CH. II] 



LIGHTING AND FOCUSING 



69 



move the diaphragms from the illuminator or in case the, iris dia- 

 phragm 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 

 bacteria in focus. They will stand out> as clearly defined colored ob- 

 jects on a bright field. 



Fig. 66. Screen for shading the microscope and 

 the face of the observer. This is very readily con- 

 structed as shown in the figure by supporting a wire 

 in a disc of lead, iron, or heavy wood. The screen is 

 then completed by hanging over the bent wire, black 

 ■cloth or paper 30 x 40 cm. The lower edge of the 

 screen should be a little below the stage of the micro- 

 scope and the upper edge high enough to screen the 

 eyes of the observer. 



§ 120. Shading the Object.— To get the clearest image of an 

 object no light should reach the eye except from the object. A 

 handkerchief or a dark cloth wound around the objective 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 

 immersion objectives. The screen (Fig. 66) is the most satisfactory 

 means for this purpose, as the entire microscope above the illuminat- 

 ing apparatus is shaded. 



§ 121. 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 (§ 125), then if the fluid is cedar oil, wet one corner of a fresh 

 piece in xylene or chloroform and wipe the front lens with it. Im- 

 mediately 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 mixture proceed as above, but use 

 water to remove the last traces of glycerin. 



