2 Transactions of the Society. 



for now every maker aimed at the construction of a stereoscopic 

 Microscope. In fact, at that period a plethora of new types 

 appeared simultaneously, some of which produced pseudoscopic, 

 while some attained orthoscopic effects. These effects were ob- 

 tained in some cases by means of double Microscopes, in others 

 by the use of a single objective, in which case a geometrical or 

 physical division of the pencils of rays was produced. The history 

 of these different types, numbering about twenty altogether, which 

 appeared in the course of a very few decades, is admirably compiled 

 by M. von Kohr * in his work of reference, " The Binocular Instru- 

 ments." 



While the Continental workers did not take very kindly to 

 these types, the stands of English instruments were for a long 

 time regularly provided with binocular fittings. The type most 

 widely used was the one permitting the binocular arrangement to 

 be thrown out of use in order to change to the ordinary monocular 

 method of observation. This device could, however, be used only 

 for quite low-power systems, otherwise two images were obtained 

 differing widely in their coefficient of light intensity. With all 

 forms of this construction the quality of the image suffered more 

 or less deterioration, so that after the purely aesthetic pleasure 

 of seeing stereoscopic views had passed by, it was realized even 

 in England that for scientific investigations the monocular Micro « 

 scope was always superior to these types. Later on, in Germany, 

 E. Abbe f devised an arrangement with his stereoscopic eye-piece, 

 which threw all previous types into the shade. Nevertheless, 

 this eye-piece appears even to this day to enjoy a very restricted use. 

 It is well known there has been in existence since 1897 a perfect 

 type of instrument for low magnifications, namely, Greenough's 

 Microscope. 



As a matter of fact, in view of optical law, the importance of 

 viewing stereoscopically diminishes in the same ratio as the magnifi- 

 cations and apertures in use are increased. Already, with medium 

 magnifications and apertures, the penetrating power reaches values, 

 so far as they can be calculated from purely dioptric data, which 

 approach the limits of resolution of the Microscope, so that no 

 further information of any importance bearing on the spatial 

 structure of the specimen can be obtained. Taking physiological 

 and psychological effects into consideration, the foregoing results 

 will differ widely, which will be explained later. Many micro- 

 scopists, especially in England, retained the binocular construc- 

 tion even with higher magnifications, in order to be able to use 

 both e}'es, as this was said to be less tiring. In spite of this, 



* The Binocular Instruments. Berlin, 1907. 



t Description of a New Stereoscopic Eye-piece, with general remarks on the 

 conditions governing Micro -stereoscopic Observation. Kais. Zeit. f.Mikr.,ii. (1880) 

 pp. 207-34 ; see also Ges. Abhand., i. pp. 244-62. 



