ABSORPTION OF LIGHT, ETC. 



37 



but I will not go further into that, because what I have said 

 is merely an introduction to the use of the prism in the 

 simplest manner possible. 



The simplest way to form a pure spectrum experimentally, 

 and a way which suffices, provided you do not want to put 

 objects in the spectrum, but only to see it, is to suppose the 



FIG. D. 



lens to represent your eye, and the screen placed in the focus 

 of the lens to represent the retina. 



I said that if the light was passed through a hole it would 

 be brought to a point ; but if it came in through a slit, which 

 you may regard as a succession of holes in a direction perpen- 

 dicular to the plane of the paper, then after passing through 

 the lens it would be brought to a line of light standing out 

 in a direction perpendicular to the plane of the paper, which 

 is supposed to be the plane of refraction for light from the 



