Colour. The gradual removal of these suspicions led him at last to an experimentum crucis, which furnished him with the true explanation of the phenomenon. Hc took two boards, and placed one of them close behind the prism at the window, so that the light might pass through a small hole made in it for the purpose, and fall on the other board, which he placed at about 12 feet distance, having first made a small hole in it also for some of the incident light to pass through. Then he placed another prism behind this second board, that the light trajected through both the boards might pass through that also, and be again refracted before it arrived at the wall. This done, he took the first prism in his hand and turned it to and fro slowly about its axis, so much as to make the several parts of the image, cast on the second board, successively pass through the hole. in it, that he might observe to what places on the wall the second prism would refract them. And he saw by the variation of these places, that the light tending to that end of the image towards which the refraction of the first prism was made, did in the second prism suffer a refraction considerably greater than the light tending to the other end. Thus he detected the true cause of the lengthened image to be no other than that light consists of rays differently refrangible, which without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall. Thus Newton ascertained that light is composed of rays differing from each other in their refrangibility. The number of these rays he settled at seven, guided by the difference of their colour; and he informs us in his Optics that the names of the colours were not imposed by himself, but by another person whose skill in distinguishing colours was much greater than his own. The least refrangible ray was the red, and the most refrangible the violet. The order of refrangibility, and the names of the rays are as follows: Red, orange, yellow, green, blue, indigo, violet. The proportional lengths of each of these rays in the spectrum, he found by measurement to be the following. These discoveries furnished him with an explanation of the different colours of bodies, a circumstance which had not hitherto heen understood. Colours are not qualifications of light derived from refractions or reflections of natural bodies, but original and connate properties, which in different rays are different. To the same degree of refrangibility ever belongs the same colour, and to the same colour ever belongs the same refrangibility. Thus the red rays are always the least, and the violet rays the most, refrangible. And the same holds of all the others. The colours of any particular ray cannot be changed by refraction nor reflection from natural bodies, nor by any other cause that he could observe. Every possible method was tried to alter the colour of such a ray; it was refracted by prisms, and reflected from bodies possessing a different colour from itself; it was transmitted through variously coloured mediums; in consequence of this treatment it would, by contracting or dilating, become more brisk or faint, and, by the loss of many rays, in some cases very obscure and dark; but it never changed in the kind of colour which it exhibited. Yet seeming transmutations of colours may be made, where there is any mixture of different sorts of rays. For in such mixtures the common colours appear not; but by their mutual allaying one another, they constitute a middle colour, which by refraction may be resolved into all the constituents of which it is composed. Thus there are two sorts of colours, the one original and simple, the other compounded of these. The original or primary colours are red, orange, yellow, green, blue, indigo, violet. The compounded ones are made by mixtures of the simple ones; thus blue and yellow make green; red and yellow, orange; red and indigo, violet; and so on. White is the most surprizing of these compound colours. It is formed by a mixture of all the simple colours in a due proportion. When all the rays after being refracted by the prism are made to converge together again, they constitute light perfectly white and free from every other colour. Hence it happens that whiteness is the usual colour of solar light. In it all the rays are mixed in their due proportion; but if any one ray predominate, the light will incline to its colour. Hence the reason why the light of a candle is yellow; that of burning sulphur, blue; that of Mars, red; and so on. These facts enabled him to explain the different colours which bodies possess. They are always of the colour of the ray or rays which they reflect to the eye of the spectator. The reason why different bodies appear of one colour by reflected, and of another by transmitted light, is equally evident ; and why two transparent liquids of different colours become opaque when placed in contact. Nor is it more difficult to explain the colours produced by the prism or the colours of the rainbow. These things being so, it can no longer be disputed whether there be colours in the dark, nor whether they be the qualities. of the objects we see, nor whether light be a body. For since colours are the qualities of light, having its rays for their entire and immediate subject, how can we think those rays qualities also, unless one quality may be the subject of and sustain another; which in effect is to call it a substance. We should not know. bodies for substances, were it not for their sensible qualities, and the principle Newton's theory attacked. of those being now found due to something else, we have as good reason to believe that to be a substance also. Such is a pretty full account of Newton's first discoveries respecting light, as given by himself in the Philosophical Transactions for 1672. No sooner had it made its appearance than it was attacked with considerable violence by different foreign mathematicians, chiefly disciples of Descartes. Father Ignatius Gaston Pardies, a Jesuit, and Professor of Mathematics in the Parisian College at Clermont, was the first who entered the lists. A set of animadversions by him on Newton's theory was inserted in the S4th number of the Philosophical Transactions published the same year with the original theory.* To this Newton wrote an answer, shewing the mistakes into which Pardies had fallen. Pardics wrote a second letter stating fresh objections, which was likewise answered by Newton. Upon which the French Philosopher declared that all his objections were removed, and that he was perfectly satisfied of the truth of the Newtonian theory of light and colours. He died the year following, in the 37th year of his age. Newton's next antagonist was Dr. Hooke, who wrote a number of animadversions on the Newtonian theory, admitting the truth of most of the experiments, and explaining them by a hypothesis supposing light to be the undulations of a fluid. This paper was not published; but Newton's answer, which was very long, appeared in the same volume of the Philosophical Transactions with his preceding papers above mentioned.