102 



MARKERS AND MECHANICAL STAGES 



[Ch. Ill 



Fig. 6i. A Microscopical Speci- 

 men with a Small Ring En- 

 closing the Part of Special 

 Interest. 



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. Micro. Soc, 1894, pp. 112-118). 



§ 173. Pointer in the ocular. — This 

 is a slender rod of some sort situated 

 at the level of the real image in the 

 microscope, and it appears with the 

 specimen in the field of view. 



A pointer may be 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 sur- 

 face of the diaphragm (fig. 23-24) so that it will project about half- 

 way across the opening. If one uses this ocular, the pointer will 

 appear in the field and one can place 

 the specimen so that the pointer in- 

 dicates it exactly, as in using a pointer 

 on a diagram or on the blackboard. It 

 is not known to the author who de- 

 vised this method. It is certainly of 

 the greatest advantage in demonstrat- 

 ing objects like amcebas or white blood 

 corpuscles to persons not familiar with 

 them, as the field is liable to have in 

 it many other objects which are more 

 easily seen. 



§ 174. Mechanical stage. — For 

 high school and ordinary laboratory 

 work a mechanical stage is not needed; 

 but for much work, especially where 

 high objectives are used, a mechanical 

 stage is of great advantage. It is also advantageous if the mechanical 

 stage can easily be removed. 



The one found on the most expensive American and English micro- 

 scopes for the last twenty years and the one now present on the larger 



Fig. 62-63. Pointer Ocular 

 and Microscopic Field. 



P P The pointer attached to 

 the diaphragm of the ocular and 

 extending out into the free space 

 in fig. 62. In fig. 63 the pointer 

 is shown indicating the position 

 of a leucocyte. 



