OF THE MICROSCOPE. 119 



i 



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. It has been objected 

 to it that the object is "tilted," as it were, but this is imper- 

 ceptible in practice, and causes no difficulty. The objection 

 is. that when the object moves, many of the finer methods of 

 illumination are disturbed. Another common plan is to make 

 the nose-piece, which holds the objective, movable. This alters 

 the length of the body, and changes the magnifying power 

 every time a change is made in the focal adjustment. This 

 change is too slight to be observable, but it is sufficient to inter- 

 fere with delicate micrometric measurements.* 



To avoid this difficulty, the entire body and its attachments, 

 including the coarse movement, are carried by the fine adjust- 

 ment. In its general features, this plan is a very old one, and 

 at least a dozen modifications of it have been devised by dif- 

 ferent makers. As usually constructed, the body is raised by 

 means of a lever, the long arm of which is acted upon by a 

 delicate screw. A strong spring is arranged to lower the body, 

 and as the spring maintains a continuous action, all lost motion 

 is prevented. In the fine motion used by the Bausch & Lomb 

 Optical Co., and which was invented by Mr. Gundlach, the body 

 is suspended on two parallel springs, slides on carefully planed 

 ways, and is moved by a fine screw. 



Objectives of large aperture are so sensitive to slight varia- 

 tions in focal adjustment, that it is difficult to get a fine adjust- 

 ment sufficiently delicate. Mr. Gundlach has recently at- 

 tempted to increase this delicacy by using the well-known dif- 

 ferential screw, and with very satisfactory results. 



In judging of the quality of either a fine or coarse adjustment, 

 the points to be observed are the delicacy and accuracy with 

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



*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 quite 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. 



