EMISSION AND WAVE THEORIES. 



145 



Fresnel a few years afterwards showed to be so astonish 

 ingly fruitful in results.* 



* In the remarks here made by Arago on Malus s investigation of 

 the refractive powers of solid and liquid wax, there appears some lit 

 tle obscurity of statement, and a degree of importance attached to the 

 result as decisive between the rival theories, which it does not appear 

 to deserve. 



Perhaps for the general reader a few words explanatory of the 

 method may be necessary, in order to see the general bearing of the 

 case. 



When a ray passes out of a denser medium m into a rarer , the 

 angle of refraction r will be greater than that of incidence z, according 

 to the well-known law of the sines, which here becomes sin ru. 

 sin L But fj, being constant for the same two substances, there is a 



certain limit to i when sin r=l or r=90 or sin i=- that is, the 



P 



refracted ray coincides with the bounding surface of the media, or it 



ceases to be refracted: and if i exceed this value, sin r would be 

 greater than unity, which is impossible, or the ray cannot emerye from 

 the denser medium, but must remain wholly within it. This alone, 

 however, does not prove that it will be reflected. Experiment, how 



ever, shows that it is, and the precise angle i at which this begins to 



take place, or when sin = - for any pair of media, can be easily and 



p 



accurately determined; thus p is found for that pair of substances, 

 but , is the compound ratio of the separate refractive powers of each 

 out of vacuum or air; if, therefore, one of these is known, the other 

 is deduced. 



On this principle Dr. Wollaston s method was founded (Phil. Trans. 



SEO. SEK. 7 



