CH. IX] OBJECTS FOR MICRO-PROJECTION 269 



Large Tube (fig. 121) 



ORDINARY MICROSCOPIC OBJECTIVES 

 Large Tube (fig. 121) 



399a. The preparations listed in the above table are simply examples of 

 objects which can be shown entire with the different objectives without oculars. 

 In practice any good microscopic preparation and many living things can be 

 shown with the projection microscope. 



For the complete understanding of any specimen it is necessary to see it as 

 a whole and then by using higher and still higher powers (391) to get views of 

 finer and finer details. 



In demonstrating the finer details one can show but a very small specimen or 

 a small part of a large specimen. For large specimens it is a great advantage to 

 have objectives of different powers on a revolving nose-piece so that it takes 

 only a moment to turn from one to the other. If only the large condenser is 

 used (fig. 121) the objective remains practically stationary, but the specimen 

 must be on a movable stage so that it can be farther from the objective or 

 nearer to it depending upon the focal length of the objective (fig. 132). 



If one uses substage condensers the stage remains stationary and a long 

 focus substage condenser is used for low powers and a short one for high powers 

 and the objective is placed at approximately its focal distance from the object. 



It must be remembered that many living things are soon destroyed by the 

 intense light necessary for projection. While the circulation of the blood seems 

 an ideal demonstration with the projection microscope it is found in practise to 

 be a very poor way to demonstrate it. If this is tried the microscope in a ver- 

 tical position (fig. 147) is convenient. The screen distance should not be very 

 great (3 to 5 meters, 10 to 16 ft.). In the author's experience the demonstra- 

 tion of blood circulation under a microscope is vastly superior to anything that 

 can be done with a projection microscope. 



