RADIATION. 189 



is now interposed, and the only radiant rays which can pass 

 through are the non-luminous ones. I bring the needle of 

 the galvanometer to rest, and now we have a dark field, 

 allowing quite complete polarisation of the light ; but 

 when I turn the Nicol prism round until I see the 

 luminous image upon the near side of the iodine, which 

 will be absorbed, and not allowed to appear on the thermo- 

 pile, you will see that the needle is gradually deflected. 

 The field, which is a bright field, is also a hot field, and 

 when we turn the crystal round so that we have a, dark 

 field, we have also a cold field at the same time. Now we 

 have an intense amount of heat getting through ; by turn- 

 ing round the dark field you will see that the needle has 

 returned, and we shall have the cold field back again. 

 When it has returned, I interpose between the two prisms 

 the piece of mica which you saw produce so remarkable an 

 effect in the case of light. This experiment is a little more 

 delicate, but still I hope we shall be enabled to get a 

 sensible amount of heat allowed to pass through just as 

 we saw a very sensible amount of light pass through 

 before. You see the needle has moved through a consider- 

 able angle since the mica was placed there. Then we will 

 remove the mica, and you will see it return to zero. These 

 experiments were followed by a number of others ; and by 

 varying these experiments with the mica polarisers and 

 rock-salt apparatus Principal Forbes proved nearly all the 

 elementary facts of the undulatory theory of light to be 

 true with respect to heat. Perhaps the most striking was 

 the circular polarisation of light. M. Fresnel, by a most 

 wonderful application of mathematical analysis to physics, 

 viz., by an interpretation which he gave to the remarkable 

 mathematical symbol \/ - l, which had been found in some 

 researches of Sir Wm. Hamilton's and others to have a 

 peculiar significance, and by interpreting the significance of 

 the symbol in a peculiar way, was led by the equation 

 he had deduced to a most remarkable discovery, the circular 

 polarisation of light by what is called Fresnel's rhomb, 

 which consists of a piece of glass of a peculiar shape. It 

 is found that when light, polarised in a certain plane, falls 

 on one of those surfaces, and, after two reflections, is 

 allowed to pass out again, the light is in the peculiar con- 

 dition which is called circular polarisation, and the physical 



