CH. IX] EXHIBITION WITH PROJECTION MICROSCOPE 271 



An experiment with the iris partly closed and then wide open will 

 show the necessity of observing this rule. 



The microscopic slides should be in order and properly masked 

 ( 384) and marked in some way so that the operator can tell which 

 edge up they should be placed on the stage. 



It is also a great advantage to have marked on the microscopic 

 specimen the objective or objectives that should be used in pro- 

 jecting it to bring out the structural details which it is desired to 

 show. 



FIG. 149. SLIDE Box TO HOLD PREPARATIONS FOR DEMONSTRATION. 

 (Cut loaned by the Spencer Lens Company). 



For ease in getting hold of the slides to be exhibited, either a 

 shallow tray can be used or a slide box (fig. 148, 149). As with 

 lantern slides, it is advantageous to have the microscopic specimens 

 so placed that they can be grasped easily, and put on the stage as 

 desired without hesitation. 



Some teachers, including the senior author, have found it 

 advantageous to manage the projection themselves, giving the 

 explanations from the position of the lantern. 



The best way to point out the parts in the screen image to be 

 especially noted is to have a slender pointer about two meters (six 

 feet) long, like the upper two-thirds of a bamboo fishing rod, and 

 to hold this out in the beam of light. The shadow appears on the 

 screen sharply, and one can point out details with the same clear- 

 ness as by using a pointer on the screen. It is easier also, because 

 the speaker does not get his eyes dazzled by looking into the light 

 beam, as so often happens when standing near the screen in the 

 usual lecture position. 



SPECIAL DEMONSTRATIONS WITH HIGH POWERS 

 401. Substage condenser in projection. As indicated in 

 359 the authors of this book believe that projection for large 

 audiences and with low objectives is best accomplished without 



