CH. VII] 



CLASS DEMONSTRATIONS 



microscope is limited. With it one can show organs both adult and 

 embryonic, and the general morphology. For the accurate demon- 

 stration of cells and cell structure the microscope itself must be used. 

 As a general statement concerning the use of the projection micro- 

 scope for demonstration purposes, it may be said that it is entirely 

 satisfactory for objects and details which show under the microscope 

 with objectives up to 16 mm. equivalent focus. For objects and 

 details requiring objectives higher than 16 mm. focus in ordinary 

 microscopic observations, the projection microscope is unsatisfactory 

 with large classes. 



5_ 

 10 



Fig. 159. Slide of several sections with a black mask. The mask is per- 

 forated over the sections to be demonstrated with the microscope or Hie pro- 

 jection microscope. It is put on the back of the slide and not on the cover- . 

 glass. 



Unless one has a mask something like this the light is so dazzling that it 

 is almost impossible to find the proper sections. It is easily removed by plac- 

 ing the slide on wet blotting-paper. 



Fig. 160. Projection Microscope. This figure illustrates a modern pro- 

 jection microscope with an arc light for radiant. Such a projection micro- 

 scope is available whenever there is an electric current; and is of the greatest 

 use in projecting microscopic objects ranging from 60 mm, to fa of a millimeter 

 so that a large class can see all the details. Its use in drawing was pointed out 

 in \ 200. (From Gage, Origin and Development of the Projection Microscope.) 



