106 THE USE OF THE MICROSCOPE 



drop), since the aperture of the objectives is usually less 

 than 0,1, The image is injured, however, by the rays 

 having to pass through the uneven cover of a Petri dish. 

 It may be possible, in the case of fungus cultures especially, 

 to have circular pieces of plate glass to replace the ordinary 

 covers of Petri dishes, at least during observation. In all 

 cases where the binocular is used without a stage, the 

 main axis is best kept at right angles to the surface of the 

 water or glass. 



3, Small objects, mostly invisible to the unaided eye, 

 usually examined on a glass slide, and mostly in water, are 

 included here. For these, useful magnifications from 50 

 to 200 are needed, and these are obtained by the low-power 

 objective of the standard microscope; which is, at the best, 

 an apochromatic, with an initial magnification of 10, and 

 an aperture of 0.3. This objective will in practice stand 

 an eyepiece magnification of about 20 times. The objects 

 for this lens, and for the achromatic objective of corre- 

 sponding aperture, should be under a cover-glass between 

 0.1 and 0,2 millimeter thick. If a cover-glass is omitted, 

 the tube may be pulled out for an appropriate short 

 distance for correction. (This distance can be determined 

 by the star test.) The water-immersed condenser, with a 

 large diaphragm on the ground-glass disc before the source 

 of light, can be employed for this low power without other 

 change from its use with the high power, except increase 

 in the size of the diaphragm on the radiant. This allows 

 of a clear cone of uniform light of nearly 0.3 aperture being 

 used, which brings out the full value of the objective with- 

 out flooding (which is not the case with the concave mirror). 

 The images of many of these objects (even if stained) are 

 improved by using yellow-green light; but where the natural 

 colors are to be noted, a filter of the bluish daylight glass 

 is to be inserted between the tungsten lamp and the mirror. 

 The Bitukni (or other) eyepiece attachment, for oblique 

 vision with horizontal stage, may often be required in view- 

 ing objects in liquids. 



