362 THF. EYE. [LfSSOJl XXXVlf. 



ened situation, the great number of rays which 

 would enter, would probably much injure the fine 

 contexture of nerves at the bottom of the Eye, or* 

 which the image* of objects are pourtrayed; and 

 if the pupil did not expand when ihe eye was in a 

 dark place, the few feeble rays which would enter 

 the Eye, would not form any impression on the 

 retina. Hence it is a very remarkable and pro- 

 vidential circumstance, that the change in ihe pu- 

 pil should take place almost spontaneously and 

 independent of any act of the will. 



The five following associated circumstances 



w 



seem to have some influence on our judgment con- 

 cerning distance : the number of objects which 

 intervene, the degree of distinctness in which the 

 minute parts are seen, the degree of brightness, 

 the inclination of the optic axes, and the confor- 

 mation of the Eye. Thus, distance is chiefly con- 

 ceived from experience j and the more distant an, 

 object is, the less it appears : when therefore, from 

 certain circumstances, we cannot form a just con- 

 ception of distance, and when we cannot judge of 

 objects but by the image which is pourtrayed in 

 our Eyes, we are then necessarily deceived as to 

 their size. Thus every person must be aware how 

 liable we are, in travelling by night, to mistake a 

 distant tree for a bush that is near, or a bush near 

 at hand for a tree at a distance. In the same 

 manner if we do not distinguish objects by their 

 shape, and by it also judge of distance, the fallacy 

 is likely to continue: in this case, a fly, which may 

 pass before us slowly, will seem to be a bird 



at 



