252 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



(2) Eye-pieces and Objectives. 



Edinger's Pointer-Double-Ocular.* — In spite of the advantages (if 

 the pointer-ocular, L. Edinger has long felt that an eye-piece which 

 would allow of simultaneous observation by two persons, and which would 

 at the same time preserve the principle of the pointer, was a great deside- 

 ratum. The applicability of such an auxiliary to teaching purposes 

 needs no demonstration. The apparatus shown in fig. 84 has been made 

 by the Leitz firm to the designs of C. Metz, and is found to answer its 

 purpose completely. The name Pointer-double-ocular (Zeigerdoppel- 

 ocular) has been given to it. Between the collective-lens and the eye- 

 lens of an ordinary ocular and exactly over the ocular diaphragm a 

 double prism, i. ii., is inserted. Prism i. is an isosceles prism of angles 

 35°, 35°, 110°. Prism ii. is right-angled, with angles 35°, 55° and 90°. 

 The prisms are placed so that their larger sides (i.e. those opposite 110° 

 and 90°) are in juxtaposition, and in such a manner that they are 

 separated by an exceedingly thin layer of air. At this air-space, inclined 

 at 35° to the optic axis, a partial reflexion of the light-beam takes place. 



Fig. 34. 



About two-thirds of the beam passes on and about one-third is reflected. 

 The image formed in the main optic axis is therefore somewhat brighter 

 than that formed by reflexion. The images formed are of equal value, 

 because in both the full aperture of the objective is utilized. The axis 

 of the reflected image forms with the axis of the Microscope an angle 

 of 70°. Lateral observation would be quite possible if an eye-lens, the 

 same as for the vertical observation, were applied at a suitable distance. 

 But this would bring the lateral observer too near to the tube and to 

 the first observer. Therefore a lens combination is applied somewhat 

 similar to a terrestrial ocular. The tube of this telescope has a down- 

 wards inclination which imparts some difficulty to its use until the 

 observer's head has found the right position. But the Leitz firm also 

 deliver the apparatus with an upwards-inclined tube, and for this purpose 

 prism iii. is introduced. This, however, has the effect of weakening the 

 light, and the author prefers the first method. Adjustment is made at 

 the vertical eye-lens, and this is simultaneously effected through the bent 

 tube at the second eye -lens. If the second observer's eyes are not normal 



* Zeitschr. wise. Mikrosk., xxvii.(1910);pp. 336-8 (1 fig.). 



