640 SUMMARY OF CURRENT RESEARCHES RELATING TO 



if required, be completely removed. In order to connect a preparation 

 in the central part of the object-stage circle with the peripheral ring and 

 vernier, a transparent plate is firmly attached to the latter, and covers 

 over the divided circle. In ordinary cases this transparent plate would 

 be of glass, but, for heated objects, it is replaced by one of mica. The 

 object to be viewed is set in a special clamp K, to be secured to the 

 rod S. It may easily happen that the clamp holding the preparation 

 may press the mica plate hard on to the divided circle ; but, although 

 this difficulty could have been easily met, the author considers that with 

 heating arrangements it is advantageous that rotation of the Nicol 

 should take place under a tight grip, as it were, of the preparation. The 

 projecting part of the object-stage not only carries the rod S, but secures 

 that the latter shall not, in its rotation, interfere with the mirror. 



Hirschwald's New Microscope Model and Planimeter-Ocular.* — 

 This instrument is made by R. Fuess, of Steglitz, Berlin, and is shown 

 in fig. 144. An essential difference between this new model and 

 Microscopes hitherto made with combined Nicol-rotation consists in that 

 the Nicol rotated is not an analyser placed over the ocular, but that an 

 analyser inserted at N in the tube is rotated at the same time with the 

 polariser. A disadvantage of the ordinary ocular-analyser clearly is that 

 the field of view (i.e. the focal distance of the ocular used) is pushed 

 back on account of the lengthened eye-distance of the Nicol ; this 

 results in a more or less intense diminution of the field. But the new 

 construction allows the rotation of the inner analyser only (the analogue 

 of the ordinary ocular-analyser) relative either to the stationary polariser 

 P, or to the preparation, stationary or rotatory, on the object stage T. 

 In both cases the ocular and analyser rotate and the ocular-threads mark 

 the rotation-directions of the Nicol. The design also permits of two 

 other controlling movements, less frequently required : the analyser may 

 move relatively to the stationary polariser and ocular ; or, the polariser 

 may move relatively to the analyser and ocular. For these combinations 

 the requisite arrangements are as follows : — 



(a) Polariser, Analyser, and Ocular rotate in unison. The screw- 

 head b, under the stage T, is loosened. A connecting screw is inserted 

 in the large ocular rim T x . The arm o, appended to the vernier arm s 

 and rotatory about a hinge, is unlocked. For orientating the ocular 

 there are two lines scored on the ocular mount-collar T x . 



(b) Polariser remains stationary, Analyser and Ocular rotate. To 

 carry out this movement the Nicol circle T x must be set at zero, the 

 screw-head b under the stage is tightened, and the screw on the Nicol 

 circle T 1 loosened. The arm o is unlocked. Rotation takes place on the 

 rim of the Nicol circle T x . 



(c) Polariser and Analyser rotate in unison and the Ocular remains 

 stationary. The arrangement is the same as for a, but the arm o is 

 locked over the projecting screw on the ocular. 



(d) Polariser and Ocular remain stationary and Analyser rotates. 

 The arrangement is the same as for b, but the arm o is locked up as 

 in last. 



* Zentralbl. f. Mineral, 1904. p. 626; Zeit. f. Instrumentenk., xxiv. (1904) pp. 

 367-8 (2 figs.). 



