MAGNIFYING APPARATUS. 
is not desirable, however, in any case, to push the amplifying 
power of the instrument too far, because the illumination of the 
image in that case becomes inconveniently faint; and if there be 
any causes of aberration uncorrected in the lenses, whether sphe- 
rical or chromatic, their effects will be rendered more apparent. 
7 . In the mounting of the instrument, provisions are necessary 
for varying, within certain limits, the distance of the object, as 
well from the illuminating as from the amplifying lenses. If the 
object be very minute, it is necessary that it should be illuminated 
with proportionate intensity; and, therefore, that it should be 
moved very near to the focus of the illuminating lens, c c. If it be 
larger, this position would, however, be unsuitable, inasmuch as 
the light would be collected upon a small part of it, to the exclu¬ 
sion of the remainder. In that case, therefore, the object must be 
brought farther in advance of the focus, F, of the illuminating lens, 
so as to intersect the cone at a point of greater section, and thus 
to receive a light which, though less intense, will be diffused 
over its entire surface. 
The amplification required will be greater in proportion as the 
object is smaller. For very minute objects, therefore, the ampli¬ 
fying lens must be brought nearer to the object, and the screen 
must be removed farther from it, while for larger objects, the 
arrangement would be the reverse. 
8. All that has been said on the subject of the screen in the 
case of the magic lantern will be applicable to the solar micro¬ 
scope, except that, in this case, the method of showing the object 
through a transparent screen is objectionable, because of the light 
which is lost by it, and for other reasons; and, besides, it is 
useless, that method of exhibition being adapted only for phan¬ 
tasmagoria, and other similar subjects of amusement. 
9. In what has been explained above, it has been assumed that 
a beam of solar light is thrown upon the condensing lens c c, in 
the direction of its axis. Now it is evident that it could never 
happen that the natural direction of the sun’s rays would coincide 
with that of the axis of the tube of the microscope ; for, that axis 
being necessarily horizontal, or nearly so, the sun to throw its rays 
parallel to it should be in the horizon. Some expedient, there¬ 
fore, is necessary, by which the direction of a sunbeam can be 
changed at will, and thrown along the axis of the tube. 
The obvious method of accomplishing this is by means of a plate 
of common looking-glass; such a plate being conveniently mounted 
in front of the condensing lens, may always have such a position 
given to it that it will reflect the sunbeam which will fall upon it 
in the direction of the axis of the tube. 
But since, by reason of its diurnal motion, the sun changes its 
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