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 investic;ation of 

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

 tle obscurit}' 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 n, the 

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

 to the well-known law of the sines, which liere becomes sin r=u 

 sin i. But /i being constant for the same two substances, there is a 



1 

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



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

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

 greater than unity, which is impossible, or the ray cannot emerge 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 whicli this beo-Ins to 



1 ° 



take place, or when sin i= - for any_;joJ;' of media, can bo easily and 



accurately determined; thus ji is found for that pair of substances 

 but It 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 (P/«7. Trans. 



SEC. SER. 7 



