THE MICROSCOPE. 
viewed by the naked eyes, as well as in microscopes, especially if 
but one eye is employed. Thus, if we look intently for some 
time at a basso-relievo (a die of a coin, for example), illuminated 
with very oblique light , it at first appears in its true character ; 
but, after a little while, some point on which we more particularly 
direct our gaze will begin to appear in alt , the whole rapidly 
follows; in a little time the effect w r ears off', and w T e again see it 
in bas-relief; then again in alt; and so on, by successive fits. 
This deception arises from the simple circumstance that the lights 
and shades in bas-relief are very nearly like those of an alto- 
relievo of the same subject , illuminated from the opposite side ; 
our understanding in this case instantly corrects the false testi¬ 
mony of the eye, when we consider from which side the light 
comes . (If we observe with a microscope, we must remember that 
its image is inverted, and that in consequence the light must be 
considered as proceeding from the side of the field of view oppo¬ 
site to that where the source of illumination actually exists.) It 
will also be highly advisable, when we are in doubt as to the 
manner in which an instrument shows prominences and depres¬ 
sions, to verify its vision by observing some known object with it, 
of the real state of which, as to inequality of surface, we have 
been previously informed by the sense of touch, to which it has 
been well said there is no fellow.” * 
* “We usually see objects illuminated from above with the shadou's below 
the prominences; now, unless the light is below an opaque object, when 
we view it in an engiscope, we shall see the shadows above , giving the 
prominences the appearance of depressions, and producing a very unnatural 
effect.” 
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