ON LIGHT. 



277 



wall, is heard, though with diminished intensity, on the 

 other side, we have the analogue to the partial reflexion 

 of a beam of light at a transparent surface ; and on the 

 other hand, in the deadening of sound in passing through 

 woolly or puffy substances, while it is transmitted with 

 exceeding sharpness and distinctness through compact 

 solids or through water, we have the parallel to the 

 absorption of light in some media, and its copious trans- 

 mission through others. 



(61.) The explanation of refraction on the undulatory 

 theory is exceedingly simple. Suppose a plane wave to 

 sweep obliquely along the surface B E of a medium 

 capable of propagating within it the luminiferous undu- 

 lation, and let it be supposed at equal intervals of time 



(successive seconds, for instance) to assume successive 

 positions B b, c c, D d, EC, arriving in succession at 

 equidistant points B c D E of the surface. So soon as 



