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 n, the 
angle of refraction r will be greater than that of incidence 7, according 
to the well-known law of the sines, which here becomes sin r= 
sin 7 But w being constant for the same two MeneneseN; there is a 
certain limit to 7 when sin r==1 or r=90° or sin i=- that is, the 
refracted ray coincides with the bounding surface of ing media, or it 
ceases to be refracted: and if i exceed this value, sin 7 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 7 at which this begins to 
1 
take place, or when sin i= — for any pair of media, can be easily and 
accurately determined ; thus uw 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. SER. 7 
