THE MICROSCOPE. 
have that visual angle, it would follow that the smallest object 
of which the form could be rendered distinctly visible by a 
microscope of a given magnifying power, would be one whose 
linear dimensions are as many times less than the 360th part of 
an inch as there are units in the number expressing the magnify¬ 
ing power. 
It must not be forgotten, however, in considering such points, 
that the smallest object whose form can be distinctly seen at a 
given distance without a glass, depends on many conditions, some 
connected with the object, and some with the observer, as has been 
already stated. 
Many persons fall into the error of supposing that the excel¬ 
lence of a microscope is to be determined by the greatness of its 
magnifying power. On the contrary, that instrument must be 
considered the most efficient which renders the details of an object 
perceptible with the lowest power. Distinctness of definition, 
by which is meant, the power of rendering all the minute linea¬ 
ments clearly seen, is a quality of greater importance than mere 
magnifying power. Indeed, without this quality, mere magnify¬ 
ing power ceases to have any value, since the object would appear 
merely as a huge misty silhouette. 
Sufficiency of illumination is another condition which it is 
difficult to combine with great magnifying power, but which is 
absolutely necessary for distinct vision. 
If two instruments show the same object with equal distinct¬ 
ness of definition and with sufficiency of illumination, one 
having a higher magnifying power than the other, then it must 
be admitted that the one which bears, with such conditions, the 
higher power is the more efficient instrument. 
The mere magnifying power depends on the focal length of 
the lenses, the illumination on the angle of aperture, and the 
distinctness of definition on the extent to which those condi¬ 
tions have been fulfilled which confer upon the combination of 
lenses composing the instrument, the qualities of aplanatism and 
achromatism. 
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