CH. IX a] PROJECTION MICROSCOPE 265 



objective must be spread out over a relatively 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. 



The distance of the screen from the microscope depends largely on 

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

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

 This distance answers well for a class of 200 persons. If oculars are 

 used with the objective, a screen distance of 35 meters is sufficient. 



For the minute details of a projected specimen it is recommended 

 that the audience use opera glasses. These are also useful for focus- 

 ing the image on the screen. 



419. Darkening the Room. It is impossible to succeed in 

 micro-projection unless the room can be made dark, the darker the 

 better. It is especially important that the screen should be free from 

 all light except that projected upon it in forming the image. 



420. Enclosing the Projection Apparatus. It is desirable 

 to have the projection apparatus enclosed as completely as possible to 

 avoid diffusing light through the room and thus vitiating the most 

 careful darkening of all windows and sky lights. It is also desirable 

 to shut in the light from the apparatus, as it dazzles the eyes of the 

 operator and of those near it in the audience so that the image on the 

 screen cannot be satisfactorily seen. Some forms of apparatus are en- 

 closed in a metal box, others have a frame over them upon which is 

 spread black cloth like silesia. If this is made fireproof by soaking it 

 thoroughly in a solution of alum, borax and sodium tungstate it will 

 not readily catch fire. The cloth should not be too thick, otherwise it 

 will retain too much heat around the apparatus. 



One should remember the fundamental law of vision, viz, that 

 other things being equal, the clearest images are obtained when no 

 light reaches the eye except from the object. 



421. Preparations Suitable for Micro-Projection. As a 

 generalization it may be said that any specimen which shows clearly 

 and sharply under the microscope with a 1 6 mm. or lower objective 

 will also give an excellent projection image. Details which are not 

 visible with the 16 mm. objective are rarely well brought out with 

 sufficient clearness on the screen for one or two hundred people to see. 



(A) The stains showing best are those which are very transpar- 

 ent, or pure differential stains like hematoxylin. Admirable results 



