GALILEO AND THE NEW PHYSICS 



erns refraction escaped him. It remained for Wille- 

 brord Snell, a Dutchman, about the year 1621, to dis- 

 cover the law in question, and for Descartes, a little 

 later, to formulate it. Descartes, indeed, has some- 

 times been supposed to be the discoverer of the law. 

 There is reason to believe that he based his generaliza- 

 tions on the experiment of Snell, though he did not 

 openly acknowledge his indebtedness. The law, as 

 Descartes expressed it, states that the sine of the angle 

 of incidence bears a fixed ratio to the sine of the angle 

 of refraction for any given medium. Here, then, was 

 another illustration of the fact that almost infinitely 

 varied phenomena may be brought within the scope 

 of a simple law. Once the law had been expressed, it 

 could be tested and verified with the greatest ease; 

 and, as usual, the discovery being made, it seems sur- 

 prising that earlier investigators in particular so saga- 

 cious a guesser as Kepler should have missed it. 



Galileo himself must have been to some extent a 

 student of light, since, as we have seen, he made such 

 notable contributions to practical optics through per- 

 fecting the telescope ; but he seems not to have added 

 anything to the theory of light. The subject of heat, 

 however, attracted his attention in a somewhat differ- 

 ent way, and he was led to the invention of the first 

 contrivance for measuring temperatures. His ther- 

 mometer was based on the afterwards familiar prin- 

 ciple of the expansion of a liquid under the influence 

 of heat ; but as a practical means of measuring tem- 

 perature it was a very crude affair, because the tube 

 that contained the measuring liquid was exposed to the 

 air, hence barometric changes of pressure vitiated the 



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