ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 247 



object stage is completely rotatory around the optic axis, and has also 

 rectangular movements — one with slow micrometer adjustment, readable 

 to 0*01 mm., the other with quicker movement, readable to 0*1 mm. 

 The circular graduations are into 8G0° with a vernier. The rotatory 

 object-stage, by lifting the fixing screws at its sides, can be removed and 

 replaced by a vulcanite stage with a finding arrangement, and is likewise 

 graduated into 860°. The micrometer-screw has a vernier ; the third 

 Nicol is rotatory for about 90° in a collar within the tube ; there is a 

 Bertrand condenser. 



Reichert's Large Mineralogical Stand.* — This instrument, cata- 

 logued as No. 1 b, is shown in fig. 41, and is made with a rotatory 

 object-stage, divided into 360°, and crossed by two millimetre scales at 

 right angles for orientating known objects. The mirrors are hollow- 

 plane, and adjustable at various heights. The coarse-adjustment is by 

 rack-and-pinion, and the fine by a new delicate graduated micrometer- 

 screw. Both polariser and cylinder-diaphragm have a vertical rack 

 movement, and are fitted into a diaphragm-carrier of Abbe's complete 

 illuminating apparatus, in order to afford a rapid change from polarised 

 to unpolarised light. The analyser is placed above the ocular, and 

 is fitted with a graduated circle divided into 360°. It has also an 

 opening for the insertion of a quartz prism, and can be removed and 

 replaced without disturbing the ocular. The polarising Nicol is easily 

 rotatory, and the four quadrants of rotation are indicated by the click- 

 ing of a spring. The third Nicol, without any interference with the 

 adjustment of the instrument, can be applied as an analyser immediately 

 above the objective. The application of a pin ensures that the cross- 

 threads, ocular, and the graduated circle are always in connexion. The 

 screws c c' are for accurately centring the objective. Nicols with large 

 field of view, or quartz plates, can be inserted at Z. A condenser facili- 

 tates the observation of axial images of mineral sections. By drawing 

 out the lens L the rays through the objective can be changed from 

 parallel to divergent pencils ; the necessary draw-out adjustment of 

 ocular is then performed by the rack t. The iris on the Abbe condenser 

 receives the disks of calcite and mica. 



Reichert's Microscope for Determining Hardness of Substances. f 

 This instrument, which has been constructed from the designs of 

 J. A. Brinell, is shown in fig. 42. The principle of the method depends 

 upon measuring the area in square millimetres of the circular dent 

 produced in a substance when a superposed steel sphere is subjected to 

 a known pressure in kilograms. The ratio of pressure per square 

 millimetre gives the " hardness number " of the substance. The general 

 view of the instrument is given in fig. 42, and the chief parts are : — 

 (1) T (fig. 43) the tube forming a special Microscope, with cross-threads, 

 ocular and objective, working up to about 50-fold magnification ; (2) 

 M,i the object-stage, acting also as foot of the whole, with a pillar 

 carrying the rotatory upper parts ; (3) a horizontal arrangement of 

 parts — some fixed, some movable — serving for the lengthwise and 

 diagonal movements of the tube ; (4) a vernier for reading off the 



* C. Reichert (Vienna), Catalogue No. 25 (Mikroskopie, 1904) pp. 28-9, fig. 16. 

 + Tom. cit, p. 36, fig. 17e ; and Special Circular. 



