CLASS DEMONSTATIONS 



[CH. IX 



It is not known who adopted the simple device of putting the tip 

 of a cat's whisker or an eye-lash on the diaphragm of the ocular as 

 shown in Fig. 200. This ma} 7 be done with any ocular, positive or 

 negative. One may use a little mucilage, Canada balsam or any 

 other cement, and stick the eyelash on the upper face of the diaphragm 

 so that it projects about half way across the opening. When the eye- 

 lens is screwed back in place the hair should be in focus. If it is not 

 screw the eye-lens out a little and look again. If it is not now sharp, the 

 hair is a little too high and should be depressed a little. If it is less 

 distinct on screwing out the ocular it is too low and should be elevated. 

 One can soon get it in exact focus. Of course it may be removed at 

 any time. 



§ 397- — Marking the Position of Objects. — In order that one 

 may prepare a demonstration easily and certainly in a short time the 

 specimens to be shown must be marked in someway. A very efficient 

 and simple method is to put rings of black or colored shellac around 

 the part to be demonstrated. For this the Marker, Fig. 61-62, is 

 employed as described on p. 66. 





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Fig. 202. 



Fig. 202. Ring around one of the sections of a series for demonstrating some 

 organ especially well. 



Fig. 203. Figure of a microscopical 

 preparation with a ring around a small 

 part to show the position of some structural 

 feature. 



