152 Professor Forbes on the Refraction 



blished the fact of comparative non-reflection from a second re- 

 flecting plate of mica, the plane of incidence being at right angles 

 to the first ; but I had more reason than ever to be satisfied of 

 the value of the simple and effective method of transmission 

 through thin mica plates. In fact, it was only by the aid of 

 that method that I could have advanced to the still more delicate 

 inquiries which, by the constancy of my first results, I Mas en- 

 couraged to undertake. 



§ 4. On the Depolarization and Double Refraction of Heat 



by Crystals. 



46. The analogies which have hitherto guided us from the 

 laws of light to those of heat, suggest that it is far from impro- 

 bable that the influence of crystallized bodies upon polarized 

 light, which produces the most splendid and most varied, but, at 

 the same time, amongst the most determinate phenomena of op- 

 tics, may have a counterpart in the science of heat. The simpler 

 of these, of course, it is our object first to verify ; and, to a cer- 

 tain extent, this is all that is necessary, in order to complete the 

 analogy of heat and light in this particular case ; for the condi- 

 tions essential to their production in the case of light, are on all 

 hands admitted to depend on the susceptibility of the principle 

 of light to undergo certain modifications in certain circumstances, 

 extremely limited in number, and which then produce, as neces- 

 sary consequences, all the subsequent effects. If we find that 

 heat undergoes the same changes under the same circumstances, 

 so far as we can detect them, there is the highest probability in 

 favour of the extended analogy ; for if there be a necessary se- 

 quence in the one case, it must be inferred also in the other. 



47. When polarized light is caused to pass through a crys- 

 tallized body possessing the power of double refraction, it hap- 

 pens, in a great majority of the conditions under which the ex- 

 periment may be made, that the light, on emerging from the 

 crystal, has undergone some change. This change may, for in- 



