82 BINOCULAR MICROSCOPES [Ch. Ill 



If one closes the diaphragm until \ or f of the aperture of the 

 objective is used, the image will be a combined color and refraction 

 image. 



§ 140. 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 card-board. Unless one 

 has a very favorable light the shading of the object is of the greatest 

 advantage, especially with the homogeneous immersion objectives. 

 The shield (fig. 33) is the most satisfactory means for this purpose, 

 as the entire microscope above the illuminating apparatus is shaded. 

 This screen also shades the face of the observer. 



§ 141. Cleaning homogeneous objectives. — After one is through 

 with a homogeneous objective, it should be carefully cleaned as fol- 

 lows : Wipe off the homogeneous liquid with a piece of the lens paper 

 (§ 158); 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. 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 mixture proceed as above, but use water to 

 remove the last traces of glycerin. 



Binocular Microscopes 



§ 142. For a binocular arrangement which shall be equally good 

 for all powers up to and including the highest oil immersion, the 

 following fundamental requirements must be met: 



1. The light to each eye should be of equal intensity. 



2. The optical path of the light to each eye should be of the same 

 length, so that the magnification of the two images will be the same. 



3. The numerical aperture should not be cut down or disturbed in 

 any way. 



4. The diffraction effects should be the same as for the monocular 

 microscope. 



5. The oculars must be laterally adjustable for the pupillary dis- 

 tance of different observers. 



