OF NEWTON'S OPTICS. 



19 



the other. One of the prisms was slowly spectrum produced by the other. The 



turned on its axis, until the spectrum two spectra thus appeared to form one, 



produced by it was thrown upon the which, in its colours, differed from either 



Fig. 19. 



of them, the red of the one being mixed 

 with the violet of the other, the orange 

 with the indigo, and so on. This mixed 

 spectrum was now viewed through a 

 third prism, held as represented in the 

 figure. The effect which ensued was, the 

 separation of the spectra, which assumed 

 the cross position shewn in the figure. 

 This experiment is another variety of 

 that which we last described ; the in- 

 clined positions assumed by the spectra, 

 with respect to their first position, was 

 explained in (24) to arise from the 

 different refrangibilities of the rays. 

 In the present experiment, this inclined 

 position is given at the same time to two 

 spectra. The inclinations are in opposite 

 directions with respect to the first po- 

 sition, because the lights which form the 

 spectra are disposed in an opposite order. 

 (29.) The following experiment, men- 

 tioned by Newton, is another beautiful 

 example of the analysis of mixed lights. 

 A circular piece of white paper A, about 

 one inch in diameter, was placed before 

 a black wall, and using the two prisms 

 mentioned in the last experiment, the 

 paper A, Jig. 20, was illuminated at 

 the same time with the red light p. 2 o 

 from the one, and a deep violet 

 light from the other. By this 

 mixture the paper assumed a 

 rich purple colour. The circle 

 A was then viewed through 

 a prism at some distance, and 

 the appearance exhibited was 

 two circles, R and V, the cir- 

 cle R, nearer to the paper 

 being red, and the more re- 

 mote one, V, violet. The prism 

 in this case refracted the red 

 and violet light, mingled in 

 the circle A, through different 

 angles ; the red being least re- 

 frangible was removed to R, 

 and the more refrangible vio- 

 let light carried so far as V. 



To confirm this, the apertures before 

 the prisms, which cast the red and violet 

 lights on A, were in turns covered, so as 

 alternately to deprive the circle A of 

 these lights. It was accordingly found 

 that the circles R and V alternately va- 

 nished, plainly proving that all the light 

 of R came from the prism which cast 

 the red light on A, and that all the light 

 of V came from the prism which cast 

 the violet light on A. 



By turning one of the prisms at the 

 window upon its axis, the circle A was 

 successively illuminated with all the pris- 

 matic lights, while the other prism, being 

 stationary, constantly projected on it an 

 intense red light. The effect produced 

 to an eye viewing these changes through 

 the third prism was, that the circle V 

 changed its colour according to the 

 change in the light used with the red 

 upon A. But the change of colour was 

 not the only alteration observed in V. 

 Its position was also changed. When 

 the blue light was mixed with the red, 

 it appeared nearer to R. Still nearer 

 when green was mixed with red on A. 

 In a word, the circles R and V always 

 exhibited the colours mixed upon A, 

 and their separation from A and from 

 each other always corresponded to the 

 separation of the prismatic spectrum 

 from the direct course of the light, and 

 to the separation of the two lights in the 

 spectrum from each other. 



From all these experiments no doubt 

 could remain that lights which differed 

 in colour differed also in refrangibility. 

 One test more however remained, ana- 

 logous to that which was applied to 

 the colours of natural objects in (19). 

 If the difference of refrangibility be ad- 

 mitted, it will necessarily follow that the 

 same double convex lens will have dif- 

 ferent foci for differently coloured lights, 

 the focus of the more refrangible light 

 being nearer to the lens than that of the 

 p. Q 



