of Light ; and a neiv Theory of Vision. 433 



such a part of it that it may pass out perpendicularly through 

 the further parallel surface, it is evident there can be no refrac- 

 tion, because there is no angle of incidence, the direction of the 

 ray is in the line of the power of attraction at both surfaces, and 

 therefore cannot be affected by it. It is evident from this, that 

 under these circumstances, a doable refracting substance ought 

 not to exhibit two images any more than a single refracting sub- 

 stance; because, if the rays falling but a little oI)iique upon the 

 surface could be made visible to the eye in one instance, it must 

 in the other, for the law is precisely the same in both cases ; and 

 even if it were so, it would not produce a double image, but the 

 one image would be extended over the whole of the retina*, so 

 that the object would not be doubled but only magnified. 



2. The second peculiarity of Iceland crystal, its double image, 

 after what I have just said, will be easily explained by a re- 

 ference to the plate. Let K L M N (Plate V.) represent a 

 piece of common glass, and a ray from the object A striking 

 upon the first surface, and passing on through P by the ordinary 

 reflection into the eye at E. The image of the object then will 

 appear at P ; but there will be only one image, because all the 

 other rays from this object will be refracted outside of the eye, 

 and consequently cannot be visible. Now if instead of glass the 

 rays have to pass through a double refracting substance, while 

 the ray R enters the eve from P by the ordinary refraction, the 

 substance possessing the greatest power of refraction will refract 

 the ray r down to Q in a greater angle, and from thence into 

 the eye, so that two images of one object must always appear 

 whenever the substance possesses a double refracting power, and 

 the rays from it are oblique and not perpendicular. 



The reader will understand that I am not here showing the 

 power of refraction that each substance possesses, but am merely 

 explaining the case. 



The reader will understand that I do not pretend, in either of 

 these figures, to give the exact (juantum of the power of rcfrac- 

 tio!i that each substance possesses ; my object is nscrely to show 

 the principle of doul)le refraction ; and as long as there are two 

 refracting substances in the same medium possessing different 

 degrees of power, there must always be two images let the dii- 

 ference of power be what it will. 



3. In order to explain the third peculiarity, that the distance 

 between ttie images increases in proportion to the thickness ot" 

 the plate, let K L M N represent a thin plate of Iceland crystal, 

 and the rays Mr refracted into ^tlie eye at e from the last stir- 

 facc at p and q, where their distance asunder is very small ; but 



• Tliis would be the case if the commonly receiyed theory of vision was 

 truf, as I shall [nescni Iv make very evident. 



Vol. 55. No. 26().' June 1820. R r if 



