GENEKAL PEELIMINAEY PEEPAKATIONS 183 



possible aberration, due to the lens with which the test is made, 

 is then entirely eliminated. 



One point of considerable importance, affecting both the 

 dark-slide and the focussing-screen, is that they should drop 

 into their position at the back of the camera easily without jar 

 or dislocation of any part of the apparatus. The method of 

 inserting either the one or the other in a long groove 

 should therefore be avoided, and it is sufficient that they 

 drop in at the back either in a very short groove, or even 

 better, are merely held in position with a simple form of catch. 

 It is also important to see that the shutter of the dark-slide 

 moves very easily and sweetly when it is being withdrawn ; 

 nothing is more disconcerting than for any unforeseen jar 

 to disturb any part of the apparatus as the result of removing 

 the focussing-screens, inserting the dark-slide, or withdrawing 

 the shutter, and too much attention can hardly be given to these 

 points to ensure that all the movements are perfectly smooth and 

 can be made with the least possible risk of disturbance. As 

 the room in which the work is being carried out should be either 

 partially or completely darkened, there is very little risk of 

 extraneous light entering the camera at this point ; so that it is 

 not nearly so essential to ensure that the junction between 

 camera and dark-slide is absolutely light-tight, as in the case 

 of a camera used in the field. 



Focussing-screens. The camera should be provided with 

 two focussing-screens : one of ground-glass and the other of plain 

 glass the latter to be patent plate to ensure that there is no 

 inequality of surface. The ground-glass screen should be finely 

 but well ground. It is not recommended that the ground-glass 

 screen or its substitute should be extremely fine, for in the 

 writer's experience this is a point that may be carried too far. 

 If a screen is too translucent, then such a large amount of light 

 is lost by direct transmission that the image itself loses in 

 brilliancy. On the other hand, with too coarse a screen the 

 grain of the screen itself becomes so painfully obvious when 

 focussing that the fine details of the image are lost. It must, 

 however, be pointed out that a great deal of the focussing of 

 ordinary photo-micrographic work is best carried out on the 

 ground-glass screen, so that careful attention to this point is 

 advisable. 



