DISCOVERY OF THE LAW OF REFRACTION. 381 



The explanation of the rainbow and of its mag- 

 nitude, afforded by Snell's law of sines, was perhaps 

 one of the leading points in the verification of the 

 law. The principle, being once established, was 

 applied, by the aid of mathematical reasoning, to 

 atmospheric refractions, optical instruments, dia- 

 caustic curves, (that is, the curves of intense light 

 produced by refraction,) and to various other cases; 

 and was, of course, tested and confirmed by such 

 applications. It was, however, impossible to pursue 

 these applications far, without a due knowledge of 

 the laws by which, in such cases, colours are pro- 

 duced. To these we now proceed. 



