THE MICROSCOPE. 
this provision with a proper assortment of object-pieces and eye¬ 
pieces, all possible gradations of power between the highest 
attainable, and the lowest which is applicable, can be obtained. 
The actual magnitude of the space which can be presented at 
once to the view of the observer, will vary with the magnifying 
power; but in all cases it is extremely minute. Thus, with the 
lowest class of powers, where it is largest, it is a circular space, 
the diameter of which does not exceed the 8th or 10th of an inch; 
it follows, therefore, that no object, the extreme limits of whose 
linear magnitude exceed this, can be presented at once to the 
view of the observer. Such objects can only be seen in their 
ensemble, by means of less powerful magnifying glasses, or with 
the naked eye. 
22. The field of view, with powers from 100 to 300, varies in 
diameter from the loth to the 40th of an inch ; from 300 to 500 
it varies from the 40th to the 70th of an inch; and from 500 to 
700 from the 70th to the 100th of an inch. 
It will thus be understood, that even with the moderate power 
of 700, an object to be included wholly within the field of view, 
must have a magnitude such as may be 
included within a circle whose diameter 
does not exceed the 100th of an inch. 
These observations will be more clearly 
h appreciated by reference to the annexed 
diagram, fig. 12, where A is a circle 
jl whose diameter is the 6th of an inch; 
B one whose diameter is the 12th of an 
inch; c the 25th, d the 50th, and E the 
m 100th. 
But when still higher powers are used, 
the actual dimensions of the entire space. 
50 comprised within the field of view will be 
so very minute, that an object which 
_i- would fill it, and still more, smaller 
objects included within it, would not only 
* be altogether invisible to the naked eye, 
but would require considerable micro¬ 
scopic power to enable the observer to see them at all. 
The actual dimensions of the field of view, which correspond to 
each magnifying power, vary more or less in different instru¬ 
ments. Those which I have given above, are taken from a 
microscope made by Charles Chevalier, which is in my possession. 
The difference however in this respect, between one instrument 
and another, is not considerable, and the above will serve as a fair 
illustration of the limits of the field of instruments in general. 
26 
Fig. 12. 
