OF NEWTON'S OPTICS. 



Fig. 12. 



upon the screen equal to the size and 

 shape of the aperture. At present we 

 will consider the aperture circular. To 

 find, then, the magnitude of the illumi- 

 nated spot upon the screen, first describe 

 a circle A B, jig. 12, whose diameter is 

 equal to that of the 

 hole. Then, adding to 

 the radius of this cir- 

 cle another line, A C, 

 whose length is such 

 as would subtend at 

 the hole an angle 

 equal to the sun's se- 

 midiameter, with the 

 whole distance O C 

 describe a circle. This circle will be the 

 magnitude of the illuminated spot on 

 the screen. 



(22.) Instead of allowing the beam of 

 light to pass directly from the aperture 

 to the screen, let it be intercepted by a 

 prism ABC near the hole, with its re- 

 tracting angle B presented downwards. 

 The refraction by the surfaces of this 

 prism might be expected to deflect the 

 beam from its original course, and raise 

 it to an higher position, as represented 

 in Jig. 13. The part of the screen on 



Fig. 13. 



15 



Fig.14. 



3 



r 



which the refracted ray falls, is, accord- 

 ingly, elevated by the effect of the prism, 

 but this is not the only effect produced. 

 The breadth of this spectrum, as it is 

 called, is exactly equal to the diameter 

 of the illuminated spot which would be 

 projected on the screen, if the prism 

 were removed. But, instead of a cir- 

 cular image being projected on the 

 screen, the illuminated part assumes 

 an oblong form, such as RV, fig. 14. 

 The sides are straight, and the ends 

 semicircular the length being perpendi- 

 cular to the axis of the prism. 



It appears, therefore, that of the rays 

 which pass through the prism, some are 

 refracted to a higher part of the screen 

 Hum others, those toward the end V 

 being more elevated by the refraction 



than those towards the end R. 

 From this Newton inferred, 

 that of the light which passed 

 through the prism, some was 

 more, and some less refracted, 

 those rays which passed to- 

 wards the highest points, V, 

 being more refracted than 

 those which passed towards 

 the lowest points R. 



The oblong form given to the 

 illuminated part of the screen, 

 was not the most curious 

 or surprising effect produced 

 by the prism. This image or K 

 spectrum exhibited the most beautiful se- 

 ries of colours, each depicted on the screen 

 with a degree of splendour and inten- 

 sity far exceeding those of the colours of 

 any natural object. Beside these the most 

 brilliant colour which nature presents, 

 or the most refined efforts of art could 

 exhibit, would seem faded and dim. The 

 lower extremity, R, exhibited the most 

 dazzling red, and above this, in regular 

 succession, were ranged the colours, 

 orange, yellow, green, blue, and indigo, 

 the upper termination, V, being violet. 

 These colours were not separated by 

 distinct limits, but the tints seem to melt 

 imperceptibly one into another, it being 

 impossible to determine exactly where 

 any one ended and the next began. Thus 

 the red was tinted off insensibly into the 

 orange, the orange into the yellow, and 

 so on. 



From these observations, it is appa- 

 rent that the red light, and all that por- 

 tion of light which partook of this cha- 

 racter, being deflected to the lower part 

 of the spectrum, is less refracted than 

 the blue light, and those colours of the 

 same class which are refracted to the 

 upper part. It would therefore seem to 

 be an obvious inference, that the solar 

 beam incident on the prism, was a mix- 

 ture of different kinds of light ; that the 

 prism acting on the component parts re- 

 fracted some of them in a greater, some 

 in a less degree ; those partaking of the 

 blue character being more refracted than 

 those of the red kind. This inference 

 seems also to harmonize with the former 

 experiments made upon coloured bodies 

 (18, 19), whereby it was proved that 

 red light is less refrangible than blue. 



(23.) Newton repeated the preceding 

 experiment in another way. The prism 

 being placed as before, at the aperture 

 in the window shutter, he placed his eye 

 behind it, so as to receive the rays emerg- 

 ing after the second retraction from the 



