THE OBJECT 107 



4. This class includes objects which are usually examined 

 with the high dry objective, after having been found 

 with the low power of the standard microscope. Many 

 objects in water can be examined in this way, in the absence 

 of a good water-immersion objective, and of practice in its 

 use. For all these, the correct cover-glass thickness is 

 of importance; but in cases where the liquid between the 

 cover-glass and the slide is more than 0.01 millimeter 

 thick, the cover should be slightly thinner, rather than 

 thicker, than 0.17 millimeter. With a thinner cover it is 

 easier to make the required correction for a thicker layer of 

 liquid, for the one compensates the other, more or less. 

 Whereas, with a thicker cover, the tube length has to be 

 reduced, both for the cover and for the layer of liquid over 

 the object; and there is usually, in most monoculars with 

 drawtubes, only 2 centimeters or less of reduction available 

 (from 160 to perhaps 140 millimeters). The same applies 

 to correction collars on dry objectives. 



The correction of the condenser is important with 

 this magnification, for with the 40 objective of 0.85 aper- 

 ture, a condenser cone of 0.8 can sometimes be used with 

 advantage; and this is only to be obtained from a well- 

 corrected condenser, preferably the same water-immersion 

 condenser used for the higher powers. After practice, it is 

 convenient and easy to use a correction collar on this 

 objective. 



For this same magnification, however, we may also 

 use the apochromatic objective 20, of 8-millimeter focal 

 length, with aperture of 0.65. This objective will stand 

 an eyepiece magnification of about 25, and has several 

 advantages over objectives of smaller focal lengths. 



5. This class includes all objects which are observed 

 with apertures up to 1.25, with the high-power, water- 

 immersion objectives. These objects are in water or a liquid 

 of similar refraction, where the stratum between the cover 

 and the slide is more than about 10 microns thick. If the 

 object can be put in optical contact with the cover-glass, 

 or within 10 microns of the cover, oil-immersion objectives 



