PROCEEDINGS OF THE SOCIETY. 207 



(7) Finish. — All black (permanently acid-resisting), including 

 mounts of objectives, and milled heads of coarse adjustment. 



Accessories. — Additional objectives, double nose-piece, iris diaphragm, 

 eye-piece scale. 



II. (b) The range of advanced work in Chemistry is so wide that 

 no single microscope can satisfactorily cover the whole ground. The 

 requirements for different kinds of work are also so wide apart that if 

 an instrument were designed to satisfy all these requirements it would 

 probably not he very good for any of them. One or other of the 

 microscopes for advanced work for which specifications have been 

 drafted will be suitable for different special classes of advanced chemical 

 work. For general purposes the specification of the " Instrument for 

 advanced Pathological work " (Type II. supra) may be adopted as a 

 basis, with the following modifications : — 



(1) The microscope should be provided vfith polarizers and analysers 

 capable of being rotated simultaneously. 



(2) The stage should be fixed and not less than 4i inches in diameter, 

 fitted with centra], detachable, perforated, spoon-shaped, spring clip with 

 turned-up edges, or a pair of movable lateral clips with dimpled ends. 

 [If a rotating stage is desired the arrangement in the Petrological type 

 {supra) may be adopted.] 



(3) The suh-stage should allow the use of a high power as well as 

 of a fairly low power condenser — e.g. one of the well-known achromatic 

 condensers of N.xl. • 9, either with or without the top lens ; more- 

 over, when this top lens has been removed it should still be possible to 

 use the polarizer even when the instrument is in the upright position. 



(4) Mirrors in the usual gimbal fitting, 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 polarizing 

 prism. 



III. Draft Specification for a Metallurgical Microscope of simple 

 type, intended for students and for general use in works and laboratories. 

 Price not to exceed £15. 



(1) Stand. — Form of foot immaterial, but stand must be strong and 

 rigid. The limb, if inclinable, should be provided with an efficient 

 clamp. Rigidity is of great importance when large or heavy specimens 

 have to be examined, or when the stage is to be used at a long distance 

 from the objective. 



(2) Stage. — Coarse focussing by rack and pinion essential ; ample 

 bearing surface important. Fine adjustment must be on the stage. 

 Stage to be plain with central opening for transmitted light, and a stop 

 to close the opening when not required. The stage should be provided 

 with a removable ridge against which the specimen or the object-carrier 

 may rest when the instrument is in the inclined position. Strong spring 

 detachable clips required. The movement of the stage should be 

 sufficient to allow of a free working distance between the objective and 

 t!ie surface of the stage of not less than 4 inches, in order to allow of 

 the use of a 8-inch objective with a specimen of fair thickness. 



Facilities should be provided for the addition as an extra of a 

 mechanical stage with concentric rotation and centreing screws. 



