NINETEENTH ANNUAL 31 EE TING. 113 



but it is with never-failing pleasure that lovers of nature to-day look back upon the 

 experiments of that young philosopher, fellow of Cambridge, just turned twenty- 

 four, whose mechanical ingenuity from boyhood and love of mathematics had al- 

 ready attracted favorable attention. Having bought a prism in the town, he took 

 it home to his rooms to study the laws of refraction. The chamber was darkened, 

 and the prism was placed in a ray of sunlight, which entered through a small, round 

 hole cut in the shutter for that purpose. The diverted rays fell upon the wall, and 

 then, for the first time recorded in the annals of science, was spread out that mag- 

 nificent band of color — the solar spectrum. To understand Newton's mingled sur- 

 prise and delight, we must remember that the only recognized function of the prism 

 at that time was its power to bend a ray of light in accordance with Suell's law 

 of refraction. The complex character of sunlight was unsuspected, dispersion un- 

 dreamed of ; nor did the truth dawn in Newton's mind at once. The unexpected 

 display of color he attributed, as was in keeping with the teaching of the day, to 

 some subtle action of the glass, and, having procured a second prism, he tried to 

 increase the effect by passing the ray through it also. The further divergence of 

 the spectral tints and their reunion in the original white of the sunbeam, according 

 to the positions of the prisms, afforded the necessary hint, and Newton advanced by 

 rapid strides to two epoch-making discoveries: That sunlight is the mixture of a 

 vast number of colored rays differing from each other by insensible gradations of 

 hue; and that these colors, being differently refracted, may be separated (dispersed) 

 by the prism. 



To appreciate the complete novelty of thase results and the effects they were 

 destined to produce upon the science of that day, we must remember that optics was 

 in its very infancy; that the telescope itself , but little changed from the early forms 

 devised by Galileo and the Dutch opticians, had been known but half a century, and 

 that the first important step in its improvement was at this very time about to be 

 taken by Newton himself. Although it was a period of great scientific activity, em- 

 bracing such names as Gregory, Hooke, Sir Christopher Wren, and Boyle in Eng- 

 land, and La Place, Leibnitz, Hugghens, and Bernouilli on the continent, the system 

 of natural philosophy still in vogue was the Cartesian, the absurdities of which, day 

 by day more manifest, were nevertheless inadequate to dethrone it until these and 

 other remarkable advances had taken root, and the Newtonian philosophy, nearly 

 half a century later, had finally become established in the universities. It was the 

 beginning of a struggle against the extreme dogmatism of the period — a struggle 

 which was to end in the introduction of experimental demonstrations, and so in the 

 planting of the germ from which have sprung the great laboratory systems of the 

 present time. 



Starting from the complex character of sunlight as a foundation, the science of 

 color in the hands of Newton and his followers made rapid progress, until the vari- 

 ous ways in which, in nature, certain components of sunlight become isolated and 

 appear as color, have become well known. We are thus able to divide all colors into 

 two great classes — objective and subjective, and have learned that objective colors 

 are formed either by the absorption of other rays or by interference; while subjective 

 colors are due to physiological processes by means of which the eye is rendered more 

 sensitive to certain colors than to the other components of the light which reaches 

 the retina. 



The blue of the sky is due, then, to absorption or to interference, or it is of sub- 

 jective character. Newton ^ himself thought it due to interference, but it is cus- 



^ Newton; Optice, London, 1706. 

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