THE MICROSCOPE. 
small insect at the distance of a yard. The same object may he 
viewed at different distances, and will have different visual mag¬ 
nitudes, these magnitudes being greater as the distance is less. 
The visual diameter of a small object, seen from the distance of a 
yard, is three times less than when seen from the distance of a 
foot. It appears, therefore, that the “ visual magnitude of an 
object seen in the usual way with the naked eye,” is a term of 
comparison which, without some further condition to limit it, has 
no fixed meaning, and consequently leaves the “ magnifying 
power” of which it is made the standard, altogether vague and 
indefinite. 
65. The visual magnitude therefore which is made the standard 
of magnifying power, must be the visual magnitude at some 
arbitrary distance conventionally assumed. As we have already 
stated, it has been generally agreed, since micrography has taken 
the rank of a special branch of science, to adopt ten inches as the 
standard distance. This distance is recommended not merely on 
account of the arithmetical facility which arises out of its decimal 
character, but because it agrees sufficiently for all practical 
purposes with the standard derived from the measures of other 
countries. In France, for example, the standard usually adopted 
is twenty-five centimetres, which is equal to 9*427 inches, being 
less than ten inches by only about the sixth of an inch. 
According to this convention, then, the magnifying power of a 
microscope would be the number of times the visual diameter of 
the object viewed with the microscope is greater than its visual 
diameter viewed by an eye placed at ten inches from it. Thus, 
if the visual diameter of an object seen at the distance of ten 
inches be fifteen minutes of a degree, and the visual magnitude of 
the same object seen with a microscope be two and a half degrees, 
or 150 minutes, the magnifying power will be ten. 
But an objection will even still be raised. The object may be 
so small that at the distance of ten inches it would not be visible 
at all with the naked eye. Nay, it may be, and in the case of 
microscopic objects often is, so minute that it would not be per¬ 
ceptible to the naked eye at any distance, however small. In 
that case it maybe asked, What is to be understood by “ its visual 
magnitude at the distance of 10 inches ?” 
This point will require some explanation. There is a certain 
limit of magnitude within which an object will cease to make any 
sensible impression of its magnitude or form upon the eye. This 
minor limit of magnitude varies with different individuals, and, 
in the case of the same individual, with different objects according 
to their colour, illumination, the ground on which they are pro¬ 
jected, and many other conditions which it is not here necessary to 
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