Chap. 41.] How tbe ~Eye judges ofDiftanct. 401 



the object is at a very great diftance, it will feem to 

 go the fame way, as when a perfon runs by moon- 

 light, the moon appears to accompany him. 7th. If two 

 or more objects move with the fame velocity, and a 

 third remains at reft, it will appear in motion while the 

 moving ones feem at reft ; this is exemplified by the 

 moon and the clouds. 



There are fix natural methods, by which we judge 

 of the diftance of objects from the eye. ift. By the 

 angle which is made by the optic axes. For want of 

 this direction it has been obferved, that perfons who 

 are blind of one eye frequently mifs their mark in 

 pouring liquor into a glafs, &c. 2.ndly, and I think 

 moft generally, by the apparent magnitude of ob- 

 jects. By depending upon this method we are very 

 frequently deceived in our cftimates of diftance by 

 -any extraoidinary large objects, as in travelling to a 

 great city, church, or caftle, we fancy them nearer 

 than they really are. This furnifhes. us with a rea- 

 fon why animals and other fmall objects fcen conti- 

 guous to large mountains appear exceedingly fmall j 

 for we imagine the mountain to be nearer to us thaa 

 it actually is. When we look down alfo from a 

 high building, the objects beneath us appear much 

 fmaller than they would at the fame diftance on 

 the level ground j the reafon is plainly, becaufe we 

 'have no diftinct idea of diftance in that direction, 

 and therefore judge by the impreffions upon the 

 retina, whereas cuftom has corrected our judgment 

 in the other cafe. The third method of determin- 

 ing the diftance of objects is by the force and vivid - 

 nefs of the colours, and the fifth is analogous to it, 

 namely, by the different appearance of the minute 

 parts. When thefe appear diftinct, we judge the ob- 

 ject to be near, and the contrary when they appear 

 VOL. III. Dd faint 



