Il8 ELEMENTARY CHEMICAL MICROSCOPY 



or stopper S. To the outer end of this stopper is fastened a 

 metal head M, whose circumference is graduated, each division 

 being equal to two degrees. These graduations are not intended 

 for accurate measurement but merely to serve as a guide in rotat- 

 ing the material cemented to the knob k at the inner end of S. 

 For use the cell and stopper are placed in a metal mounting B 

 and laid upon the stage of the microscope. The cell C is filled 

 with a liquid of such refractive index as to practically obliterate 

 the usual heavy black contour bands. Leakage is prevented by 

 holding the stopper tightly in place by the tension spring /. A 

 curved finger, fastened by the screw A, holds the glass parts in 

 the metal mounting and allows easy removal for cleaning. An 

 index mark i upon the tube T furnishes a means of determining 

 the amount of rotation of the object attached to k. The instru- 

 ment is provided with two cells, one 10 millimeters deep and one 

 1 5 millimeters deep, and a special condensing lens K for observa- 

 tions with converging polarized light. 



A Simple Device for Orientation, often perfectly satisfactory, 

 consists in cementing the object to the point of a needle or 

 tiny glass rod and inserting the other end of the needle or rod 

 into a mass of plasticine. The needle or rod can be moved in 

 any direction and secured in place by gentle pressure of the 

 fingers upon the plasticine. Solid angles of tiny crystals may 

 thus be computed by measuring the plane angles of the different 

 faces in turn. 1 



Lens Holders. Frequently low magnifications are required 

 in preparing or separating material for microscopic study, but 

 placing the objects upon the stage of the compound microscope 

 is inconvenient or impossible. Recourse may then be had to 

 magnifiers held in some sort of easily adjustable stand. The 

 author has found a stand of the general style shown in Fig. 62 2 

 to be the most useful. The lens holder itself, consisting of a 

 spring clip C, renders the stand applicable to a wide variety of 

 uses other than merely supporting lenses. The hinged arms and 

 .thumb-screw admit of adaptation to any position and to all 



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



2 Made by the Bausch & Lomb Optical Co., Rochester, N. Y. 



