178 ELEMENTARY CHEMICAL MICROSCOPY 



since his object is merely to ascertain whether the crystal under 

 examination is, or is not, a certain compound. In simple inor- 

 ganic analyses angle measurements are rarely resorted to, but in 

 the examination of organic compounds and in the case of mix- 

 tures of inorganic and organic substances, the measurement of 

 angles may often prove a most rapid means of differentiation. 



Only thin, well-formed crystal plates with practically perfect 

 edges should be selected for measurement. Avoid high magni- 

 fications. The rotating stage having been previously centered, 

 the preparation is moved with the fingers until the selected 

 crystal is brought under the cross-hairs of the eyepiece. One of 

 the bounding edges of the angle sought is made exactly parallel 

 to and almost in coincidence with one of the cross-hairs ; the posi- 

 tion of the graduated circle of the stage is noted and the stage is 

 rotated until the other bounding edge of the angle becomes par- 

 allel with the same cross-hair. The graduated stage circle is 

 again read. The difference between the two readings is the 

 angle sought. 



If it is known that the cross-hairs in the eyepiece are exactly 

 at right angles, a slightly quicker method consists in measuring 

 the complement of the angle and deducting it from 90 degrees. 

 Or, if the angle be obtuse, measure the amount that is greater 

 than 90 degrees. This method does not necessitate as careful 

 centering of the stage, and can, therefore, be used with high 

 powers with sufficient accuracy for analytical work. It is essen- 

 tial in all measurements of crystal angles that the instrument 

 be most carefully focused upon an edge, and that care be taken to 

 avoid error due to the projection of an image of another edge 

 through the crystal. In the case of very transparent crystals 

 it is sometimes difficult to tell which is the proper line (edge) to 

 employ, unless the crystal is thin. 



For the measurement of solid angles where several planes 

 meet, the crystals must be of sufficient size to permit their being 

 turned first in one position, then in another. Cementing to the 

 point of a needle (method of Kley 1 ), imbedding the head of the 

 needle in a cork and cementing the cork to a glass slip will permit 



1 Kley, Rec. trav. chim. Pays-Bas, 19 (1900), 13. 



