PRACTICAL HISTOLOGY. 



FIG. 32. Nose-piece, made by Swift. 



rotated round a vertical axis, so as to bring the one or the 

 other lens under the tube. In the original form of this 



apparatus both lenses re- 

 mained vertical. Powell 

 and Lealand, however, im- 

 proved it by causing the 

 lens not in use to project 

 free from the stage. The 

 form shown in Fig. 32 is 

 a still further improve- 

 ment made by Swift, in 

 which the objective is 

 readily centred accurately 

 under the tube. The cap 

 over the lens not in use 

 should always fit closely 

 to prevent the entrance 

 of dust. The nose-piece may be made to carry three or 

 even four lenses. 



47. The Adjustments. These are two in number, 

 the coarse and thejzne. The former moves the tube with 

 the lenses rapidly, the latter moves it slowly up or down, 

 so that the objective may be quickly brought to its focus 

 with the former, and then very accurately by the latter. 

 The fine adjustment is almost always a fine screw (H, Fig. 

 i) ; the fingers are constantly kept upon it during the 

 examination of an object, in order to slightly change the 

 focus and examine every stratum of it thoroughly. If, 

 therefore, the screw be not sufficiently low in position to 

 allow the arm to rest on the table, the fatigue becomes very 

 inconvenient. The arrangement in Fig. 'i serves the pur- 

 pose fairly well. 



In such a microscope as that shown in Fig. i the coarse 

 adjustment is effected by seizing the lower milled head on 

 the tube (D), and giving it a rapid spiral motion. The 

 only disadvantage of this is, that the tube is sometimes apt 

 to jerk, and so to bring the lens suddenly down on the 

 object. Moreover, unless the tube be kept quite clean, 

 verdigris accumulates and it becomes stiff. Still, the pre- 



