LIGHT. 115 



effects of lateral induction be the same a striking analogy 

 with one of the effects observed in the propagation of light 

 and sound. The effects observed by MM. Fizeau and Foucault, 

 of the slower progression of light in proportion as the trans- 

 mitting medium is more dense, seem to me in favour of the 

 view here advocated ; as a greater degree of heat would be 

 produced by light in proportion to the density of the medium, 

 force would be thus carried off, and the molecular system 

 disturbed, so that the progress of the motion should be more 

 slow ; but so many considerations enter into this question, and 

 the phenomena are so extremely complex, that it would be 

 rash to hazard any positive opinion. 



Dr. Young ultimately came to the conclusion that it was 

 simplest to consider the ethereal medium, together with the 

 material atoms of the substance, as constituting together a 

 compound medium denser than pure ether, but not more 

 elastic. Ether, or highly attenuated matter, might thus be 

 viewed as performing the functions which oil does with tracing- 

 paper, giving continuity to the particles of gross matter, and 

 in the interplanetary spaces forming itself the medium which 

 transmits the undulations. 



Since the period when Huyghens, Euler, and Young, the 

 fathers of the undulatory theory, applied their great minds to 

 this subject, a mass of experimental data has accumulated, all 

 tending to establish the propositions, that whenever matter 

 transmitting or reflecting light undergoes a structural change, 

 the light itself is affected, and that there is a connection or 

 parallelism between the change in the matter and the change 

 in the affection of light, and conversely that light will modify 

 or change the structure of matter and impress its molecules 

 with new characteristics. 



Transparency, opacity, refraction, reflection, and colour 

 were phenomena known to the ancients, but sufficient attention 

 does not appear to have been paid by them to the molecular 

 states of the bodies producing these effects ; thus the transpa- 

 rency or opacity of a body appears to depend entirely upon 

 its molecular arrangement. If striae occur in a lens or 

 glass through which objects are viewed, the objects are dis- 

 torted : increase the number of these striae, the distortion is 



