CH. 171] 



CLASS DEMONSTRATIONS 



199 



well the adage: "necessity is the mother of invention," for what 

 teacher has not been in despair many times when trying to make a 

 student see a definite object and neglect the numerous other objects 

 in the field. So far as the writer has been able to learn, Quekett 

 was the first to introduce an indicator ocular with a metal pointer 

 which was adjustable and could be turned to any part of the field 

 or wholly out of the field. See Fig. 152, § 140. 



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

 fine hair on the diaphragm of the oeular as shown in Fig. 153. This 

 may be done with any ocular, positive or negative. One may use a 

 little mucilage, Canada balsam or any other cement to stick the 

 hair on the upper face of the diaphragm so that it projects about 

 halfway across the opening. When the eye-lens of the Huygenian 

 ocular 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. 



§ 269. 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 some way. An 

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

 around the part to be demonstrated. For this the Marker, Figs. 

 70-72, is employed. For temporary marking an ink line may be 

 put on with a pen. 



Fig. 156 



Fig. 157 



Fig. 156. Ring around one of the sections of a series for demonstrating 

 sonic organ especially well. 



Fig. 157. Figure of a microscopic preparation with a ring around a 

 small part to show the position of some structural feature ; 



