DISCOVERY BY EXAMINING PECULIAR TRUTHS. 489 



of double refraction in a crystal of Iceland spar largely 

 aided the discovery of the general laws of light. In 

 the year 1669, Erasmus Bartolinus published a work on 

 the subject, and also discovered by observation the fact 

 that one of the images was produced according to the 

 ordinary law of refraction, and the other according to an 

 extraordinary and new law, and varied also in different 

 positions of the crystal. It was by investigating peculiar 

 phenomena that Huyghens discovered polarisation of light. 

 He says : ' Before I quit the subject of this crystal I 

 will add one other marvellous phenomenon, which I have 

 discovered since writing the above ; for though hitherto 

 I have not been able to find out its cause, I will not, 

 on that account, omit pointing it out, that I may 

 give occasion to others to examine it.' He then states 

 the phenomena, which are, that when two rhombo- 

 hedrons of Iceland spar are in parallel positions, a ray, 

 doubly refracted by the first, is not further divided when 

 it falls on the second ; the ordinarily refracted ray is 

 ordinarily refracted only, and the extraordinary ray is 

 only extraordinarily refracted by the second crystal, 

 neither ray being doubly refracted. The same is still 

 the case if the two crystals have their principal planes 

 parallel, though they themselves are not parallel. But 

 if the principal plane of the second crystal be perpen- 

 dicular to that of the first, the reverse of what has been 

 described takes place ; the ordinarily refracted ray of the 

 first crystal suffers, at the second, extraordinary refraction 

 only, and the extraordinary ray of the first surfers ordinary 

 refraction only at the second. Thus, in each of these 

 positions, the double refraction of each ray at the second 

 crystal is reduced to a single refraction, though in a dif- 

 ferent manner in the two cases. But in any other position 

 of the crystals, each ray, produced by the first, is doubly 



