THE ABSORPTION OF, LIGHT AND THE 

 COLOURS OF NATURAL BODIES. 



TWO LECTURES. 



BY PROF. STOKES. 



LECTURE I. 



THIS subject is one which does not admit very well of experi- 

 mental illustration before a large class ; in fact, with all the 

 appliances of the electric light, I should only be able to show 

 you, comparatively imperfectly, what you can each see for 

 yourselves by experiments which you can make quietly in 

 your own chambers, requiring, I may say, hardly any apparatus 

 at all. The foundation of what I have to say rests on Newton's 

 discovery of the compound nature of white light, with which 

 I presume you are already familiar. You know that when a 

 beam of light is allowed to fall upon a prism, it is decom- 

 posed into the different kinds of light of which it consists 

 which are bent round in passing through the prism to a 

 different degree. 



Supposing a beam of sunlight reflected horizontally into a 

 room through a small hole and allowed to fall on a prism close 

 by, if the light were of one kind, the beam would be simply 

 bent round as shown in this diagram [referred to], and instead 

 of a circular spot being painted on the wall as at A, it would 

 be as at B. But on making the experiment you have actually 

 an elongated coloured image. The cause of that is, the light 

 is not of one kind, but consists of a variety of kinds differing 

 from one another by the colour with which they impress 

 the eye, and by their ref rangibility or capability of being bent 



