CH. IX] 



CLASS DEMONSTRATIONS 



247 



§ 396. 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 micrometer is 

 magnified with the real image and appears as a part of the projected 

 image (§ 170). Bj t rotating the octdar or the pointer any part of the 

 real image may be pointed out as one uses a pointer on a wall or black- 

 board diagram. By means of the indicator eye-piece one can be cer- 

 tain that the student sees the desired object, and is not confused by the 

 multitude of other things present in the field. The method of its use 

 is indicated in Fig. 201. This device has been invented manj^ times. 

 It illustrates 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. 199, § 126. 



Fig. 199. Fig. 200. Fig. 2or. 



Fig. 199. Indicator ocular with -metal pointer like the one devised by 

 Quekett (Leitz ; catalog). 



Fig. 200. Indicator ocular with an eyelash {cilium) on the ocular diaphragm 

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

 ing. On the opposite side are shown two rays from the microscope to indicate that 

 the real image is formed at the level of the ocular diaphragm. 



Fig. 20 1 . Field of the microscope with a mammalian blood preparation to show 

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



