SUMMARY OF CURRENT RESEARCHES. 



701 



Fig. 150. 



connected with the stage, and the movable parts and tube depend on 

 the pillar. The disadvantage common to all these models is that the 

 micrometric movement, after a 

 time, gets worn, and the fine ad- 

 justment no longer works with 

 accuracy. A second disadvantage 

 is in the position of the micro- 

 meter screw, which ought to be 

 placed vertically below the stage. 

 This arrangement was actually 

 adopted by the Parisian firm of 

 Trecourt, Bouquet, and Ober- 

 hauser in 1830 ; and Oberhauser, 

 after his separation from his 

 partners, introduced the same 

 arrangement in all his instru- 

 ments between 1847 and 1857. 

 The only Continental firm which 

 makes larger stands with the 

 micrometer below the stage, as 

 the author thinks it should he, is 

 that of W. and H. Seibert, of 

 Wetzlar. 



Fig. 149 shows Seibert's large 

 model with three adjustments. 

 The coarser adjustment is by 

 rack-and-pinion ; the fine, with 

 parallelogram movement, is under 

 the stage ; the third adjustment 

 is an extremely slow motion for 

 delicate observations with the 

 highest powers. Fig. 150 shows 

 the two fine adjustments. The 

 least fine of these is a micro- 

 meter screw, and its action se- 

 curely moves the whole pillar 

 and tube on the foot ; its rotation 

 about ten times as coarse as 

 that of the third adjustment. 



(2) Eye-pieces and Objectives. 



Kreidl's New Stereoscopic Loup * (fig. 151).— The four totally re- 

 flecting planes m, m', n, n' of the four prisms P, P', p, p' act as mirrors, 

 by means of which the rays o c and o c', coming from the point o of the 

 object, are reflected after their passage through the achromatic magni- 

 fying lenses abc, a' be'. After two reflections these rays emerge 

 parallel or nearly so. An object at o, the common focus of the two 

 central rays, is magnified to the right eye in the direction A' o', and to 

 the left eye in the direction Ao. The angle enclosed by the two 



* Zeitschr. wise. Mikr., xviii. (1901) pp. 10-4 (1 fig.). 



