62 - Dr. North on the Binocular Microscope. 
of knowledge, and with as much accuracy and satisfaction as a 
view of a real object with both eyes. 
At first thought it seems strange that the stereoscope was not 
sooner invented. A full detail of the reasons why in the progress 
of the human mind this invention is so late, and how it is that 
we are able to do without it, would open up a wide field of inter 
esting considerations, and include the philosophical and aesthet- 
ical principles of the art of painting.* It will be sufficient to 
hint at these uses of vision by which the eye is the mind’s instru- 
ment for mathematical or strictly scientific perception and knowl 
edge. Such knowledge depends on measurement; solids are 
analyzed and measured by means of planes; the third dimension 
is in a plane at right angles to that of the length and breadth; it 
is length or breadth in another direction.’ In judging of length 
or breadth by the eye, we measure by a process of halving and 
repeated bisection, and make the eye a mathematical instrument; 
but in the case of an actual solid whose position we can change, 
if we would obtain the same accuracy of knowledge in regard to 
the third dimension, we must turn the object over until depth be- 
comes length or breadth. If the object cannot be turned over, 
the dimension which, as it lies, is that of depth, cannot be meas 
ured accurately by the eye, and cannot, so to speak, be mathemat 
ically perceived ; hence stereoscopic vision, either when it is natu 
ral, as being the simultaneous act of both eyes or as that obtained 
by successive focal changes of a single eye, or when it is eflecte 
by suitable optical instraments, cannot be what we call mathe- 
matical vision or that which is attended with measurement. — 
But optical instruments are useful not only for their assistance 
satisfaction and pleasure are also important considerations. This 
is especially true of the microscope, an instrument as essentially 
necessary and indispensable in science as the telescope, and equally 
accurate and reliable with it in all its revelations. 
Inexperienced persons are embarrassed, when judging of ob 
jects under the microscope, by their appearing as flat pictures; 
by an inability to pereeive depth, and by the care and slowness 
with which the mind must be employed when the focus ¥ 
discrimination 18 peel 
by a watch-maker, engraver, by any extremely accurate mechanic, or by a natura, 
. i i ployed. ta 
or other scientific man, but one eye Is employ Spectacles which are lenses of 
low power for both eyes, are used solely because an individual’s eyes do not see at 
certain average distances as well as mankind’s in general—never strictly § eaking 
for magnifying an object too small for good eyes. First the closest inspection by « 
ingle eye is employed and at the shortest working distance e eye; if the ob: 
ject be still too minute, a single hand lens, and never a pair of them, are resorted 1 
Attempts have been made io introduce watchmaker’s magnifiers in spectacle frames; 
yet they have not come into use. de bey copes pies 
: 
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