The President's Address. By Prof. P. Martin Duncan. 147 



so far as I can gather from a reference to the printed records of the 

 Society, has so much hght been thrown upon subjects which are of 

 the first importance in microscopy. 



I propose, then, briefly to recall the evidences of progress in 

 this part of our science, which the past year affords. 



The Able Theory of Microscopical Vision. 



As a notable feature may be mentioned the greatly increased 

 interest which has been awakened in the important contribution 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 extra- 

 ordinary nature of the experiments, was until lately confined 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 " diSraction " 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 different in structure and different objects 

 may seem to be identical, entirely according as their difi'raction 

 images are made dissimilar or similar by artificial appliances 

 between the objective and eye-piece. 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, i. e. 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 to the actual nature of such objects, as 

 in the case of ordinary vision. 



Minute objects, or parts of objects, only a few multiples of the 

 wave-lengths, are, however, imaged in an entirely different way, viz. 



L 2 



