THE MICROSCOPE. 
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, who is familiar with the appearance of the 
full moon, will he 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 2oth; 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, 3|-'. 
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 
Fig. 32. 
