CH. IX] PROJECTION MICROSCOPE 249 



PROJECTION MICROSCOPE 



One of the most useful and satisfactory means at the disposal of 

 the teacher of Microscopic Anatomy and Embryology for class demon- 

 strations is the Projection Microscope. With it he can show two 

 hundred as well as one person 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 

 exacth* 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. 201), 

 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 gen- 

 eral morphology. For the accurate demonstration of cells and cell 

 structure the microscope itself must be used by each student personally. 



If no ocular is used a 3 mm. or \i inch objective is about as high a 

 power as can be employed in a room holding two hundred. Even with 

 an oil immersion fair demonstrations can be made, but up to the pres- 

 ent time really successful demonstrations are usually made with powers 

 below 3 mm. rather than above. If projection oculars are used one 

 can hardly go bej'ond an 8 mm. objective with real satisfaction. And 

 finally it should be remembered that the Continuous Current Arc Lamp 

 must be employed for satisfactory results. Alternating currents are 

 wholly unsatisfactory for this purpose. 



§ 39S. Projection Microscope. — This is an arrangement of the 

 microscope so that an image of the object under the microscope is 

 thrown upon a screen of some kind. The picture on the screen is 

 looked at precisely as one looks at the pictures thrown on the screen 

 by an ordinary magic lantern. Indeed the projection microscope is a 

 magic lantern with short focus objectives. One of the first uses of the 

 microscope was to throw the images of various objects on a screen so 

 that they could be seen by several persons at once, and the light used 

 was sunlight. Hence those early projection microscopes were called 

 solar or sun microscopes. If sunlight were available at all times and 

 could be controlled, it would be universally employed ; but as it is not 



