PHYSICS OF THE NINETEENTH CENTURY. LIGHT. 461 



from air to water being as a matter of experimental fact -f, or 1*333, it 

 follows, if the undulatory theory be correct, that light moves with less 

 velocity in water than in air in proportion as 3 is less than 4. Here 

 we see that the undulatory theory perfectly accounts for the law of the 

 sines in refraction. 



We have postponed to this place an account of the discovery of 

 double refraction, in order that the subject might be considered in the 

 present connection. The facts are briefly these : A Danish physician 

 published in 1669 an account of certain crystals found in Iceland, 

 which possessed the property of causing all objects viewed through 



FIG. 207. ICELAND SPAR SHOWING DOUBLE REFRACTION. 



them to appear double. Fig. 207 shows the effect produced when a 

 single line of print is placed behind a crystal of Iceland spar. It 

 became evident when this property of Iceland spar was discovered, that 

 the laws of refraction as stated by Snell (page 155), were not applicable 

 to Iceland spar. Huyghens investigated- the new case, as we have 

 already stated on page 228, and he found that by a certain geometrical 

 conception he was able to arrive at a very satisfactory explanation of 

 the phenomena of double refraction, and to apply to them the formulae 

 of mathematics as rigorously as in the case of ordinary refraction. 

 The hypothesis by which Huyghens explained the peculiarities of re- 

 fraction in Iceland spar, supposed that a ray of light passing into the 

 crystal is divided into two parts, one of which is capable of propaga- 

 ting its undulations in the crystal with equal velocity in every direc- 

 tion, so that the wave -fronts to. which it gives rise are spherical at 



