mean of two microscope readings is desired; and since, in this 

 relation, the value of one division is 2", the mean value sought in 

 seconds is simply the sum of the two drum readings. Other com- 

 binations can be employed by which each sub-division on the drum 

 can be made to represent 5", or 1", as the case may be. 



Adjustments 



1. To secure distinct vision of the cross-wires, the eyepiece 

 must be moved out, or in, until the wires are clearly and sharply 

 defined. This adjustment is independent of all others. It differs 

 for different persons and is the first one to be attended to in using 

 the microscope. 



2. To make an even 

 number of turns of the screw 

 equivalent to a given space, 



Movable X measure the image of the 



(Wres \ space with the screw. If the 



image is too small, the 

 objective must be brought 

 nearer to the graduation and 

 thecrosswiresmovedfurther 

 from the objective; opposite 

 motions of the parts must 

 be made if the image is too 

 large. The tubes carrying 

 the objective and microm- 

 eter box permit such motion. 

 A few trials will make this 

 adjustment sufficiently 

 close. In making this 

 adjustment care must be 



Fig. 84 



taken to avoid parallax which occurs when the cross wires and the 

 image of the object viewed are not in the same plane. It is detected 

 by moving the eye from side to side while looking through the 

 ocular. If the twin wires and image show any relative motion 

 parallax exists. It may be removed (supposing the ocular adjustment 

 to be made) by moving the whole microscope nearer to, or further 

 from, the graduations. 



3. To bring the zero of the comb scale into coincidence with 

 the cross wires when the micrometer drum reads zero, move the 

 comb scale by means of the adjusting screw at the end of the 

 micrometer box. This adjustment need not he very exact since the 

 only office of the scale is to count whole revolutions. It may be 

 also accomplished by moving the micrometer head on the screw 

 shaft since the head is usually held fast by means of a lock nut on 

 the shaft. 



146 



