THE COARSE ADJUSTMENT 31 



in Figs. 16, 17, etc., is that most generally adopted, and is 

 very convenient, but has the drawback in common with 

 one or two other devices for the same purpose that 

 reliance cannot be placed on a proper centration of the 

 objectives when it is used. The sliding objective changers 

 of Zeiss shown on the stand at Fig. 15, and in section 

 at Fig. 11, allow of an almost instantaneous change of 

 objectives, and at the same time permit of their accurate 

 centration by means of screws. A slider is attached to 

 the microscope tube, and each objective is screwed into 

 another provided with centring screws, and so can be 

 rapidly slipped into position. 



The Mirror. The mirror is always provided with both 

 plane and concave reflecting surfaces, and should be 

 carried on a swinging arm capable of universal move- 

 ments. Some sort of catch to show when the arm is in the 

 plane containing the optical axis is useful, and there should 

 be some means of raising or lowering the mirror to focus 

 the light when using the concave side without a con- 

 denser. 



The Coarse Adjustment. The focussing arrangements 

 are the most important mechanical details of a micro- 

 scope, and the fineness of motion which they impart to the 

 tube must be suitable to the power of the objective in 

 use. On most stands there are a coarse and a fine adjust- 

 ment, the former actuated by a diagonal rack and spiral 

 pinion. But some students' microscopes are focussed by 

 means of a sliding sleeve, a poor substitute, however, 

 for a good mechanical arrangement. 



The coarse adjustment should be sufficiently delicate 

 to focus low-power objectives, and if well made it should 

 work very smoothly and without an} 7 shake. There must 

 also be no tendency for the tube to slide down by 

 its own weight. The range of motion allowed ought 

 to be sufficient to permit of the use of long-focus 

 objectives. An extra range for lenses of 3" focus or 



