THE EYE, AND THE PHENOMENA OF VISION. 



355 



an olject is from the eye, the less will be the inclination of the lines, and tho 

 less the angle of vision. The nearer an object is to the eye, therefore, the 

 larger it will appear. 



Fig. 288. 



Thus the trees and houses far down a street or avenue appear smaller than 

 those near by, and tho size of a vessel seen at sea diminishes with the increase 

 of distance, as is shown in Fig. 288. Tho moon, on account of its proximity, 

 appears much larger than any of the stars or planets, although '\t is, in fact, 

 very much smaller. 



Fia. 289. 



- Q 



Let A B, Fig. 289, represent a planet, and C D the moon. The angle of 

 Tision which the planet A B makes with the eye at G, is evidently less than 

 the angle which the moon subtends at the same point. To a spectator at Gr, 

 therefore, A B, though much the larger body, will appear no larger than 

 E F; whereas the moon, C D, will appear as large as the hne C D. 



When will an "^l^. When ail object is so remote, or so 

 object appear gmaU that llncs drawn from its extremities 



as a mere ' 



point r form no appreciable angle at the eye, the ob- 



ject appears as a mere speck or point. 

 How »mnii an ^^^ ^^^j with an Ordinary amount of light, 

 •bject is visible can 866 an object which occupies in the field 



to the eye ? '' J- ^ 



of view a space of only the sixtieth of a de- 

 gree (or one minute). 



This space is about the 100th of an inch in a circle of twelve inches diameter, 

 the eye being supposed to be in the center of the circle. Now a body smaller 

 than this at six inches from the eye, or any thing, however large, placed so 

 iar from the eye as to occupy in the field of view less space than this, is invis- 



