CH. VI 7] 



CLASS DEMONSTRA TIONS 



197 



it usable for nearly all the purposes required in a laboratory. 

 This instrument is a type of such apparatus which has grown up 

 with the needs of advancing knowledge. 



§ 268. Indicator or Pointer Ocular. — This is an ocular in 

 which a delicate pointer of some kind is placed at the level where 

 the real image of the microscope is produced. It is placed at the 

 same level as the ocular micrometer, and the pointer like the micro- 

 meter is magnified with the real image and appears as a part of the 

 projected image (Fig. 154). By rotating the ocular or the pointer 

 any part of the real image may be pointed out as one uses a pointer 

 on a wall or blackboard diagram. By means of the indicator eye- 

 piece one can be certain that the student sees the desired object, 

 and is not confused by the multitude of other things present in the 

 field. This device has been invented many times. It illustrates 



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Fig. 152 



Fig. 153 



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Fig. 154 



Fig. 152. Indicator ocular with metal pointer like the one devised by 

 Quekett (Leilz' catalog). 



Fig. 153. Indicator ocular with a fine hair from a camel's hair brush on 

 the ocular diaphragm to serve as a pointer (P). This projects about half way 

 across the diaphragm opening. On the opposite side are shown two rays from 

 the microscope to indicate that the real image is formed at the level of the ocu- 

 lar diaphragm. 



Fig. 154. Field of the microscope with a mammalian blood preparation 

 to show the use of the indicator (P) for pointing out a white blood corpuscle. 



