ILLUMINATION 5 



board 4 or 5 inches long, with an opening to fit over the tube of the microscope, 

 leaving the other end to block the vision of the unused eye, will prevent the strain. 

 This apparatus can be purchased in vulcanite. 



Warm Stages. A warm stage for the study of living protozoa may be extempor- 

 ized by taking a piece of copper about the size of the stage and with a strip projecting 

 out anteriorly for 5 or 6 inches. The under surface of the plate is covered with 

 flannel and a hole about i inch in diameter cut out of the center. The proper amount 

 of heat is applied by a flame impinging on the tongue-like projection of the copper 

 plate. 



At present there are electrically heated warm stages, connected with the desk 

 socket by a wire and plug, which are most convenient. In fact they are almost as 

 satisfactory as the more expensive and less convenient warm chamber or microscope 

 oven surrounding the microscope. 



Illumination. Direct sunlight or excessively bright light is to be 

 avoided. If such conditions must exist a white shade or muslin curtain 

 drawn across the window is a necessity. Light from the north and from 

 a white cloud is the most desirable. South of the equator a southern 

 light. In the tropics a piece of plate glass fitted into the lower part of 

 a wire screen frame gives good lighting, keeps out dust, and does not 

 interfere greatly with the circulation of the air. 



The technic in connection with proper illumination is probably more important 

 than any other point; unless the light is utilized to the best advantage, the best 

 results cannot be obtained. In examining fresh blood preparations or hanging drops 

 the concave mirror should be used and the light almost shut off by the iris dia- 

 phragm so as to give a contour picture. In examining a stained blood or bacterial 

 preparation, the Abbe condenser should be properly focused so as to best illuminate 

 the stained film. In many instruments set-screws are provided which check the ele- 

 vation of the Abbe condenser when the proper focus is reached. Inasmuch as the 

 light from the condenser should come to a focus exactly level with the object studied, 

 it is evident that a fixed position for the condenser would not answer when slides of 

 different thickness were used. Always use the plane mirror when examining stained 

 bacterial or blood films, as a color image is desired. Ordinarily in examining tissue 

 sections, the Abbe condenser should either be put out of focus by racking down or by 

 the use of the concave mirror and the narrowing of the aperture of the iris diaphragm. 

 Swing-out condensers are now made which are very convenient. The proper em- 

 ployment of illumination only comes with experience, and one should continue to 

 manipulate his mirrors, diaphragm, and condenser until the best result is obtained. 

 Then study the specimen. 



For microscopical work in a laboratory not properly supplied with windows or for 

 night work the frosted incandescent bulb is very satisfactory. 



An objection to artificial light is that one working almost entirely 

 with sunlight forms standards and when using a different light is some- 

 what confused in interpretation of the microscopical picture. 



