1882.J 



MICROSCOPICAL jour:n"al. 



Ill 



naturally throws great light also on 

 the conditions for effective-micro- 

 stereoscopic vision. It is obviously 

 only when an object can be complete- 

 ly seen in all three dimensions at one 

 adjustment of the focus, that a true 

 stereoscopic image of it can be ob- 

 tained. So long as only a single layer 

 of inappreciable depth is visible simul- 

 taneously with any distinctness, no 

 stereoscopic apparatus, however per- 

 fect, 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 con- 

 siderable thickness can be seen as 

 solids. By reason, however, of the 

 rapid decrease of the depth of vision 

 to which I have referred, the thick- 

 ness of the objects which can be seen 

 in relief, rapidly and disproportion- 

 ately decreases as the power is in- 

 creased, so that only very thin ob- 

 jects 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 ap- 

 proach more and more to simple 

 plane sections, the absolute depth 

 with a power of 1,000 times amount- 

 ing only to a micromillimetre. 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 considerations also show 

 the importance of using the lowest 

 power sufficient to recognize the 

 object. 



Whilst the reduction in depth lim- 

 its effective stereoscopic observation, 

 Professor Abbe properly points out 

 that there is a compensating advan- 



tage in ordinary microscopic observa- 

 tion, in that as the depth -perspective 

 becomes more flattened the images 

 of different planes stand out from 

 each other with still greater distinct- 

 ness, 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 ob- 

 ject, of a fineness and sharpness that 

 no instrument could produce by me- 

 chanical means." 



Another novel point was the de- 

 monstration of the very material dis- 

 tinction between ordinary stereosco- 

 pic vision and that with the micro- 

 scope. 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 mi- 

 croscopical vision, in which we have 

 only the other element, a relative dis- 

 placement of successive layers in the 

 image. That these displacements 

 are seen in the microscope, depends 

 entirely on the peculiar exaggeration 

 in the amplification of the depth of 

 an object which is not found in ordi- 

 nary vision. 



The paper " On the Conditions of 

 Orthoscopic and Pseudoscopic Effects 

 in the Binocular Microscope" is also 

 a most useful contribution 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 misconcep- 

 tion that had arisen as to the sup- 

 posed necessity for the rays from the 

 two halves of the objective crossing 

 in order to get proper orthoscopic ef- 

 fect. If the delineating pencils have 

 been reflected an even number of 

 times in the same plane, the rays 

 must cross, but otherwise not. 

 ( To be continued. ) 



o 



The Preparation of Diatoms. 



BY R. S. WARREN, M. D. 



Directions for the preparation of 

 diatoms are rather meagre in books. 



