CH. IX] PROJECTION MICROSCOPE 261 



"be adjustable. This is because the arrangement needs to be somewhat 

 different for different objectives. 



The guiding principle is that the specimen should be lighted by a 

 •converging cone of light, and it should be lighted by the entire cone of 

 light traversing the lamp condenser. If one uses a white card it is 

 easy to determine the position and size of the cone of light. If it is 

 too large for the specimen, either the lamp condenser is too near the 

 radiant or the specimen is too close to the lamp condenser. 



If one uses an achromatic condenser the lamp condenser and the 

 achromatic condenser should be so arranged that the entire cone of 

 light traversing the lamp condenser can enter the achromatic condenser. 

 If the cone is too large they are too close together, or the lamp con- 

 denser is too near the radiant. If the cone is too small then the lamp 

 condenser is too far from the radiant or the achromatic condenser, 

 or perhaps both faults are present. One must remember in all his ex- 

 periments that a converging cone of light should be used and not a 

 diverging one. The specimen must then not. be beyond the focus of 

 the lamp condenser. 



If one wishes to make micro-projection a success it will be nec- 

 essary to give the apparatus the requisite time and thought. Try to 

 understand the conditions of success and continue experimenting until 

 you have learned to make it possible for the machine to do its best for 

 you. The satisfaction of showing a class real things is sufficient re- 

 ward for all the trouble. 



' §420. Screen and Screen Distance. — For a screen nothing is 

 so good as a dead-white, smooth wall. A lusterless, white cloth screen 

 answers well also. It is an advantage to have this entirely opaque, so 

 that none of the light can pass through it. One must remember that 

 the light passing through the minute lenses of the objective must be 

 spread out over a great space even with low powers, and over a 

 much greater with high powers, so that one cannot afford to have any of 

 the light lost by transmission through the screen. All are agreed that 

 for micro-projection a translucent screen with the projection apparatus 

 behind it is not desirable, although for ordinary lantern slide projection 

 it answers fairly well. 



The distance of the screen from the microscope depends largely on 

 the size of one's audience. The writer has found a distance of eight 

 meters (26 feet) good for both low and high power projection. This 

 distance answers well for a class of 200 persons. 



