ON LIGHT. 251 



deposits, with every possible intermediate gradation of 

 size. But if in all this series any particular size were 

 found entirely and universally deficient, throughout the 

 whole series of formations traceable to that source, we 

 should conclude, not that a mass of that size is an im- 

 possibility in rerum natura> but that owing to some un- 

 known cause in the nature of a previous sifting, every 

 pebble or grain of that size had been already separated, 

 or otherwise arrested in limine, and might expect else- 

 where to find it in the case of some other series of 

 geological formations. So it is with the sun's light. 

 Certain definite and marked degrees of refrangibility are 

 wanting in its spectrum, indicated by the dark lines 

 which cross it. But if absent in solar light, they exist in 

 the light of flames, and of other luminous sources, which 

 in their turn are again deficient in other degrees which 

 yet abound in the solar rays. Refrangibility, then, 

 taken as a property of light generally, is a quality sus- 

 ceptible of indefinite gradation, from the one extreme of 

 the spectrum to the other. 



(35.) If we limit our consideration to some one 

 medium glass, for instance we find each particular 

 degree of refrangibility associated, first, with a deter- 

 minate and invariable index of refraction, which de- 

 termines its place in the spectrum by determining the 

 amount of deflexion it shall undergo in passing through 

 the prism ; and, secondly, with an equally determinate 

 and invariable tint in the scale of " prismatic colour," 

 the red corresponding to the least and the violet to the 

 greatest refractive index. The truth of these proposi- 



