482 Mr. E. H. Cook on the 



here we undoubtedly have a well-marked effect produced by 

 heat on the substance, whatever it is, which transmits the un- 

 dulatory movement. Assuming the existence of the luniini- 

 ferous aether, we explain this as follows : — The heat produces 

 a more rapid oscillating movement among the particles of the 

 body, the amplitude of the vibrations which the particles exe- 

 cute being at the same time increased. The freedom of the 

 aether particles in which the others are immersed to oscillate 

 is changed owing to this; and this change is of such a nature 

 as to alter the inclination of the optic axes to each other. 

 The only part of this explanation which it is difficult to com- 

 prehend is the most important part of it, viz. why the oscilla- 

 tion of the molecules of the body should alter the elasticity of the 

 containing medium. When we remember the relatively large 

 space which exists between the molecules of bodies, and there- 

 fore the large volume of the contained aether compared with 

 the size of the vibrating molecule, it is difficult to understand 

 the reason why so small a vibrating mass should affect so large 

 a containing one. Again, we must remember that the effect of 

 the heat is simply to increase a previously existing motion, and 

 not to call into action a new species of motion. It is therefore 

 less likely that this increased motion should affect the containing 

 medium. Another consideration which makes it difficult to 

 accept this explanation is, that one of the best-marked proper- 

 ties of the aether is its extreme mobility, which would there- 

 fore allow of the vibration of molecules in it without resistance 

 and also without affecting it. The acceptance, then, of this 

 explanation is attended with various difficulties. 



Assuming, however, that the transmission of the undulatory 

 movement of light takes place by the vibration of the particles 

 of the body, the effect of the alteration of temperature is very 

 natural, and, in fact, is just what would be expected. The 

 increased oscillation of the particles by a rise in the tem- 

 perature causes an alteration of a certain kind to occur in the 

 mode of vibration possible to those particles. It is unneces- 

 sary for us to inquire what the precise nature of this altera- 

 tion is, as it is easily seen that an alteration is possible. The 

 change may be analogous to the change in the clang-tint of a 

 note produced by the admixture of different harmonics with 

 the same fundamental. 



Secondly, the researches of Brewster have shown that some 

 crystals possess two axes for light of one colour, but only one for 

 light of another. Thus Glauberite possesses two axes inclined to 

 each other at an angle of 5° for red light, but only one for violet. 

 These experiments may therefore be taken to prove that for red 

 light the molecules of the crystal are symmetrically arranged 



