202 PKOCEEDINGS OF THE SOCIETY. 



Type III. — An instrument designed especially for research work, price 

 £25 to £35 ; the character of this to depend on the individual 

 requirements of the different workers, 



1. Stand. — With spiral rack and pinion coarse adjustment, and to 

 he equally stable in vertical or horizontal positions. The centre of mass 

 should be kept as low as possible consistent with freedom of access to 

 the sub-stage. 



2. TiiM. — Minimum diameter of body tube to be 50 mm. internally, 

 v^'ith double draw tube, so that either long or short tube objectives can 

 be used. Second draw tube to be 100 mm. long, and both draw tubes 

 to be graduated in mm. 



3. Fim Adjustment. — Lever type. Lateral milled heads on both 

 sides of stand, of small diameter, but with long milled surface. Fine 

 adjustment must be graduated. 



4. Stage. — Circular rotating, with centreing screws, and provided 

 with a clamp. The mechanical part should be graduated with verniers 

 and be detachable, and so fitted that when removed a plane surface is 

 left. As an alternative, it should be built in with separate and inde- 

 pendently operating milled heads. 



5. Objective Changing. — Should have sliding objective changers, 

 which should be provided with centreing screws. 



6. Sub-Stage. — Should have centreing screws. Rack and pinionio 

 be exceedingly well fitted and made. Sub-stage iris diaphragm should 

 be rotatable, and should have rack-work lateral motion. 



7. Condenser. — An achromatic immersion condenser which can be 

 used dry. 



8. Dark Ground Illuminator. — Type in which there are spherical 

 reflecting surfaces. Some device in upper surface which, without inter- 

 fering with the satisfactory working of the accessory, will indicate the 

 axial centre, 



9. Objectives. — The following battery of lenses should form the 

 foundation, which would, of course, need addition for the particular re- 

 search work engaged upon : 16 mm., 8 mm., 3 mm., or 2 mm., focal length. 

 Each objective should be engraved with the tube length, the N.A., and 

 the magnification at the distance for which the objective is corrected, 

 expressed by X ... . Mounting of immersion lenses to be distinctive, 

 preferably black. 



10. Oculars. — T^vo to form the basis of equipment, of 40 mm. and 

 25 mm. focal length. Each ocular should be engraved with the initial 

 magnification, the focal length in mm., and with a magnification 

 number arrived at by dividing 250 by that focal length, and expressed 

 by X ... , 



Geneeal Remaeks. 



The Committee desire to endorse the following observations : — - 

 The popularity of Continental models depends on — 



{a) The remarkably high and uniform quality of the lenses. 



