206 PKOCEEDINGS OF THE SOCIETY. 



for insertion of accessories in focus, cross wires to be accurately 

 parallel to the planes of polarization. The ocular to be engraved with 

 the equivalent focal length. 



8. Diaphragms with holes of different sizes, and others with central 

 stops for dark-ground illumination corresponding with the objectives. 



9. Gypsum Plate, showing red or violet of the first order. 



ACCESSOEIES. • 



The following accessories are to be regarded as additional and not 

 included in the price specified. Some of them will not be required by 

 ordinary students. 



10. Beck Lens, which can be placed on ocular, to be capable of 

 being focussed. It should be provided with cross wires and a slot, and 

 should be capable of rotation and graduated for every 5°. 



11. Bertrand Lens in metal carrier for insertion in the slot above the 

 objective. 



12. Extra Analyser, which can be placed either on the ocular or the 

 Beck lens, capable of rotation and graduated for every 5°. 



13. Quartz Wedges, ordinary, graduated, combination and double. 

 The form especially recommended is a graduated combination wedge 

 with projecting gypsum plate, the graduation being in hundreds of 

 micro-millimetres of relative retardation.* 



14. Mica Steps, progressing by relative retardation of 100 micro- 

 millimetres. 



15. /SVa^e i/zcro»?e/(?r in metric units, and to include sub-divisions of 

 100 and of 10 microns with an accuracy of not less than 5 p.c. 



16. Squared Ocidar Micrometer in metric units, with an accuracy of 

 not less than 5 p.c. 



17. Nakamura Plate. (A modification of the plate in the Bertrand 

 ocular.) 



II. {a) Draft Specification of a simple form of microscope for 

 general use in Chemical Laboratories when something rather better 

 than a hand magnifying glass is required. The price complete for the 

 microscope specified, with one objective and ocular in case, is to be 

 about £3, without the accessories. 



(1) Stand. — Fixed upright form, on either tripod or horseshoe base. 

 The limbs should be shaped so as to form a convenient handle. 



(2) Stage. — A plain square stage with removable spring object clips. 

 The stage should be blacked brass so that it can be temporarily covered, 

 when corrosive fluids are being used, with a separate plain plate of 

 glass supplied with the instrument. Standard-sized under-stage fitting, 

 to take iris diaphragm if required. 



(3) Mirrors. — Concave adjustable in two directions on arm, fixed as 

 to distance from stage. 



(4) Focussing Arrangement. — A good coarse adjustment only, with 

 range sufficient to focus a 50 mm. objective. 



(5) Objective. — One low power only, but any usual power between 

 50 mm. and 15 mm. as desired, 



(6) Ocular. — One lower power only — e.g. 30 to 40 mm. focal length. 



