CH. IX] PROJECTION MICROSCOPE 263 



with and without the oculars one can decide quite accurately the best 

 combination to select. On the whole the writer has found it better to 

 employ a sufficient variety of objectives and not use the projection 

 oculars. It is somewhat easier to obtain a brilliant image without the 

 oculars. 



§422. 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. 



§423. Enclosing the Projection Apparatus. — It is desirable 

 to have the projection apparatus closed 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. 



§ 424. 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 16 mm. 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 

 have been obtained with hematoxylin and eosin, and the various car- 

 mines when differentiated. Every method of staining which gives 

 either sharply differentiated results or transparent colors produces 

 preparations adapted to projection. A weak, or washed out appearance 

 under the microscope is sure to be even less satisfactory on the screen. 



(B) The thickness of the sections may vary from \jj. to 40/^. But 

 one must remember that thick sections are adapted for low powers 



