GASEOUS REFRACTIVE INDICES 



By C. CUTHBERTSON 



Fellow of London University, U?iiversity College 



Light, which travels with a velocity of 186,300 miles a second 

 through empty space, is retarded in passing through transparent 

 material bodies. If the surface of the medium is not normal to 

 the direction of the beam of light, this direction is changed, and 

 the light is said to be refracted. The refractive index of the 

 material is defined as the ratio of the sine of the angle of 

 incidence to that of the angle of refraction ; but it is simpler to 

 think of it as the ratio of the velocity of light in vacuo to its 

 velocity in the medium. This ratio is usually indicated by the 

 letter ^ ; and, since the velocity of light in vacuo is usually 

 greater than in a material medium, /a is a number greater than 1. 

 In water it is about 1*33, in crown glass about 1*5, and in the 

 diamond 2*42. In passing from one medium to another the 

 frequency of the periodic disturbance which constitutes light 

 remains constant, and, consequently, the change of velocity must 

 be due to a change in the distance between two similar states 

 of the disturbance, which is called the wave length. Hence, the 

 refractive index, in its simplest terms, is the ratio of the wave 

 length in vacuo to the wave length in the substance, and/4-1 

 expresses the ratio of the retardation to the length of the 

 retarded wave, or, in other words, the fraction of its own 

 retarded length by which a wave is shortened in passing through 

 a transparent substance. 



It seems probable, a priori, that the study of refractive indices 

 may give valuable information with regard to the constitution 

 of matter. Light is an electro-magnetic disturbance; and even 

 if we do not go so far as to hold, with some physicists, that 

 matter is nothing but a mode of electricity, it is becoming more 

 evident every day that the constitution of the atom is intimately 

 connected with electrical phenomena. It is, therefore, to be 

 expected that the amount of retardation of light in passing 



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