ON LIGHT. 243 



ator and denominator of the fraction expressing it, so 

 that, for instance, in the passage of light out of water 

 into air, the "law of the sines" is expressed in the same 

 general terms, but the " refractive index" (by which is 

 meant the number expressing the proportion in question) 

 has to be changed into its numerical reciprocal. In the 

 case supposed, when light passes out of air into water, 

 the proportion of the sines is that of 1336 to 1000, or 

 almost exactly 4 to 3; and the "refractive index" is 

 accordingly expressed by the fraction , or the almost 

 exactly equivalent decimal i'336. In the reversed case, 

 then, when the transmission is out of water into air, it 

 will be 1=075, or more precisely 0749. 



(26.) As a matter of experiment, it is found that be- 

 tween transparent media, or substances capable of being 

 traversed by light, there exists a very wide diversity in 

 this ratio of the sines of the two angles in question, or 

 in the numerical values of the "refractive indices." 

 Thus when light passes out of air into the less refractive 

 species of plate-glass, the index instead of % is | or i -5 ; 

 into sulphur (which in its crystalline form is transparent), 

 2-0; and into diamond, or the mineral called octohe- 

 drite, 2-5. In fact, each particular transparent sub- 

 stance, solid, liquid, or gaseous, has its own peculiar, and, 

 so to speak, characteristic index of refraction, which is 

 found to stand in relation to its physical habitudes in 

 many other respects, especially with its chemical com- 

 position, and its state of aggregation and density. 



(27.) Even common air, in respect of a vacuum, has 

 its refractive index viz., i -0003 the effect of which is 



