48 THE ELEMENTS OF VEGETABLE HISTOLOGY 



power field before changing to higher powers, speci- 

 mens should always be located and viewed under low 

 powers even if the high powers are subsequently used. 

 Focusing with High Powers. Raise the tube by 

 means of the coarse adjustment so that the objective 

 is about 10 mm. above the specimen, and place the 

 high power objective (No. 7, No. 8, or 3 mm.) in posi- 

 tion. In the best instruments, the objectives are 

 par-focal, or are so constructed that when one has 

 focused the low-power objective, the other objectives 

 may be immediately turned into position and will 

 be in approximate focus. Adjustments must be very 

 accurate on par-focal objectives and the lenses can 

 only be used on the instrument to which they have 

 been fitted by the maker. As the lenses of high 

 power objectives are smaller than those of low power, 

 and admit less light, the diaphragm opening should 

 be slightly increased. Lower the objective, using the 

 coarse adjustment, until it almost touches the cover- 

 slip, then slowly focus upward until the object is 

 visible. A final regulation of the diaphragm is usually 

 necessary; and the student should keep focusing with 

 the fine adjustment so as to obtain good views of all 

 parts of the specimen. As previously stated, the lens 

 systems of microscopes are corrected for use with a 

 certain tube length, and allowance is made for the 

 refractive effects of the cover-slip. Modern objectives 

 are usually corrected so that they work best with No. 

 2 cover-slips, which range in thickness from 0.17 mm. 

 to 0.25 mm. Differences in cover-slip thickness may 

 be compensated for by varying the tube length, increas- 

 ing it for covers of less than standard thickness and 

 decreasing it for those thicker than standard. 



