RADIATION OF HEAT. 



413 



light ; and in a word, all the phenomena explained in optics, concerning the 

 reflection of light by surfaces, whether plane or curved, are found to accompa- 

 ny the reflection of the non-luminous calorific rays. This is actually found to 

 take place, whether the non luminous rays be those which are obtained by re- 

 flecting the solar light by the prism, or produced from a heated body 



Fig. 3. 



In the experiments of Berard, the question of the identity of the calorific 

 and luminous rays was submitted to tests even more severe. There are certain 

 crystallized bodies called double refracting crystals, which produce peculiar 

 effects on the rays of light transmitted through them. Let A B, fig. 4, be the 

 surface of a piece of Iceland spar, or carbonate of lime, which is one of this 

 class of bodies, and let L L' be a ray of light striking obliquely on the surface 

 of this crystal ; if the crystal were common glass this ray would be bent out 

 of its course, and would pass through it in another direction ; but, in the case 

 of Iceland spar it is observed that the ray L L' is divided into two distinct 

 rays, which proceed in two different directions, L/ M, L' M', through the 

 crystal. Let a non-luminous calorific ray, taken from the lower end of the 

 spectrum, be in like manner transmitted to the surface of such a crystal, it 

 will be found, that, in penetrating the crystal, it will be divided into two rays, 

 and that these two rays will be deflected according to the same laws, exactly 

 as a luminous ray is under the same circumstances. 



Fig. 4. 



A luminous ray thus, after its transmission through a double refracting crys- 

 tal, is observed to have received a peculiar physical modification, which is 



