EYE-PIECES. 
lenses, haying their plane sides turned towards the eye ; they are 
set in opposite ends of a brass tube, varying in length from two 
inches downwards, according to their focal lengths, the distance 
between them and, consequently, the length of the tube being 
always equal to half the sum of their focal lengths. 
The higher the power of the eye-piece, and consequently the 
shorter the focal length of the eye-glass, the less will be the length 
of the tube in which they are set. 
This tube is called the eye-piece. 
It will be apparent from what has been explained, that the 
magnifying power of the instrument will depend conjointly on 
those of the object-piece and eye-piece. 
21. In the prosecution of microscopic researches, the use of 
very various magnifying powers is indispensable; the higher 
powers would be as useless for the larger class of objects, as the 
lower ones for the smaller. But even for the same object, a 
complete analysis cannot be accomplished without the successive 
application of low and high powers: by low powers the observer 
is presented with a comprehensive view of the entire form and 
outline of the object under examination, just as an aeronaut who 
ascends to a certain altitude in the atmosphere obtains a general 
view of the country, which would be altogether unattainable 
upon the level of the ground. By applying successively higher 
powers, as has been already explained, the smaller parts and 
minuter features of the object are gradually disclosed to view, 
just as the aeronaut, in gradually descending from his greatest 
altitude, obtains a view of objects which were first lost in the 
distance, but at the same time loses, by too great proximity, the 
general outline. 
The microscope-makers, therefore, supply in all cases an 
assortment of powers, varying from 30 or 40 upwards ; observa¬ 
tions requiring powers under 40, being more conveniently made 
with simple microscopes. For this purpose it is usual, with the 
best instruments, to furnish six or eight object-pieces and three 
or four eye-pieces, each eye-piece being capable of being combined 
with each object-piece. The number of powers thus supplied will 
be equal to the product of the number of object-pieces, multiplied 
by the number of eve-pieces. 
The powers, however, may still be further varied, by provi¬ 
sions for changing the distance between the object and eye-pieces, 
within certain limits. For this purpose, the tube of the instru¬ 
ment is sometimes divided into two, one of which moves within 
the other, like the tube of a telescope, the motion being produced 
by a fine rack and pinion: in this case the eye-piece is inserted in 
one of the tubes, and the object-piece in the other. By combining 
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