342 On the Adjustment of the Eye to see 



First Olservation. 



If we look attentively at a remote object, which subtends 

 a very small angle at the eye, that object will be seen more 

 distinctly than any other object that may be near it ; because 

 those ray^; thit enter the eve near the axis of vision are So 

 nearlv parallel, on account of the great distance of the ob- 

 ject, that the eve can distinguish no difference between these 

 and parallel rays ; and those rays passing through the cry- 

 slallint. lens, near its axis, and falling perpendicularly upon 

 the TL'tina, produce vision as perfect as the distance and 

 maj^nitude of the object will admit. But those rays which 

 form an angle v;ith the visual axis fall obliquely upon the 

 retina, and produce indistinct vision. 



Second Observation. 



If we look at a near and small object with attention, the, 

 pupil will contract so as to admit only those ravs that are 

 nearly parallel lo the axis of vision. Suppose, for example, 

 that we look at the point on the letter i in a printed book, 

 the remaining part of the letter, and all the surrounding 

 letters, will be seen imperfectly, for the same reason that was 

 given above. 



Third Ohservation. 



Let the same object (•) be brought within two inches of 

 thee\e, and it will appear indistinct, because the rays issuing 

 from it will enter the eye in a diverging state, in conse- 

 quence of the pupil's being too large for the object. But if 

 the pupil be contracted by means of a perforation about -J^ 

 of an inch in diameter, placed before the eye, made either 

 in a thin piece of metal or a slip of paper, the object will be 

 seen distinctly, because no rays will then enter the eye, 

 except those that are nearly parallel to the axis of vision. 



Hence it is evident, that we have no perfect vision, but 

 by rays which enter the eye in a direction parallel to tlie 

 axis of the crystalline lens, and fall perpendicularly upon 

 the retina; and as all rays that fall upon a convex lens 

 parallel to its axis are converged lo the same point, the fol- 

 lowing conclusion must be strictly true, viz. 



When 



