THE MICROSCOPE. 



B 

 Fig. 1. 



will therefore appear nearly twice as long as C D, being seen 

 under twice the angle, and in the same proportion for any 



greater or lesser difference 

 in distance. The angle in 

 question is called the angle 

 of vision, or the visual an- 

 gle. 



The angle of vision must, 

 however, not be confounded 

 with the angle of the pencil 

 of light by which an object 



is seen, and which is explained in Fig. 2. Here we have drawn 

 two arrows placed in relation to the eye as before, and from 

 the centre of each have drawn lines exhibiting the quantity 

 of light which each point will send into the eye at the respec- 

 tive distances. 



Now if E F represent the diameter of the pupil, the angle 

 E A F shows the size of the cone or pencil of light which enters 

 the eye from the point A, and in like manner the angle E B F 

 is that of the pencil emanating 

 from B, and entering the eye. 

 Then, since E A F is double E 

 B F, it is evident that A is seen 

 by four times the quantity of 

 light which could be received 

 from an equally illuminated 

 point at B; so that the nearer 

 body would appear brighter if 



it did not appear larger; but as its apparent area is increased 

 four times as well as its light, no difference in this respect is 

 discovered. But if we could find means to send into the eye a 

 larger pencil of light, as for instance that shown by the lines 

 G A H, without increasing the apparent size in the same pro- 

 portion, it is evident that we should obtain a benefit totally 

 distinct from that of increased magnitude, and one which is in 

 some cases of even more importance than size in developing the 

 structure of what we wish to examine. This, it will be here- 

 after shown, is sometimes done; for the present, we wish merely 

 to explain clearly the distinction between apparent magnitude, 



Fig. 2. 



