PROFESSOR MARTIN DUNCAN’S ADDRESS. 335 

EXTRACT FROM 
PROF. MARTIN DUNCAN’S ADDRESS 
To THE MEMBERS OF THE RoyvaAL MICROSCOPICAL SOCIETY, 
FEBRUARY 8TH, 1882, 

( Continued from page 310.) 
Micro-Stereoscopic Vision. 
_ determination of the depth of vision (in monocular obser- 
vation) naturally throws great light also on the conditions for 
effective micro-stereoscopic vision. It is obviously only when an 
object can be completely seez in all three dimensions at one adjust- 
ment of the focus, that a true stereoscopic image of it can be 
obtained. So long as only a single layer of inappreciable depth is 
visible simultaneously with any distinctness, no stereoscopic appa- 
ratus, however perfect, can bring into view the form of the whole 
of the object. 
Now with low powers we have large visual depth, so that objects 
of considerable thickness can be seen as solids. By reason, how- 
ever, of the rapid decrease of the depth of vision to which I have 
referred, the thickness of the objects which can be seen in relief, 
rapidly and disproportionately decreases as the power is increased, 
so that only very thin objects are suitable with even the medium 
powers, the absolute depth, in the case of an object magnified 300 
times, not amounting to a hundredth of a millimetre. With still 
higher powers the images of solid objects (though the decrease in 
depth is no longer so irregular) necessarily approach more and 
more to simple plane sections, the absolute depth with a power of 
1000 times amounting only to a micro-millimetre. For medium 
and high powers, therefore, the only objects suitable for the stereo- 
scopic binocular, are those which present, within a smad/ depth, a 
sufficiently characteristic structure, that is, which have sufficient 
salient points for stereoscopic effect. We can, however, increase 
the depth of vision by using narrow illuminating pencils, and by 
mounting the objects in some highly refractive substance. The 
above considerations also show the importance of using the /owest 
power sufficient to recognize the object. j 
Whilst the reduction in depth limits effective stereoscopic 
observation, Professor Abbe properly points out that there is a 
compensating advantage in ordinary microscopic observation, in 
that as the depth-perspective becomes more flattened the images 
of different planes stand out from each other with still greater 
distinctness, so that “ with an increase of amplification the Micros- 
CC 
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