THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



YOL. I. 



New Yokk, November, 1880. 



No. 11. 



A New Stereoscopic Ocular. 



In a long and very instructive 

 article in tiie Zeitschrift fur Mi- 

 hroskopie^ Dr. E. Abbe has de- 

 scribed a new binocular arrange- 

 ment which he has devised, and 

 which seems to possess several im- 

 portant advantages over those in 

 use at the present time. It may be 

 ])riefly described, with the aid of 

 the illustration, as follows, omitting 

 any special reference to the me- 

 (ihanical arrangements for setting 



and adjusting the prisms : a and h 

 Fig. 27 are two prisms so combined 

 that they form a thick, plain plate 

 of glass, the continuity of which is 

 interrupted by a very thin stratum 

 of air between the prisms, inclined 

 to the axis at an angle of 38°.5. 

 The thickness of the layer of air is 



less than 0.01""". The rays from 

 the objective, when they reach the 

 air, are partly transmitted and part- 

 ly reflected. The transmitted rays 

 pass on through tlie prism «, and 

 fonn an image of the object in the 

 ocular B. The reflected beam, as 

 shown in the flgure, emerges from 

 the lateral face of the prism 5 at an 

 angle of 13° from the horizontal, 

 and suffers total reflection from the 

 face of prism h into the ocular B' ^ 

 the axis of which is inclined about 

 13° to the axis of the primary tube. 

 The distance of the two oculars 

 from each other is adjusted l)y 

 means of the screw at Z>, or by 

 drawing out the oculars them- 

 selves. 



The oculai*s are of the ordinary 

 two-system kind ; but on account 

 of the different distances which the 

 direct and the reflected rays re- 

 spectively have to travel to reach 

 the eye, the difference in the mag- 

 nification thus produced is corrected 

 by using oculars of different con- 

 structions. 



The most usual form of binocular 

 microscope is restricted in its appli- 

 cation to rather small magnifica- 

 tions; Tolles' and Stephenson's 

 binoculars, however, are not sub- 

 ject to this objection, but the 

 former greatly increases tlie length 

 of the tube, or, when this defect is 

 corrected, the optical action of the 

 objective is deranged because it is 

 then used with a shorter tube 

 than it was intended for. Stephen- 



