IN OPTICS. 113 



the sphericity of the cornea and crystalline, the 

 various refrangibility of the different kinds of 

 light, and the width of the pupil at night, can 

 be supposed to allow; for I find, that, if I now 

 pass a needle across the axis of the right eye, 

 its shadow will not be seen. But should I, after 

 this, withdraw my accurate attention from the 

 star, and view it in the state of sight we have, 

 when we are said to be in a reverie, in which, 

 though our eyes are open, we are yet scarcely 

 conscious of seeing surrounding objects, the 

 appearance to the right eye expands itself, and 

 if a needle be again passed before this eye, its 

 shadow will be observed to move over the star, 

 in a direction contrary to that of the needle 

 itself; a sure indication that the rays of light 

 now tend to a focus behind the retina. In the 

 same state of things, the appearance -of the star 

 to the left eye contracts, and if a needle be 

 held before the eye, no shadow is seen ; a sign 

 that the rays are collected to a focus on the 

 retina ; whereas they had formerly crossed one 

 another before they reached that membrane. 



Upon the whole then it is manifest, from the 

 experiments I have related, that my left eye 

 can collect to focuses in the retina, rays which 

 proceed from objects at every distance what- 

 soever, not less than seven inches; that my 

 right eye can collect to focuses in the retina, 



i 



