340 
tion. Further, the optical disadvantages of such 
a plan would be considerable, for the nearer to 
the principal focus of the lens the object is 
brought, the more obliquely will the rays fall 
upon its surface, and the greater, therefore, will 
be the errors of aberration. This method of 
augmenting the power of a microscope has 
been adopted in spite of these disadvantages,” 
but it is not found to answer. Nevertheless, it 
is capable of being made of great utility, as 
we shall presently show, to a limited extent. 
A much more generally convenient method of 
varying the power of the microscope 1s to 
employ, as object-glasses, lenses of different 
foci ; and thus, as the same distance between 
the image and the lens is constantly maintained, 
whilst that of the object varies, the number of 
times that the latter is amplified is changed in 
a like proportion. In whatever mode addi- 
tional amplification be obtained, two things 
must always result from the change; the por- 
tion of the surface of the object, of which an 
image can be formed, must be diminished, and 
the quantity of light spread over that image 
must be proportionably lessened. In the use 
of high magnifying powers, the compound 
microscope has the great advantage over the 
simple, that the object need not be brought to 
nearly the same proximity with the lens, and 
that much more of it can be seen with comfort 
at a time. The long focus and large ——— 
with which the eye-piece is usually made pre- 
vent even the prolonged use of the instrument 
from acting prejudicially on the visual powers, 
except in cases of peculiar tendency to nervous 
disorders of the eye. And as the power of the 
eye-piece as well as that of the object-glass 
can be raised, there are scarcely any limits to 
the magnifying power that may be obtained. 
Practically, however, there are limits, arising 
from the fact that, as the amplification is greater, 
the aberrations will be increased in even an 
augmented proportion; so that these com- 
pletely antagonise the benefit otherwise deri- 
vable from the employment of high powers. 
The aberrations can only be diminished by 
contracting the aperture of the object-glass ; 
and this renders the image so dark that no real 
advantage is gained. Moreover, the imperfec- 
tions necessary to the best compound micro- 
scope, in which ordinary lenses are employed, 
are further augmented by the slightest error in 
the centering of the lenses, so that their axes 
do not coincide. 
In addition to the two lenses of which the 
compound microscope has been stated essen- 
tially to consist, another is usually introduced 
between the object-glass and the image formed 
by it. The ordin urpose of this lens is to 
ange the course of the rays in such a manner 
that the image may be formed of dimensions 
not too great for the whole of it to come within 
the range of the eye-piece, and consequently 
to allow more of the object to be seen at once. 
* We have seen a microscope, constructed by 
Chevalier, in which the tube was capable of being 
drawn out to the length of between three and four 
feet! 
MICROSCOPE. 
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Section of Compound Microscope, with field-glass 
¥ introduced. bine 
A A, the image which would be formed by the — 
object-glass alone; B B, the — formed by — 
the interposition of the field-glass F F; the an 
of this image is within the range of vision of the 
eye-glass E E, and the field of view is therefore : 
increased. fee ; 
7 
Hence it is called the field-glass (fig. 163). 
It may be so adjusted, however, in regard — 
to the eye-glass, as to correct its errors mm 
almost a perfect d ; and it is now, therefore, — 
usually considered as belonging to the ocu- — 
lar end of the instrament,—the eye-glass and — 
the field-lens being together termed the eye- — 
piece. Various forms of this eye-piece have — 
