2,5() OPTICS. 



crystalline humour, yet they can only see distinctly 

 within certain limits; and nearer than that, objects 

 appear confused. But these limits are not the 

 same in different people, A good eye can see dis- 

 tinctly when the rays fall parallel upon it; and 

 then the principal focus is at the bottom of the 

 eye. 



A man can judge of a small distance with a 

 single eye, by frequently observing how much 

 variation is made in the eye to make the object 

 distinct; and from this a habit of judging is ac- 

 quired. But this cannot be done at great distances, 

 because, though the distance be varied, the change 

 in the eye becomes then insensible. 



But one can judge of greater distances with 

 both eyes, than with one. For the eyes being 

 at a distance from one another, as long as 

 that distance has a sensible proportion to the dis- 

 tance of the object, one gets a habit of judging, by 

 the position of the axes of the eyes, which are 

 always directed to that point. For different dis- 

 tances require different positions of the axis, which 

 depend on the motions of the eyes, which we feel. 

 But in very great distances, no judgment can be 

 made from the motion of the eyes, or their internal 

 parts. Therefore we can only guess at the dis- 

 tances from the magnitude, colour, and the position 

 of interjacent bodies. 



Whatever light falls upon that part of the retina 

 where the optic nerve, D, springs, makes no im- 

 pression; and therefore, if the picture of an object 

 falls thereon, it is not perceived, and that object is 

 invisible. This will appear by placing a small 

 bright object before you, and looking at it with 

 one eye; then moving one eye laterally towards 

 the contrary side, (towards the left, if it be the 



