694 SUMMARY OF CURRENT RESEARCHES. 



MICROSCOPY. 



A. Instruments, Accessories, &c* 

 CD Stands. 



Seibert's New Microscope for Crystallography and Petrography. 

 — In this large model (fig. 144), simplicity of construction, economy in 

 cost, and suitability of purpose, bave been especially kept in view. The 

 stand is hinged for inclination and clamped by a small lever. The 

 coarse adjustment is by rack-and-pinion, and the fine by a micrometer 

 screw, whose head is graduated for thickness measurements. The draw- 

 out tube has millimetre graduations. The circular object-stage (dia- 

 meter 105 mm.) is rotary, graduated, and provided with a vernier and 

 centering screws. The stage is marked with two perpendicular radii 

 for orienting the object, and the illumination is by plane and concave 

 mirrors. There are two analysers. One is applied above the ocular, 

 and has graduations and a vernier. The other can be inserted into the 

 tube directly over the objective ; the consequent change in the focal 

 length of the objective is compensated by a long focus lens on the top 

 of the prism, so that, after the insertion of the analyser, no new adjust- 

 ment of the objective is required. The prisms of the analysers have 

 right-angled end plaues, and give a very large field. The polariser is 

 placed in a small sleeve under the stage, and has a lever arrangement 

 for adjustment of height. The rotation of the nicol is controlled by 

 a screw working in a groove. Over the polariser is a doublet condenser 

 for axial images. A Bertrand lens can be used for magnifying the axial 

 images, and for this purpose is inserted from above into the tube as far 

 as the diaphragm of the draw-tube. The oculars are specially con- 

 structed for polarisation work. A screw engages in a notch in the tube, 

 so fixing the direction of the cross threads, and this direction is made 

 visible by a stroke on the outer rim of the mount. Other strokes mark 

 the angles of 45°. The illustration shows a spring attachment of the 

 objective. This arrangement, which is not peculiar to this particular 

 class of instrument, gives accurate adjustment combined with easy and 

 rapid facility of exchange. 



Beck's London Microscope. — Fig. 145 represents this instrument, 

 which was exhibited at the meeting of the Society held on October 15th. 

 It has inclining joint, first-class fine and coarse adjustments, a divided 

 draw-tube, removable mechanical stage, giving 2-in. horizontal and 

 1-in. vertical movements, with finder divisions, rack-and-pinion focus- 

 sing substage, ebonite top stage, double mirror, and is absolutely rigid. 

 The back leg of the base slides in, the side legs close up, the stage and 

 mechanical stage both remove and pack in the bottom of the leather case, 

 and the triple nose-piece swings round. 



* This subdivision contains (1) Stands; (2) Eye-pieces and Objectives; (3) Illu- 

 minating and other Apparatus; (4) Photomicrography; (5) Microscopical Optics 

 and Manipulation ; (6) Miscellaneous. 



