THE SOLAR MICROSCOPE. 
By the interposition of such a medium, the object may be pre¬ 
vented from receiving any increased temperature whatever. 
It happens that water, which is the most convenient medium 
for this purpose, is very imperfectly pervious to heat, and is ren¬ 
dered almost completely athermanous by dissolving in it as much 
alum as it is capable of holding in solution. The object, therefore, 
is perfectly protected from the effects of heat, by placing between 
the slider and the condensing lens a cell, consisting of two parallel 
plates of glass, fixed at about an inch asunder, and filled with such 
a saturated solution of alum. The light intercepted by this is alto¬ 
gether inconsiderable, while the whole of the heat is stopped by it. 
6. The magnifying part of the solar microscope consists of an 
achromatic lens, or combination of lenses, of very short focal 
length; this being brought before the object, at a distance from it 
a little greater than its focal length, will produce a highly mag¬ 
nified optical image of the object, upon a screen placed at a proper 
distance before it. 
In the case of the magic lantern, it is not indispensable to incur 
the expense of achromatic lenses, and even the expedients to cor ¬ 
rect the spherical aberration are hut little attended to. The 
magnifying powers used in that instrument not being great, and 
the objects exhibited not requiring extreme accuracy of delinea¬ 
tion, the expense which would be incurred in producing large 
lenses free from the aberrations is not necessary. But in the case 
of microscopic objects, where great magnifying powers are applied, 
lenses in which the aberrations are not corrected would produce 
images so confused and indistinct as to be altogether useless. 
Achromatic combinations, therefore, in which the spherical aber¬ 
rations are also corrected, are in this case indispensable. 
As in the magic lantern, the same lenses may be applied, so as 
to produce different magnifying effects. If the distance of the 
lenses from the object were so great as twice their focal length, 
the image would be projected upon the screen at a distance in front 
of the lens also equal to twice its focal length, and would in that 
case be exactly equal to the object, and consequently there would 
be no amplification at all. As the lenses, however, are moved 
nearer to the object, the distance at which the image would be 
formed and its magnitude would be increased, and this increase 
would go on without practical limit, until the distance of the lens 
from the object would become equal to its focal length, in which 
case the image, having been enlarged beyond bounds, would alto¬ 
gether disappear. 
In practice, therefore, the focus of the lens is brought to such a 
distance from the object, that the image upon the screen shall 
have a magnitude sufficient for all the purposes of exhibition. It 
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