PROCEEDINGS OF THE SOCIETY. 205 



1. Stand to be substantial and secure, with ample space between 

 table and stage ; horizontal axis for tilting ; and instrument capable of 

 being used and standing firmly in the vertical, inclined, and horizontal 

 positions. 



2. Rotating Stage to be not less that 4|- in. diameter, graduated in 

 degrees, with vernier reading to 5'. Also fitted with central, detach- 

 able, perforated, spoon-shaped, spring clip with turned-up internal edges ; 

 or a pair of movable lateral slips, with dimpled ends. 



3. Mechanical Tube Length of 160 mm. with the following adjust- 

 ments :— 



i. Coarse Adjustment by diagonal rack and pinion, graduated 

 so that one interval corresponds to a complete turn of 

 the fine adjustment. 

 Fine Adjustment graduated up to 10 microns on circumfer- 

 ence of milled head ; the screw should be sufficiently 

 accurate throughout its acting length to enable measure- 

 ments to be made within the above limits, 

 ii. Clamp for fixing objectives to nose-piece, or alternatively 



the ordinary double nose-piece, 

 iii. The Necessary Centreing Screws to be fitted on nosepiece 

 only, and a slot provided between the centreing nose- 

 piece and the aperture of the analyser for the insertion, 

 of accessories, tt -45° to the cross wires. 



4. Sub-Stage Fittings and Accessories, Optical and Mechanical. 



i. Mirror to be plane and concave, in the usual gimbal fittings 

 so attached as to provide an adequate range of distance 

 between mirror and stage in view of the range of focussing 

 of the sub-stage and the presence of the polarizer, 

 ii. Condenser to be in one fitting with polarizer, but removable 

 either as a whole or in part ; the whole fitting to be 

 adjustable vertically, 

 iii. Polarizer to be carried in swing-out frame ; to be capable of 

 rotation on the microscope axis, with spring catch at zero 

 reading, the rotation to be graduated at degrees 0, 90, 

 180, and 270, and the plane of vibration in the zero 

 position to be left and right. 

 iv. A Slot for the insertion of accessories in a position at 45° to 

 the cross wires above the polarizer, and sufficiently below 

 the condenser to allow a slight lowering of the condenser 

 to bring an image of a diaphragm or other accessory to 

 coincide with the object. 

 h. Objectives 30 mm. and 6 mm., showing no optical evidence of 

 strain ; the 6 mm. objectives to be of not less than 0"8 N.A. Objective 

 barrels to be of such length as to give approximate focus on interchange. 

 The N.A. and equivalent focal length to be plainly indicated on each 

 objective. 



6. Analyser to slide in and oat of lower part of tiibe and to be 

 accurately crossed with the clicked polarizer. 



7. Ocular to be fitted with cross wires in the form of spider lines, or 

 ruled glass plate accessible for cleaning, slotted at 45" to cross wires 



