PRINCIPLE OF THE INSTRUMENT. 
o from it: this image will be inverted with relation to the object; 
its top corresponding with the bottom, and its right with the left 
side of the object, and vice versa: the linear magnitude of this 
image will be greater than that of the object, in the proportion of 
o L to o l, and consequently its superficial magnitude will be 
greater than that of the object, in the proportion of the squares of 
these lines. 
The image o o, thus formed, may be considered as an object 
viewed by the observer, through the magnifying glass e e, and 
all that has been explained, relating to the effect of such a lens, in 
our Tracts on “ Magnifying Glasses ” and “ Optical Images,” will 
be applicable in this case. The observer will adjust the eye-glass 
e e, at such a distance from o o, as will enable him to see the 
image most distinctly, and the impression produced upon his sense 
of vision will be that the image he looks at, is at that distance 
from his eye, at which he would see such an object most dis¬ 
tinctly without the interposition of any magnifying lens; let 
this distance be that of a similar image o' o', and the impression 
will be that the object he beholds has the magnitude o' o'. 
The distance of most distinct vision with the naked eye, and 
the distance from the image at which the eye-glass must be 
placed to produce distinct vision, both vary for different eyes, but 
they vary almost exactly in the same proportion, so that the 
magnifying effect of the eye-glass upon the image o o, will be the 
same, whether the observer be long-sighted or short-sighted; in 
estimating the magnifying power, therefore, of such a combina¬ 
tion, we may consider, in all cases, the distance of the eye-glass 
E E from the image o o, to be equal to its focal length, and the 
distance of o' o' from the eye-glass, to be 10 inches. (See 
“ Magnifying Glasses,” 8.) 
To estimate the entire amplifying effect of such a microscope, we 
have only to multiply the magnifying power of the object-glass 
by that of the eye-glass ; thus, for example, if the distance of the 
image o o from the object-glass be 10 times as great as the 
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