The President's Address. By Prof. P. Martin Duncan. 155 



of the rapid decrease of the depth of vision to which I have re- 

 ferred, the thickness of the objects which can be seen in rehef, 

 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 lOUO 

 times amounting only to a micro-millimetre. For medium and 

 high powers, therefore, the only objects suitable for the stereoscopic 

 binocular, are those which present, within a small 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 con- 

 siderations also show the importance of using the lowest power 

 sufficient to recognize the object. 



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 Microscope 

 acquires more and more the property of an optical microtome, which 

 presents to the observer's eye, sections of the object of a fineness and 

 sharpness that no instrument could produce by mechanical means." 



Another novel point was the demonstration of the very material 

 distinction between ordinary stereoscopic vision and that with the 

 Microscope. The perspective shortening of the lines and surfaces 

 by oblique projection, which is an important element of solid vision 

 with the naked eye, is wholly wanting in microscopical vision, in 

 which we have only the other element, a relative displacement of 

 successive layers in the image. That these displacements are seen 

 in the Microscope, depends entirely on the peculiar exaggeration in 

 the ampHfication of the depth of an object which is not found in 

 ordinary vision. 



The paper " On the Conditions of Orthoscopic and Pseudoscopic 

 Effects in the Binocular Microscope " is also a most useful contribu- 

 tion to the theory of micro-stereoscopic vision, establishing as it 

 does the true criteria for both classes of effects, and at the same 

 time clearing up a misconception that had arisen as to the supposed 

 necessity for the rays from the two halves of the objective crossing 

 in order to get proper orthoscopic effect. If the delineating pencils 

 have been reflected an even number of times in the same plane, the 

 rays must cross, but otherwise not. 