† This answer is a most masterly performance. He shows the absurdity of Dr. Hooke's hypothesis in a very clear manner, proves that his own doctrine is totally unconnected with any hypothesis, and merely expresses matter of fact. Finally he refutes Dr. Hooke's opinions about colours, in such a manner as to leave no room for reply. Dr. Hooke accordingly, notwithstanding the pertinacity with which he usually adhered to his opinions, did not venture to continue the contest. + Newton's next antagonist was an anonymous Frenchman, who called himself M. N. Some animadversions by him were published in the Philosophical Transactions for 1673. To these animadversions Newton immediately replied. This reply was followed by another letter from M. N. upon the same subject, to which Newton having replied at considerable length, and pointed out the mistakes of his antagonist, the controversy dropped. The next antagonist was Franc. Linus, a peripatetic philosopher of some eminence in those days, but quite incapable of contending with Newton.§ To this only a very short answer was at first deemed necessary: Linus replied a second time, and after his death the controversy was still kept up by his pupil Mr. Gascoigne. To all of these • Phil. Trans, Vol. VII. p. 4087. + Vol. VIII. p. 6086. Phil. Trans. 1676. Vol. X. p. 499. + Page 5084. ý Phil. Trans. 1674. Vol. IX. p. 217. Newton made appropriate answers, which were published in the Philosophical Transactions immediately after the animadversions of Linus himself. Mr. Gascoigne having employed Mr. Lucas, of Liege, to repeat the experiments to which Newton in his answers had appealed, that gentleman published an account of the result, together with a new set of animadversions upon the Newtonian doctrine.* The dispute between Linus and Newton turning chiefly upon the length of the spectrum, which Linus denied, we do not think it necessary to enter into minute details. To Mr. Lucas's animadversions Newton made an appropriate reply, pointing out the proper method of repeating his experiments, and shewing how he might satisfy himself of the truth of his doctrine of the different refrangibility of light, and of the nature of colours. Thus terminated this long controversy, which had been carried on with considerable keenness for about four years. On all sides, however, the utmost good breeding was observed, unless some of the remarks of Dr. Hooke be considered as exceptions; and all his opponents uniformly expressed the greatest respect for the abilities of Newton. This controversy however had an unfortunate effect upon Newton's mind, and prevented him from laying his discoveries before the world as he had intended to do. An omission which, in a subsequent period of his life, occasioned other disputes of a very disagreeable nature; and which must have been the more unpleasant to him, as he must have been conscious that they proceeded entirely from his long hesitation in laying his mathematical discoveries before the world. tics. The first part of Newton's Optics, as he informs us in the preface, was drawn Newton's Op up in the year 1675, and read before the Royal Society. The last part was added about twelve years afterwards; it was not published till the year 1704.† A second edition, with some augmentations, was published in 17 17. It was translated into Latin by Dr. Samuel Clarke; and no doubt has made its appearance in almost every language in Europe. It consists of three parts. In the first the different refrangibility of the rays of light is demonstrated by experiment. The second part treats of the colours of thin plates, of colour in general, and of some curious particulars respecting refraction and reflection. The third part treats of the inflection of light. The whole is terminated by a most important set of queries respecting the most abstruse parts of natural philosophy and chemistry. Any remarks upon this admirable book would be quite unnecessary, as it has long enjoyed that reputation to which it is so justly entitled. As an analytical investigation, it constitutes the finest model hitherto offered to the * Phil. Trans, 1676. Vol. XI. p. 692. + Newton seems to have declined giving his work to the public till after the death of Dr. Hooke, probably in order to avoid reviving his disagreeable dispute with that caustic philosopher. Refraction and reflection ex plained, world. Various attempts have been made by philosophers to reduce the number of primary colours to three; namely, red, yellow, blue; and to prove that all the other four are only mixtures of these three. Some of these attempts have been exceedingly ingenious; and, in a metaphysical point of view, might be admitted as satisfactory, if it were possible to get over the Newtonian experiments, which show the impossibility of resolving the orange, green, and violet, into more simple colours, and the objections which Newton started to the admission of a nearly similar hypothesis advanced by Dr. Hooke. in Newton adopted the opinion that light consists of straight lines proceeding rays from luminous bodies, in contradistinction to the opinion of the undulations of a fluid advanced by Descartes, Huygens, and Hooke; but he does not pertinaciously insist upon this opinion being adopted, and his Optics are entirely independent of it. It will be proper to notice, here, the way in which Newton explains refraction and reflection in his Optics. It is exceedingly ingenious; but to a certain extent hypothetical, and by no means supported by the same evidence as the different refrangibility of light, and the theory of colours; both which are established beyond the possibility of being overturned. Newton begins by repeating some experiments first made by Grimaldi, and which it will be necessary to mention. He made a very small hole in a plate of copper, and allowing a ray of light to enter through this hole he suspended a hair in the ray. The shadow of the hair received at a certain distance was much larger than it ought to have been. Newton measured it, and found it 35 times broader than it ought to have been by the simple divergence of the rays. Hence it was obvious that the hair had acted upon the light, and pushed it out of its direction. Here then is an example of bodies repelling light. That this increase of width in the shadow might not be ascribed to an atmosphere surrounding the hair, he plunged it in water contained between two glass plates, and found the effect not in the least altered. Again, he took two sharp metallic edges; placed them parallel, and brought them gradually within th part of an inch of each other. The ray of light that passed between them was now divided into two parts, which were separated from each other, and thrown into the shadows of the respective knives. Here it is obvious, that the light had been drawn out of its former direction by the sharp metallic bodies, and that it had been drawn towards them. This then is an example of light attracted by bodies. From an attentive consideration of the phenomena, Newton shows that these attractions and repulsions, which are not sensible at moderate distances, become exceedingly strong at very small distances, or in case of actual contact; and hence he infers, that they do not follow the law of the inverse of the square of the distance, but that they approach much more nearly to the inverse of the cube of the distance. |