580 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



(2) Eye-pieces and Objectives. 



Hartwieh's New Ocular Micrometer.* — Prof. Hartwich, of Zurich, 

 has designed his instrument to facilitate measurements in those cases 



Fig. 107. 



where the general tint of the preparation renders difficult the perception 

 of the micrometer lines. The sliding part, instead of containing two 

 movable threads, has only one movable b ; the 

 second thread a being fixed over the zero of the 

 scale (fig. 108). The movable thread is worked by 

 an exterior micrometer screw (fig. 107). It cannot 

 be brought nearer to the fixed thread than the first 

 graduation, and thus the minimum reading is unity; 

 in the other direction it can be moved to the fullest 

 extent (50 divisions). The object must be adjusted 

 on the stage so that one boundary is under the 

 fixed thread ; b is then moved into position. The 

 eye-piece is then raised, if necessary, in order to 

 read the distance between the threads. In another form of the instru- 

 ment both threads are movable, and each is manipulated by a separate 

 micrometer screw. 



Hartwieh's new Micrometer Ocular with Fixed Object-Stage.j — 

 Messrs. Zulauff and Tschokke, the makers of Hartwieh's micrometer 

 above described, have adapted the instrument for the use of a Microscope 

 with fixed object-stage. The arrangement for a movable thread remains 

 unaltered. It is pushed forward by the screw a (fig. 109), and pushed 

 back by the spiral spring &, when the screw is reversed. Instead of a 



* Zeitschr. wiss. Mikr., xvii. (1900) pp. 156-S (2 figs.), 

 t Tom. cit.. pp. 432-5 (2 figs.). 



