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XiV. An Instrument for Grinding Sect ion-plates and Prisms of Crijsteds of Artifeicd 
Preparations Accurately in the Desired Directions. 
By A. E. Tutton, Assoc. B.C.S., Demonstrator of Chemistry at the Royal College of 
Science, South Kensington. 
Communicated by Professor Thorpe, F.R.S. 
Received January 11,—Read I'ebruary 1, 1894. 
The most difficult operations in connection with the investigation of the optical 
properties of the crystals of artificially-prepared substances, which are usually 
endowed with a much lower degree of hardness tlian the crystals of naturally-occurring 
minerals, are those which involve the preparation of the necessary section-plates and 
prisms. It is of primary importance that the plates should be truly parallel to the 
desired plane, or perpendicular to the desired direction in the crystal, and that they 
should possess plane surfaces truly parallel to each other. The prisms should likewise 
possess two ])]ane surfaces, inclined to each other at an angle wdiich may not usually 
exceed 70°, and whose edge of intersection is always required to be parallel to a given 
direction in the crystal ; moreover, the two surfaces may -with advantage be sym¬ 
metrical to, or one of them parallel with, a given plane in the crystal. It is not too 
much to say that the accuracy of the determinations of the optical constants of 
crystals depends fundamentally upon the degree of precision with whicli these 
requirements are attained. 
The preparation of section-plates and prisms of these relatively soft and friable 
crystals, when, as happens in the large majority of cases, tlie crystals do not exhibit 
the desired planes, or do not present tliem sufficiently prominently developed to 
enable them to be utilised as plates and prisms, must of necessity be carried out by 
grinding. In very few cases, indeed, are the crystals of artificial preparations 
endowed with sufficient hardness to withstand a preliminary cutting, by means of an 
extremely fine fretsaw, or thin wire lubricated with oil or a solvent for the crystal¬ 
lised substance. The crystals usually require delicate handling, their relative softness 
or brittleness, together with the development of cleavage, rendering them particularly 
liable to fracture and splitting. Moreover, owing to their gi’eater freedom from 
distortion, striation, and facial curvature, the smaller crystals are always to be 
preferred for the purposes of accurate investigations, and the preparation of sections 
and prisms from small crystals must necessarily be carried out entii’ely by grinding. 
19.12.94 
