CH. VI] THE PROJECTION MICROSCOPE 197 



elevated. One can soon get it in exact focus. Of course it may 

 be removed at any time. 



310. 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 (fig. 59-60) is 

 employed. For temporary marking an ink line may be put on with 

 a pen; or a glass pencil may be used. 



THE PROJECTION MICROSCOPE 



311. Projection Microscope. One of the most useful and 

 satisfactory means at the disposal of the teacher of Microscopic 

 Anatomy and Embryology for class demonstrations is the Projec- 

 tion Microscope. With it he can show hundreds of students 

 as well as one, the objects which come within the range of the 

 instrument. 



It is far more satisfactory than microscopic demonstrations, for 

 with the projection microscope the teacher can point out on the 

 screen the structural features and organs which he wishes to demon- 

 strate, and he can thus be certain that the students know exactly 

 what is to be studied. Unless one employs a pointer ocular (fig. 117), 

 there is no certainty that the student selects from the multitude of 

 things in the microscopic field the one which is meant by the teacher. 

 Like all other means, however, the projection microscope is limited. 

 With it one can show organs both adult and embryonic, and the 

 general morphology. For the accurate demonstration of cells and 

 cell structure the microscope itself must be used. As a general 

 statement concerning the use of the projection microscope for demon- 

 stration purposes, it may be said that it is entirely satisfactory for 

 objects and details which show under the microscope with objectives 

 up to 1 6 mm. equivalent focus. For objects and details requiring 

 objectives higher than 16 mm. focus in ordinary microscopic ob- 

 servations, the projection microscope is unsatisfactory with large 

 classes. 



