129 



distance and, applying- the eye to the tube, elevate 

 the objective by the coarse adjustment until it is 

 in focus. In the lower power objectives, in which 

 working distance is considerable, this may appear 

 unnecessary, but is nevertheless strongly recom- 

 mended, principally to retain the systematic 

 method. In the medium and hig-h powers it is very 

 important as the working distance is small. In 

 these, the front of the objective is brought down 

 nearly in contact with the cover glass and the dis- 

 tance should be judged by holding the head down 

 to the level of the stage and observing- the distance 

 as the objective is brought downward. The rack 

 and pinion is infinitely superior to the sliding- tube, 

 as the distance can be traversed with absolute cer- 

 tainty without any liability to slip. Then apply 

 the eye to the eyepiece and rack upward, slo\vly 

 however, that the point at which the focus is 

 reached may not be overlooked, which may easily 

 be the case in transparent and faintly colored 

 objects. After a better acquaintance with the 

 working distance of the objective the proper dis- 

 tance can be judged quite closely. 



After the focus has been found the fine adjust- 

 ment should be brought into action. This should 

 only be used after the focus has been obtained with 

 the coarse adjustment, in order to focus through 

 the different planes or depths of the object. Its 

 range of movement is necessarily short and at one 



