6 PKACTICAL PHOTO-MICROGKAPHY 



steadiness at least, was preferable, and have been inclined 

 to construct their stands with a more stable base, in some 

 cases reproducing the English method. The consequence 

 will probably be that some combination of the two methods 

 will be evolved. Should it result in the elimination of the 

 want of stability that usually obtains in stands of the Conti- 

 nental type, then much will have been accomplished. 



In choosing a microscope for photography the following 

 points must be considered, and indeed for any serious work 

 are essential. The foot should rest on three points, which 

 should be as widely separated as possible, and should be heavy : 

 in fact, the instrument should be so built that it requires no 

 clamping down to its support when in a horizontal position. 

 The body -tube, containing the eye-piece and objective, should 

 have a coarse adjustment actuated by rackwork, which should 

 move the tube to and fro with absolute precision. There 

 should be no evidence whatever of backlash that is, inclination 

 for the teeth on the wheel that engage in the rackwork to shake 

 or fail to move the tube the moment the head of the screw is 

 turned ; while at the same time the movement should be 

 sufficiently easy to avoid the necessity for anything like undue 

 strain when the tube is being moved. 



The rackwork should move in the direction of the optic 

 axis only ; so that if an object on the stage is observed while 

 the rack is moved, there should be no evidence whatever of 

 lateral displacement of the image. 



The fine adjustment is of primary importance, and any want 

 of precision in its design or construction renders an otherwise 

 good instrument practically useless. There are many types of 

 fine adjustment : in fact, each maker pins his faith to some 

 particular method, but it is not within the scope of this work 

 to go into the details of design. It should be tested by putting 

 an object on to the stage of the microscope, and an observation 

 should be made with an objective of fairly high power. The 

 fine -adjustment screw should be lightly but firmly grasped 

 on either side by the thumb and, preferably, the second finger, 

 so that an even pressure is exerted without strain. On turning 

 the fine -adjustment screw no evidence of movement of the 

 object should be discernible : it should simply come in and 

 out of focus as the screw is gently revolved. It is nowadays 



