OF NEWTON'S OPTICS. 



23 



which are more refrangible are 

 so more reflexible, and vice versa. 



CHAPTER III. 



the Methods of obtaining Homoge- 

 neous Light. 



(33.) BY the results of the experimental 

 investigations described in the last chap- 

 ter, Newton was convinced that solar 

 light was not simple and homogeneous, 

 but was a mixture or composition of 

 many lights, differing from each other 

 in certain respects, but more particularly 

 in refrantribility. This quality he adopt- 

 ed as a test for pure unmixed or homo- 

 geneous light. A beam, every ray of 

 which was equally refrangible, and 

 which, therefore, did not admit of being 

 dilated by a prism, he considered to be 

 pure homogeneous light. Although the 

 parts into which the solar beam was de- 

 composed were shewn, by the experi- 

 ment described in p. 17, to be inca- 

 pable of further dilatation; yet this 

 method did not give all the purity to the 

 light which philosophical exactitude de- 

 manded, for the reasons which we shall 

 now explain. 



It will be recollected that the aperture 

 in the window-shutter (p. 14) casts a 

 circular illuminated spot on the opposite 

 wall, the diameter of which is equal to 

 the diameter of the hole, together with a 

 line, which being drawn upon the wall 

 would subtend at the hole an angle 

 equal to the apparent diameter of the 

 sun. But this circular spot is not uni- 

 formly bright. It is more faintly illu- 

 minated at its edges than at its centre, 

 the cause of which will be easily under- 

 stood. Let O O',fg. 25, be the hole in 



Fig. 25. 



the window shutter ; S S' the sun's dia- 

 meter. A ray of light from S', passing 

 the upper boundary O of the hole, falls 

 upon a screen at P ; and a ray from S, 

 passing the lower edge O' of the hole, 

 falls upon the screen at P'. Now, it is 

 apparent that no part of the sun's disc, 

 except the lowest point S', can shine 



upon P, the upper part of the window 

 shutter intercepting the light from all the 

 other points ; and that no part of his 

 disc, except the point S, can shine upon 

 P', the lower part of the window shutter 

 intercepting the light. Hence it appears 

 that the points P P', and the entire of 

 the edge of Ihe illuminated circle on the 

 screen, will be more faintly illuminated 

 than any of the parts nearer to its centre. 



The ray from S' passing above the 

 lower edge of the hole at O' will illumi- 

 nate L, and thus the point L will be 

 exposed to light from the entire disc 

 of the sun. The same may be said 

 of L/, and of all intermediate points. 

 The several points from L to P' 

 will be exposed to light from only a 

 part of the sun's disc, that part being 

 smaller the more distant the point is 

 from L, so that the light becomes gra- 

 dually more faint from L to P'. The 

 same may be said of the light from L' 

 to P. From this it appears that the 

 circular illuminated spot on the screen 

 is composed of a small circle whose 

 diameter is L L', uniformly illuminated, 

 surrounded to the distance L P' by a 

 ring of light of gradually decreasing 

 brightness, and fading away until it be- 

 comes insensible. This ring is called 

 the penumbra *. 



Now since the effect of the prism, as 

 has been already proved, is to stretch 

 out this luminous circle into an oblong 

 form, the breadth being the same, and 

 the sides and ends being illuminated 

 by the rays which form the penumbral 

 ring surrounding the unrefracted circle, 

 it follows that the sides and ends of the 

 spectrum will be bounded by a penum- 

 bral skirt of the breadth of P L' ; and 

 such, in fact, was the result of the ex- 

 periments. 



In the experiments which Newton 

 now desired to institute, it was necessary 

 that the light should be obtained of as 

 uniform an intensity as possible, and 

 therefore it was necessary to remove or 

 very much diminish the penumbral fringe 

 which we have just described. But 

 it was still more necessary that light 

 should be obtained which was perfectly 

 pure and homogeneous, and it so hap- 

 pened that the same cause which pro- 

 duced the fringe and varied the intensity 

 of the light, also impaired its purity. 



* The hole may be so small, that its apparent di- 

 ameter at the screen will be less than that of the sun. 

 In this case the part of the figure L I/ will disappear, 

 and the whole spot on the screen will have the cha- 

 racter of penumbra, the centre being the most lumi- 

 nous point. 



