THE MICROSCOPE. 



Fig. 32. 



is just half a degree, which means, that the angular divergence of 

 lines drawn from the eye to the extremities of the diameter is the 

 same as that of two lines drawn from the centre of a circle to the 

 extremities of an arc, which is the 720th part of the entire circle. 

 Every one, therefore, wh$ is familiar with the appearance of the 

 full moon, will be as familiar with the meaning of a visual angle 

 -of half a degree, and, consequently, of a degree as they are with 

 the real magnitude of an inch or a foot. 



The distance of the moon has been ascertained to be 120 times 

 its own diameter, and it is evident that any circular disc whatever, 

 whose distance from the eye is 120 times its own diameter, will 

 have a visual angle equal to the diameter of the moon, and there- 

 fore to half a degree ; and, consequently, one whose distance is sixty * 

 times its own diameter, would have a visual angle of a degree. 



Thus, in fig. 32, there are five white discs shown upon a black 

 ground : the diameter of the first is the 6th of an inch ; that of 

 the second, the 12th ; that of the third, 

 the 25th; the fourth, the 50th; and 

 the fifth, the 100th. If these be held at 

 ten inches from the eye, the first disc, 

 A, will have a visual angle of 1 ; the 

 second, B, one of 30' ; the third, c, 

 about 15' ; the fourth, D, 7|' ; and, in 

 fine, the fifth, E, 3f '. 



It follows, therefore, that an object 

 which when viewed with a magnifying 

 power of 1000, appears with the same 

 visual diameter as the moon, or as the 

 disc B, fig. 32, placed at 10 inches from 

 the eye, must have a real diameter no 

 greater than the 12000th part of an 

 inch. 



Having familiarised himself with 

 some such standards of visual magnitude 

 -as these, and once knowing the magnifying power of his instru- 

 ment, an observer can easily make a rough estimate of the real 

 magnitudes of the objects under view. 



67. But for this, as well as many other purposes of microscopic 

 research, it is necessary that the actual magnifying power of the 

 instrument be ascertained. 



The most simple and direct means of accomplishing this are 

 supplied by the camera lucida. 



* More strictly 57*3 times ; but the round number will be sufficient for 

 the above illustration. 

 62 



