188 PEACTICAL PHOTO-MICEOGKAPHY 



mounted on a simple tube or box, or in some similar way, 

 so that the distance of the eye-pieces from the focussing-screen 

 is equal to the distance at which they are placed in the reflex 

 camera for which they are originally intended. 



Position of Microscope in Relation to Camera. A point 

 of primary importance, although it is not often referred to, is 

 to ensure that the optical system is in perfect alignment, both 

 vertically and horizontally, with the camera. This is not 

 always the case, even though the apparatus is made especially 

 for the purpose. If the microscope and other optical parts 

 swing out of centre, as already described, it may happen that 

 they do not return exactly to a position in which the optical 

 axis is directly in alignment with the centre of the camera ; 

 or, again, the microscope itself when brought into a sup- 

 posed horizontal position may not be truly horizontal. Any 

 want of truth in either of these directions may be determined 

 with exactness by observing an image on the focussing-screen 

 of the camera. As a test-object for this purpose nothing is 

 better than an ordinary glass microscopic slip, on which two 

 fine lines have been ruled with a diamond at the centre of the 

 slip exactly at right angles to one another. These are focussed 

 in the microscope with a medium-power objective, and the 

 image is projected on to the ground-glass screen so that 

 the point of intersection of the diamond lines is truly in the 

 centre of the screen of the camera. On moving the back of 

 the camera, carrying the focussing-screen, backwards and 

 forwards from its greatest to its shortest length, no alteration 

 whatever in the position of the image should be observed. This 

 condition is in practice very rarely exactly complied with, 

 and more often than not it will be found that some alteration, 

 either laterally or vertically, occurs in the position of the image. 

 Careful observation will at once show in which direction there 

 is any deviation from correct alignment of the apparatus, and 

 steps must be taken to rectify this. It is evident that 

 the direction of the movement of the object on the screen 

 will be in the opposite direction to the direction of error 

 of alignment. Any such error will often be traced to the 

 microscope itself, since in placing it in position on the base- 

 board, reliance is usually placed on the tripod foot itself being 

 exactly true. Vertical deviation may also generally be traced 



