OF NEWTON'S OPTICS. 



red half, which plainly proved that the 

 rays from the blue half were more de- 

 flected from their original direction than 

 those of the red half. Had the rays been 

 equally refrangible, the parts of the 

 paper would have maintained the same 

 relative position, although the place of 

 the entire paper would appear to be 

 elevated. 



Fig. 7. 



In order to confirm the conclusion 

 deduced from this experiment, Newton 

 viewed the same paper with the refract- 

 ing angle B, Jig. 8, of the prism pre- 

 sented downwards. He found, as he 

 expected, that the blue half of the paper 

 was lower than the red, being more de- 

 pressed, owing to the greater refraction 

 of the rays. 



Fig. 8. 



upon the light. The greater that refract- 

 ing power, other things being the same, 

 the nearer to the lens will be the focus 

 into which the rays are collected. It was 

 also well known that the place of the 

 focus might always be detected by the 

 presence of an image of an object placed 

 before the lens. We have already stated 

 (p. 8), that a certain degree of indis- 

 tinctness will attend this image, when 

 the lens is spherical ; but still a certain 

 point of greatest distinctness will always 

 be found, which may be regarded as the 

 place of the focus, were all spherical 

 aberration removed. Now, if it be true 

 that light of different colours is differ- 

 ently refrangible, and that according to 

 the result of the experiments already 

 described, red light is less refrangible 

 than blue light, it would follow that the 

 images of red and blue objects, formed 

 by the same lens, ought to be found at 

 different distances from it. 



To apply this test to the coloured 

 paper already mentioned, Newton wrap, 

 ped round it an extremely fine thread 

 of black silk, so as to form fine black 

 lines upon the red and blue ground, as 

 represented \nfig. 9 ; where, for distinc- 

 tion, the black lines are parallel on the 

 red, and cross each other on the blue. 

 He covered the wall of a dark chamber 

 with black cloth, and attached to it the 



Fig. 9. 



( 1 9.) He now determined to submit the 

 question to a different test. It was pre- 

 viously known that the position of the 

 focus, in which rays from any luminous 

 object are collected by a lens, depended 

 on the refracting power of the glass 



coloured paper, with the silk 

 thread wound round it, as 

 in the figure. Immediately 

 under this he placed a light, 

 so as to illuminate the paper 

 thoroughly, the flame not 

 rising above its lower ter- 

 mination ; and, therefore, 

 not intercepting any of the 

 light reflected from it. He 

 then placed a double convex lens M N, 



\X 



Fig. 10. 



Jig. 10, of about four inches and a quar- 

 ter diameter, at about six feet two inches 



from the paper. The image of the paper 

 was received upon a white screen, at the 



