108 Mr. A. E. Tutton. Instrument for grinding [Feb. 



III. " An Instrument for grinding Section-plates and Prisms 

 of Crystals of Artificial Preparations accurately in the 

 desired directions." By A. E. TUTTON, ABSOC. R.C.S., 

 Demonstrator of Chemistry at the Royal College of I 

 Science, South Kensington. Communicated by Professor 

 THORPE, F.R.S. Received January 11, 1894. 



(Abstract.) 



This instrument has been devised in order to replace by a method 

 of precision the difficult, wearisome, and, at the best, only approxi- 

 mate current method of grinding by hand, upon a slightly convex 

 plate of ground glass lubricated with oil or a solvent, the section- 

 plates and prisms of the relatively soft and fragile crystals of arti- 

 ficial preparations which are required for the determination of the 

 optical constants. It is possible by means of it to grind and polish 

 a truly plane surface in any desired direction in a crystal accurately 

 to within ten minutes of arc, an amount of possible error which 

 would exercise no appreciable influence upon the values of the optical 

 constants. This result may be achieved in a small fraction of the 

 time hitherto required by the most successful hand-grinding, and, 

 owing to the provision of a delicate arrangement for suitably modi- 

 fying the pressure with which the crystal bears upon the grinding 

 plane, with only the very slightest risk of fracturing even a friable 

 crystal. An arrangement is, moreover, provided by which a second 

 surface may be ground parallel, with a like degree f accuracy, to 

 the first. The sections and prisms furnished by the instrument 

 possess the further advantage of being so highly polished as to enable 

 them to be employed directly without cemented cover-glasses for the 

 determination of the optic axial angle and refractive indices, and the 

 values of these constants derived from them are no longer only ap- 

 proximate but precise. 



A short but relatively wide hollow cone, differing but slightly from 

 a cylinder, is rigidly supported vertically above a heavy base by three 

 stout pillars and a triangular cross-plate. Within it rest three 

 movable inner axes. The first carries at its upper end a horizontal 

 circle divided into half degrees, reading, with the aid of a vernier, to 

 minutes, and provided with a fine adjustment ; the axis and circle 

 are capable of rotation by a milled ebonite disc carried at the lower 

 extremity of the former. The bore of this axis is cylindrical, and 

 within it the second axis is capable of vertical motion, rotation in the 

 socket being prevented by a rib and groove. The weight of this 

 second axis and all that it carries may be wholly or partially counter- 

 balanced by a pair of levers whose fulcrum supports rest upon the 



