58 LECTURES TO SCIENCE TEACHERS. 



a fluid or solution like this perfectly free from motes, and 

 the motes which are present will reflect a certain quantity 

 of white light, and that light is principally polarised when 

 looked down on vertically from above in a plane passing 

 through the beam. When the beam is viewed by light 

 polarised in the perpendicular plane, any light which would 

 be reflected from motes is nearly got rid of, and the blue 

 light is seen in its purity. 



In this mode of observation it clearly appeared that the 

 solution of quinine belonged to the class of bodies in which 

 Sir David Brewster had discovered what he called internal 

 dispersion. Among them there is a kind of glass which 

 exhibits it in a very remarkable degree Here is a specimen 

 of the glass ; it is coloured by sesqui-oxide of uranium. He 

 noticed in this case, that the whole of the beam was un- 

 polarised, or, as he expressed it, possessed a quaquaversus 

 polarisation. 



It was twenty five years ago last Easter, when I was pre- 

 paring for my optical lectures in Cambridge, that, having 

 had my attention directed by a friend to this solution, I 

 procured some, and I was greatly struck with the remarkable 

 appearance of the phenomenon, and the question occurred 

 to me what was the cause of it. Now for my own part I 

 had the fullest confidence in the doctrine that, the light 

 belonging to a given part of the spectrum is homogeneous, 

 or all of the same kind. I am not now speaking of polari- 

 sation. It has been supposed by some, by Sir David 

 Brewster, for instance, that the light of a given part of 

 the spectrum, although no longer decomposed by the prism, 

 might be decomposable by other means, for example by 

 the use of absorbing media; and he thought he had obtained 

 white light from a particular part of the spectrum by the 

 use of a suitable absorbing medium. This has since been 

 proved to be merely an illusion of contrast. Having, as I 

 said, felt the fullest confidence in the principle that the light 

 of a given part of the spectrum is homogeneous, I felt little 

 doubt that if the principle were faithfully followed out, it 

 would lead to a solution of the problem what the nature of 

 the light which produced this e fleet was. At first I took 

 for granted that the blue light, which the prism shows 

 to be heterogeneous and not mere prismatic blue, a com- 

 pound of various colours, a little led, more green, but with a 





