THE MICROSCOPE. 
u Illumination, by cups or silver specula, does not produce these 
illusions, because they create no shade—the whole object is one 
mass of intense light; other false perceptions are, however, occa¬ 
sioned by them. Thus, all globular bodies, having polished sur¬ 
faces, reflect an image of the cups, and the spout, if there is one, 
appears as a dark spot in the centre. The eyes of insects, illumi¬ 
nated in this way, show the semblance of a pupil in the centre of 
each lens, which deception may be verified by examining small 
globules of mercury in the same manner. Spherical bodies, with 
bright surfaces, will even, on some occasions, reflect an image of 
the object-glass and its setting, on the same principle ; so that 
we must perpetually consider the laws of the refraction and reflec¬ 
tion of light, in all the conclusions we draw from the evidence 
even of the very best instruments, used with every possible 
precaution. 
“ Lastly, it must be observed, that in using microscopes, we 
must never attempt to verify an object concerning which we are 
uncertain, by increasing the depth of the eye-glass immoderately, 
so as in this way to obtain a very high power. A negative eye¬ 
glass, of about one-fourth of an inch focus, is the deepest which 
should ever be employed, even with a short body; for a microscope 
only shows a picture of an object, and the more it is amplified the 
more its imperfections are developed. It is, on this account, 
much safer to trust to moderate powers in these instruments, in 
preference to high ones, unless they are obtained through the 
medium of the depth and power of their objective part . It is the 
nature of deep eye-pieces to cause all luminous points to swell out 
into discs, and to render the image soft, diluted, and nebulous, 
at length all certain vision fades away, and the imagination is 
left to its uncontrolled operation. Single and compound magni¬ 
fiers, having to deal with the real object, may be made of any 
power which can be used; and if our eyes are strong, and habi¬ 
tuated to their use, we may place great reliance on their testi¬ 
mony ; but we must never allow them to persuade us to believe 
marvels which are manifestly impossible, or contrary to the known 
laws of nature and right reason.” 
MEASUREMENT OF OBJECTS. 
54. The determination of the real magnitude of microscopic 
objects, and that of the magnifying power of the instrument, are 
problems closely connected but not identical. Either may be 
solved independently of the other. 
* 55. If two objects be placed at the same time within the field 
of view, the real magnitude of one of which is known, that of the 
other may be at least approximately estimated by comparison. 
50 
