The Binocular Microscope. By F. Jentzsch. 3 



(hiring all this period, no one seems to have recognized clearly the 

 great importance an instrument would have which, while being con- 

 structed for binocular use, should be definitely non-parallactic, 

 but whose function should be to present to the eyes two identical 

 or congruent images, and not two pictures which differ in their 

 perspective. On the contrary, one reads frequent complaints * that 

 a particular stereoscopic instrument is no good, or is even harmful, 

 because it only produces simple binocular images. In recent years 

 interest in this question, which had died out, seems to be reviving : 

 thus, J. Amannf expressed quite definite requirements three years 

 ago with regard to a purely binocular Microscope. The instru- 

 ment which is to be described here was constructed, as far as the 

 chief features are concerned, in the winter of 1909-10. In the 

 year 1912 it was entirely reconstructed. 



2. Geometrical and Physical Division of the Eays. 



The designs hitherto adopted are unsuitable for meeting the con- 

 ditions required. It may be noted that the chief advantage of obser- 

 vation by the binocular method only becomes specially apparent 

 with very high magnifying powers and arduous examinations, such 

 as are required for dark-ground illumination and ultra-microscopy. 

 It is just in these cases that -the earlier types fail. The Greenough 

 Binocular is known to be suitable only for very low apertures, up 

 to about • 15. For higher magnifications larger apertures are 

 necessary : these require, however, very short focal distances only 

 obtainable by using one objective, therefore the division of the 

 pencils of rays must be carried out above the objective. This can 

 be done geometrically or physically, either by conducting certain 

 groups of the rays, which leave the objective towards one eye and 

 the remaining groups to the other eye ; or by splitting each single 

 ray into two parts, thus reproducing two images. 



The geometrical division can be performed in very different 

 ways. The most obvious method is to divide the circle of the 

 objective into two semicircles by using reflecting prisms (45° 

 prism, J. L. Eiddell, 1852 ; 60° prism, Nachet, 1853; or by refrac- 

 tion (Wenham, 1860). Moreover, it has also been attempted (many 

 years ago by the firm of Leitz) to divide the opening into one or 

 more annular circles or into several rectilinear zones. "With all 

 these methods of construction, that is in every kind of geometrical 

 division, a reduction of the aperture takes place, and consequently 

 there must be a diminution in the resolving power of the instru- 

 ment. Moreover, all the .spherical and chromatic defects of the 

 objective become much more marked with these diaphragms. ( It 



* Proc. R.M.S., No. 1 (1878) p. 149. 



t Das Binokulare Mikroskop. Zeit. f. wiss. Mike., xxvii. (1910) pp. 488-93. 



B 2 



