OF NEWTON'S OPTICS. 



17 



cast two spectrums R' V', r' v' t on the op- 

 posite wall so as to lie in the same right 

 line, and having the lengths at right an- 

 gles to the floor, the lowest point R', or 

 red end of one, being contiguous to the 

 highest point v', or violet end of I he other. 

 A third prism, DH, was now placed 

 with its length vertical, and of course at 

 right angles to the other two, and so as 

 to receive the rays emerging from them. 

 The two spectrums were immediately 



translated from their former positions to 

 other positions RV, rv, no longer in the 

 same line, but similarly inclined to the for- 

 mer, and therefore parallel to each other. 

 (26.) The next test to which Newton 

 submitted the problem was even more 

 conclusive and convincing than any of 

 the preceding. Through an aperture, O, 

 fig. 17, in the window-shutter he admitted 

 a beam of the sun's light which he re- 

 ceived upon a prism ABC, placed before 



Fig. 17. 



the aperture. The spectrum produced 

 by the refraction of this prism was re- 

 ceived upon a screen perforated by a 

 small hole O'. The several coloured 

 lights of the spectrum being diffused 

 over a considerable space upon the 

 screen, and the aperture O' being small, 

 the light of but one colour passed 

 through it, while the prism A B C re- 

 mained stationary ; but when this prism 

 was slowly turned round its axis, the 

 spectrum moved upwards and down- 

 wards on the screen, so as to transmit 

 in succession the lights of the several 

 colours through the aperture O'. At a 

 distance of about twelve feet from this 

 screen another was placed, having, in like 

 manner, a small aperture O". The beam 

 of coloured light transmitted through the 

 aperture O' was received upon the se- 

 cond screen, diffusing itself over a space 

 of some magnitude. A small ray of 

 this light passing through the aperture 

 O" was received upon a prism A' B' C' 

 placed immediately behind it, and by this 

 prism was refracted to a certain point 

 upon the opposite wall. 



The prism A'B'C' remaining fixed, 

 the prism ABC was turned until the red 

 end of the spectrum fell upon the hole 

 O'. A ray of red light now passed from 

 O' to O", and was refracted by the 

 prism A' B' C' to the point R on the wall. 

 This point was marked. By a slight 

 motion of the prism ABC, the orange 

 light was next brought upon O', and a 



ray of it passing in the direction 0' O" 

 fell upon the prism at the same angle as 

 the red light had before been incident. 

 This orange ray was refracted by the 

 prism A' B' C' to a point O on the wall 

 a little above the point R to which the 

 red had been brought. The yellow, 

 green, blue, indigo, and violet rays were 

 in succession transmitted in the same 

 way to the prism A'B' C 1 , all being in- 

 cident upon it at the same angle, and 

 they were severally found to be refracted 

 to the points Y, G, B, I, and V. Thus 

 it appeared that the several coloured 

 lights into which the sun-beam was re- 

 solved by the prism ABC were, under 

 the same circumstances, differently re- 

 fracted by the prism A' B 'C', each light 

 being refracted the more, the nearer its 

 situation to the violet end of the spectrum. 

 The conclusiveness of this result 

 would have satisfied an ordinary in- 

 quirer, and it would have immediately 

 been made the basis of a theory. The 

 ardour of discovery w r as, however, in 

 Newton tempered by philosophical cir- 

 cumspection", and in the unwearied 

 patience of his research, he left untried 

 nothing which could put his hypothesis 

 to the proof, and overturn it if false. In 

 the records of scientific discovery there 

 is not a more splendid instance of an 

 investigation in which theory and ex- 

 periment mutually guide and support 

 each other. In the first experiments, 

 Newton found that the coloured lights 

 C 



