es 
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274 THE NORTHERN MICROSCOPIST. 

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

The Abbe Theory of Microscopical Vision. 
S a notable feature may be mentioned the greatly increased 
interest which has been awakened in the important contribu- 
tion to the theory of the Microscope, originated by our illustrious 
Fellow, Professor Abbe. Although those views are now several 
years old, and were brought before the Society so long ago as 1877 
by our then Treasurer, Mr. J. W. Stephenson, the recognition of 
the extraordinary nature of the experiments, was until lately con- 
fined to a very small circle. Both in this country and in Germany 
and America, however, the past year has seen a great extension in 
the number of those who have followed these experiments, and 
who have appreciated the important bearing which they have on 
microscopical vision. 
I have used the term “extraordinary” because I think that 
every one who has seen these experiments will readily agree that it 
is extraordinary, in every sense of the word, to find, that merely by 
excluding a greater or less number of the “ diffraction” images 
found at the back of the objective, a great variety of entirely 
different appearances are presented by one and the same object— 
lines at a known distance apart doubled and quadrupled,—or that 
objects in reality quite unlike can be made to seem identical— 
multi-sided figures giving images of squares. In short, the same 
objects may appear to be afferent in structure and different objects 
may seem to be 7zdentical, entirely according as their diffraction 
images are made dissimilar or similar by artificial appliances 
between the objective and eye-pieee. The appearance of particular 
structure can even be “ predicted” by the mathematician, before it 
has been actually seen by the microscopist. 
_ The result of these experiments is to show that a distinction 
must be drawn, between the vision of minute objects and what may 
be termed, for this purpose, “coarse” objects, z.¢., those which are 
considerable multiples of the wave-lengths. 
The latter are imaged by the Microscope, substantially in the 
same way as by the camera or the telescope, and their images cor- 
respond point for point with the object. We are therefore able to 
draw the same inferences as the actual nature of such objects, as 
in the case of ordinary vision, 
