OP THE MICKOSCOPE. 85 



Of devices for fine movements the name is legion. An old 

 plan is to place the object upon a plate attached to the stage, 

 and move it towards the objective by means of a fine screw. 

 This is a cheap and convenient method. In general, however, 

 it is objectionable to have the object move, as it interferes with 

 many of the finer methods of illumination, etc. A very com- 

 mon plan is to make the nose-piece, which holds the objective, 

 movable. The only objection to this device is that it introduces 

 a change in the length of the body, and, consequently, a slight 

 change in the magnifying power, every time a change is made 

 in the focal adjustment. This change is too slight to be ob- 

 servable, but is sufficient to interfere with delicate micro- 

 metric measurements.* 



To avoid this difficulty, various devices have been intro- 

 duced. The most common arrangement is a very old one, in 

 which the entire body, including the arm and coarse move- 

 ment, is carried by the fine adjustment. In its general features 

 this plan is very old, and has at different times been adapted by 

 Grunow, McAllister, George Wale, Zentmayer and others. One 

 of the best, however, is a recent invention by Mr. Gundlach, in 

 which the body is hung on two parallel springs, and moved by 

 a fine screw. This plan, although theoretically defective, is 

 practically perfect. The other method, just mentioned, 

 although theoretically perfect, is, in practice, rarely as good as 

 the Gundlach device a curious, but not an unusual state of 

 things in mechanics. 



In judging of the goodness of either a fine or coarse adjust- 

 ment, the points to be observed are the delicacy and accuracy 

 with which the objective may be moved to and from the stage, 

 and the freedom from twist or apparent displacement of the 

 object. In many microscopes, when a high power is used, and 

 the body is moved for the purpose of adjusting the focus, the 



*It has been alleged that this increase or decrease of magnifying power 

 is more apparent with the higher powers than with the lower powers. 

 Indeed, it has been said that with high powers the change of magnifying 

 power is qnite perceptible. This, of course, is mere imagination, as any 

 one of an arithmetical turn of mind can see. Indeed, the facts would 

 seem to be rather the other way. 



