of JJolit ; and a neiu Theory of Vision. -^35 



wonderful," says Huvgens, (see the Edinlnirgh Philosophical 

 Journal for January) " why the rays C E and D G, incident from 

 the air upon the loner crystal, should not divide themselves like 

 the first rav A B." In another place he calls it a wonderful phe- 

 nomenon; and he accounts for there being sometimes only two 

 and at other times four im.ages, hv supposing that the ivaves of 

 light " have acquired a certain form or arrangement, in virtue 

 of vvhich, hy meeting the texture of the second crystal in one 

 position, they are capable of moving the two different matters 

 which serve for the two kinds of refraction ; while, by meeting 

 the second crystal in another position, they have not the power 

 of moving any of these matters. But in what maimer," he says, 

 " this happens, I have not been able to form any satisfactory con- 

 jecture." 



Now this wonderful phenomenon is explained in a moment hy 

 considering that the crystals are placed in such a position, with 

 respect to each other, that in one instance, the rays |)assing 

 through two different surfaces in the lower crystal enter the eye, 

 and consequently exhibit four images, while in another position, 

 the only rays that can enter the eye are those that pass through 

 the two parallel surfaces, and then only two images can be visi- 

 ble. Suppose two plates of glass to be placed, as I have repre- 

 sented them in fig. 4, and rays of light from A to 1)- striking 

 upon the first jilate in every direction. Now by the common 

 law of refraction *, the ray a would enter the eye at E ; but h c 

 and d, and all other rays from rhat object would be refracted 

 away from the eye in different angles, and could not possibly en- 

 ter it while it remained in that position ; so tliat while the plates 

 are placed in that position, with respect to each other and the 

 objects, only one image can enter the eye; and all the difference 

 between Iceland crystal and glass is, that the images are doubled. 

 If the plates however, with respect to the object and the eye, 

 should be placed as I have represented them in fig. 5, the ray a, 

 from A, will enter the lower plate at one surface, and the ray b 

 at another; and then two images will be visible at E, £is I have 

 represented them ; and when the crystals possess a double re- 

 fracting power, by the same rule there will be four. 



G. The sixth peculiarity of Iceland crystal, that sometimes one 

 image appears to move, and sometimes the other, when the cry- 

 stal is moving on its axis, may be explained in this way : Whicli- 

 cver image we fix our eyes u))on while the crystal is moving, will 

 always keep one place on the retina, and ccnsc(iuently appear to 



" In tlie two Iftst fi;{iire», as well as in the third, I have iniule the angle 

 of refraction to he half tlic anj^le of incidence : the effect, however, except 

 that the eye must he placed in a dirterent position, would he just the same 

 let thu angle of refraction be what it may. 



U r 2 be 



