by the maker, in 1671, and soon after exhibited to the king. A particular account of it was transmitted to Huyghens, and it has been carefully preserved by the Society to this day. At the same time they elected Newton a fellow.

In 1672, Newton was occupied in the construction of a reflecting microscope, upon a principle analogous to that of the reflecting telescope. About the same time, he also proposed a plan for improving his telescope, by diminishing the loss of light at the second reflection. This was done by employing, instead of the small speculum, the total internal reflection from the hypothenusal side of a right-angled prism: the light entering perpendicularly on one of the rectangular sides, being reflected at half right angles, and emerging at the other, without refraction, was thrown out to the eye glass in the side of the tube, as in the former construction.

Publication of Optical Experiments.

Having been, in 1669, appointed Lucasian professor of mathematics, on the resignation of Dr. Barrow, in that and the two following years, Newton read some lectures at Cambridge, containing an account of his researches on the unequal refrangibility of light; but they do not seem to have become much known till some time after. In a letter to Oldenburgh, secretary to the Royal Society, in 1671, he mentions his intention of communicating to that body "an account of a philosophical discovery which induced me to the making of the telescope: and I doubt not but will prove much more grateful than the communication of that instrument; being, in my judgment, the oddest, if not the most considerable detection which hath hitherto been made in the operations of nature." The communication followed soon after, giving an account of the principal experiments already described. It was received with all those marks of approbation and honour to which such a production was justly entitled : and he soon after followed it up by a second, in which some further researches on the same subject were detailed.

These researches were, in the first instance, directed to the reverse experiment of recompounding the prismatic rays into white light. This was done either by a second prism inverted, or by uniting the rays at the focus of a lens, or by a mixture of coloured powders, in the proper proportion, which, when illuminated by the sun's rays, and compared with a white paper, under the same circumstances, appeared of precisely the same whiteness. He showed also that the colours of all bodies are dependent wholly on the light reflected from their surfaces; since whatever be the original colour of a body, if no other light fall on it except one of the prismatic rays, it will appear wholly of the colour of that ray. These papers soon acquired for their author an