432 j4n Eisay on the Reflection, Refraction, mid Inflection 



6. In looking at an object through a plate of Iceland crystal, 

 if the plate be moved on its axis, sometimes one of the objects is 

 stationary, while the other appears to move round it, and some- 

 times the reverse; that is, the image that before was moving is 

 now stationary, and the one that was stationary is in motion. 



7. The images of objects seen through double refracting sub- 

 stances are always much fainter than in those mediums which 

 exhibit but one image. 



In order to prevent the possibility of a misconstruction, it may 

 be necessary to premise that rays of light proceed in every di- 

 rection from every visible object, otherwise the same object could 

 not be seen at one time by persons in different situations; so 

 that whenever we look through a glass lens at any object, the 

 whole surface of the glass is covered by rays from that object. 

 But as all these rays are refracted in different angles, according 

 to their angles of incidence and the figure of the glass, a great 

 part of them must pass outside of the eye wherever it is placed ; 

 and hence it is that objects appear to magnify as the eye is re- 

 moved further from the glass when it is convex, and diminish 

 where it is concave ; because, by the common law of refraction, 

 those rays that strike upon the extremities of a convex or con- 

 cave lens are more refracted than those that strike near the cen- 

 tre; and as these rays can only enter the eve when it is removed 

 some distance from the glass^ it follows as a natural consequence, 

 that the images of objects must grow larger as the eye recedes 

 from the glass when it is convex, and smaller when it is con- 

 cave*. Now, as all the rays that strike upon the diiferent parts 

 of a medium are refracted in different angles, only one image 

 can be seen through a single refracting surfiice, because all the 

 other rays proceeding from the same object are refracted by it 

 outside of the eye ; but in a double refracting surface, as there 

 are two refracting substances in it, one of which refracts more 

 than the other, rays from the same object will be refracted into 

 the eye by different angles, from two parts of the same surface, 

 and two images in consequence must be painted on the retina. 



With these premises, then, I proceed to explain: 



1. Why only one imxge is exhibited in a doul)le refracting 

 medium. 



Iceland crystal in form is a parallelogram with all its angles 

 oblique ; its two opposite sides consequently are parallel ; and 

 whenever a ray strikes perpendicularly upon the first surface in 



* This is the phaenomenon which Dr. Berkeley says, in his Theory of 

 Vision, has so much puzzled all the writers on optics, and can he explained 

 by none of their theories ; and yet nothing can be more simple upon my 

 principle, which will be evident when I come to show the true principle of 

 yision, which hitheitohas not been discovered. 



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