ILLUMINATION OF OBJECTS. 
green, showing faint traces of a certain reticulated skeleton of 
vegetable fibre. If it be held up before the sun, all light being 
excluded from the side presented to the eye, it will appear with a 
much paler tint of green, and the skeleton will become much 
more visible, the finer parts before invisible being distinctly 
seen. 
A stained glass-window viewed from the outside appears to 
have dark and dull colours, and might be taken to be opaque, 
showing no form or design. Viewed from the inside, forms of 
great beauty, and colours of remarkable splendour, are seen. 
When we say, therefore, that objects viewed in a microscope 
present very different appearances, according as they are illu¬ 
minated by a front or a back light, we only state a general fact 
eommon to all visible objects. 
No body can be said to be either opaque or transparent in an 
absolute sense. Bodies considered to be the most opaque, such 
as the metals, are found to be translucent when reduced to thin 
leaves. Even gold and platinum, the most dense of the metals, 
are rendered translucent under the hammer of the gold-beater, 
while glass, diamond, air, water, and similar bodies, commonly 
•considered to be transparent, are proved to absorb a portion of the 
light transmitted through them, this absorption increasing with 
the thickness of the medium. There is in fine no body which 
will not become opaque if sufficiently thick, and none that will 
not become more or less translucent if sufficiently thin. 
45. Since microscopic objects are generally of extremely 
minute dimensions, they are all, with some few exceptions, suf¬ 
ficiently translucent to be rendered visible by a back light. 
It is well known that many bodies, which are opaque or nearly 
so, may be rendered translucent by saturating them with certain 
liquids. Thus, as every one knows, paper, linen, and other 
porous bodies, which when dry are imperfectly translucent, 
become much more so when wetted or oiled, or saturated with 
white wax. 
This general physical fact has special and important application 
in the preparation of microscopic objects, which are saturated 
with various liquids, proper for each of them, by which they are 
rendered translucent. 
When a translucent object is rendered visible by a back light, 
the intensity of the light must be regulated according to its 
translucency. The more translucent it is, the less intense must 
be the light. A strong back light thrown upon a very trans¬ 
lucent object drowns it, and renders it altogether invisible. The 
light must therefore be reduced in intensity by varying the incli¬ 
nation of the reflector, the distance of the lamp from it, and by 
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