56 



LIGHTING AND FOCUSING. 



[CH. II. 



medium will also increase the sharpness of the outline. If an air bub- 

 ble preparation (§ 73) were used, one would get pure, refraction images. 

 § 101. Color Images. — Use some stained microbes, as Bacillus tuber- 

 culosis for object. Put a drop of the immersion liquid on the cover- 

 glass or the front lens of the homogeneous objective. Remove the dia- 

 phragms from the illuminator or in case the iris diaphragm is used, open 

 to its greatest extent. Focus the objective down so that the immersion 

 fluid is in contact with both the front lens and the cover-glass, then 

 with the fine adjustment get the microbes in focus. They will stand 

 out as clearly defined colored objects on a bright field. 



Fig. 58. — Screen for shading the microscope and the 

 face of the observer. This is very readily constructed 

 as shown in the figure by supporting a wire in a disc 

 of lead, iron, or heavy wood. The screen is then com- 

 pleted by hanging over the bent wire, cloth or manilla 

 paper 30 x 40 cm. The lower edge of the screen should 

 be a little below the stage of the microscope and the 

 upper edge high enough to screen the eyes of the ob- 

 server. 



§ 102. Shading the ObjecT:.— To get the 

 clearest image of an object no light should 

 reach the eye except from the object. A hand- 

 kerchief or a dark cloth wound around the ob- 

 jective will serve the purpose. Often the proper effect may be obtained 

 by simply shading the top of the stage with the hand or with a piece of 

 bristol board. Unless one has a very favorable light the shading of the 

 object is of the greatest advantage, especially with homogeneous im- 

 mersion objectives. The screen (Fig. 58) is the most satisfactory 

 means for this purpose, as the entire microscope above the illuminating 

 apparatus is shaded. 



§ 103. Cleaning Homogeneous Objectives. — After one is through 

 with a homogeneous objective, it should be carefully cleaned as follows : 

 Wipe off the homogeneous liquid with a piece of the lens paper (§ 107), 

 then if the fluid is cedar oil, wet one corner of a fresh piece in benzin 

 and wipe the front lens with it. Immediately afterward wipe with a 

 dry part of the paper. The cover-glass of the preparation can be 

 cleaned in the same way. If the homogeneous liquid is a glycerin mixt- 

 ure proceed as above, but use water instead of benzin to remove the 

 last traces of glycerin. 















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