80 LIGHTING AND FOCUSING [ CH. II 



Plain Fitting above Thread of Object-Glass o. I in. long, not to exceed 

 0.759 in. in diameter. 



Diameter (C) of Thread on Object-Glass at top of thread not to exceed 

 o 7982 in., or to be less than 0.7952 in. 



Diameter (D) of Thread on Object-Glass at bottom of thread not to 

 exceed 0.7626 in., or to be less than 0.7596 in. 



Length of Screw of Nose-Piece to be not less than 0.125 i fl - 



Diameter of Screw of Nose-Piece (A) at top of thread not to exceed 0.7674 

 in., or be less than 0.7644 in. 



Diameter of Screw of Nose-Piece (B) at- bottom of thread not to exceed 

 0.803 in., or be less than 0.8 in." 



\ 138. Standard Size Oculars and Substage Condensers. — For a considera- 

 tion of these, with measurements, see \ 53, 98. 



MARKERS AND MECHANICAL STAGES 



Markers are devices to facilitate the finding of some object or part which 

 it is especially desired to refer to again or to demonstrate to a class. The 

 mechanical stage makes it much easier to follow out a series of objects, to 

 move the slide when using high powers, and for complete exploration of a 

 preparation. Most of the mechanical stages have scales or scales and verniers 

 by which an object once recorded may be readily found again. 



£ 139. Marker for Preparations. (Figs. 70-72). — This instrument con. 

 sists of an objective-like attachment which may be screwed into the nose-piece 

 of the microscope. It bears on its lower end a small brush and the brush can 

 be made more or less eccentric and can be rotated, thus making a larger or 

 smaller circle. In using the marker the brush is dipped in colored shellac or 

 other cement and when the part of the preparation to be marked is found and 

 put exactly in the middle of the field the objective is turned aside and the 

 marker turned into position. The brush is brought carefully in contact with 

 the cover-glass and rotated. This will make a delicate ring of the colored 

 cement around the object. Within this very small area the desired object can 

 be easily found on any microscope. The brush of the marker should be 

 cleaned with 95% alcohol after it is used. (Proc. Amer. Micr. Soc, 1894, pp. 

 112-118.) 



\ 140. Pointer in the Ocular. — The Germans have a pointer ocular (Spitzen. 

 Okular), an ocular with one or two delicate rods or pointers at the level of the 

 real image, that is, at the level of the diaphragm (Figs. 26, 36, D). For the 

 purposes of demonstrating any particular, structure or object in the field, a 

 temporary pointer may be easily inserted in any ocular as follows: Remove 

 the eye-lens and with a little mucilage or Canada Balsam fasten a hair 

 from a camel's hair or other fine brush to the upper surface of the 



