44 THE ELEMENTS OF VEGETABLE HISTOLOGY 



sists of two large milled wheels moving a toothed 

 pinion which is in mesh with a rack fixed on the tube. 

 The adjustment is used with lower powers to secure 

 an approximate focus. The fine adjustment is con- 

 trolled by a milled wheel placed on top of the pillar, 

 or by small wheels one on each side of the arm. The 

 mechanism of the fine adjustment varies in different 

 instruments, but in all cases it is the most delicate of 

 the mechanical parts and should be handled with due 

 care. Turning this adjustment toward the right, or 

 toward the microscope, lowers the tube, while turning 

 in the reverse direction raises it. The fine adjust- 

 ment is used with high-power objectives and for exact 

 focus with the lower powers. With high power object- 

 ives the lens is very near the slide and care must be 

 exercised in focusing with the fine adjustment, so that 

 the objective is not forced down on the specimen. 

 Modern fine adjustments are so constructed that when 

 the objective comes in contact with the slide, down- 

 ward motion ceases. The range of tube movement 

 with the fine adjustment is limited, and the lower 

 limit will be indicated by stoppage of the adjustment 

 wheels or an automatic reversal of tube movement. 

 Fine adjustments should not be turned more than 

 five revolutions in either direction; and one should 

 particularly observe this caution when working with 

 microscopes having the fine adjustment on top of the 

 pillar. 



USE OF THE MICROSCOPE 



To obtain good results with a microscope one must 

 be familiar with the construction of the instrument 

 and must pay attention to certain apparently insig- 



