MAGNIFYING POWER. 
THE MAGNIFYING POWER. 
63. It has been well said, that a question clearly put is half 
resolved. There is no term in microscopic nomenclature so- 
familiar to the ear, and so flippant on the tongue, as the 44 magni¬ 
fying poweryet there is none respecting which there prevail 
so much confusion and obscurity. The chief cause of this is the- 
neglect of a clear and distinct definition of the term. 
It has been already shown, that the magnitudes observed with 
the microscope are visual, not real. We can say that such or- 
such an object seen in the microscope has a magnitude of so many 
degrees, but not at all one of so many inches. Strictly speaking, 
the same is true of all objects seen in the ordinary way; but in 
that case the mind is habituated to form an estimate of their real 
magnitudes, by combining the consideration of their apparent 
magnitudes with their distances. It is true that we are uncon¬ 
scious of the mental operation from which such estimates result, 
but it is not the less real. Our unconsciousness of it arises from 
the force of habit, and the great quickness of the acts of the 
mind. Every one who has been familiar with intellectual pheno¬ 
mena knows that such unconsciousness is found to attend all such 
acts as are thus habitual and rapid. 
64. But when objects are looked at in a microscope, the mind 
not only does not possess the necessary data to form such an 
estimate, but the conditions under which the visual perceptions 
are formed are so unusual, and, so to speak, unnatural, that it is 
incapacitated to form an approximate estimate even of the visual, 
to say nothing of the real, magnitude of the object of its- 
perception. 
The visual magnitude of an object, as seen in a microscope, is 
the angle of divergence of lines supposed to be drawn from the 
eye to the limits of the imaginary image formed by the eye-glass, 
which is the immediate object of perception. When we say, 
therefore, that the instrument has such or such a magnifying 
power, every one will comprehend that it is meant that this visual 
magnitude is so many times greater than the visual magnitude 
which the object would have, if it were seen in the usual way 
without the interposition of any optical expedient. 
So far all is clear, and so far there can be no difference of 
opinion on the point, provided only that the latter member of the 
sentence be clearly defined. What is the 4 4 visual magnitude- 
seen in the usual tv ay ? ” There are many ways of looking at an 
object, and 44 the usual way” depends much on the magnitude 
of the object. We can see well enough the dome of St. PauFs 
Cathedral at the distance of half a mile, while we cannot see a 
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