THE SIZE OF A TOMS. 1 83 



we understand, in palpable matter, as the differ- 

 ence between solid and fluid, or between sub- 

 stances differing enormously in density ; or such 

 heterogeneousness as differences in velocity and 

 in direction of motion, in different positions of a 

 vortex ring in an homogeneous liquid ; or such 

 differences of material occupying the space ex- 

 amined, as we find in a great mass of brick 

 building when we pass from brick to brick through 

 mortar (or through void, as we too often find in 

 Scotch-built domestic brick chimneys). 



Cauchy was, I believe, the first of mathe- 

 maticians or naturalists to allow himself to be 

 driven to the conclusion that the refractive dis- 

 persion of light can only be accounted for by a 

 finite degree of molecular coarse-grainedness in 

 the structure of the transparent refracting matter ; 

 and as, however we view the question, and how- 

 ever much we may feel compelled to differ from 

 the details of molecular structure and molecular 

 inter-action assumed by Cauchy, we remain more 

 and more surely fortified in his conclusion, that 

 finite coarse-grainedness of transparent palpable 



