184 PBACTICAL PHOTO-MICEOGEAPHY 



There are many substitutes for ground-glass that have been 

 recommended at various times, but they nearly all have the 

 disadvantage that although their grain is fine, the screen 

 itself is too translucent, thus resulting in considerable loss of 

 light. In practice it will be found that ground-glass that is a 

 little coarser than the finest that can be obtained is the most 

 efficient, and, if used in conjunction with suitable focussing- 

 glasses to be described, there are many cases where exact 

 focussing can be effected without recourse to any other screen. 

 In cases where fine detail and a particular portion only of an 

 object is to be focussed, then recourse must be had to the 

 plain-glass screen. This should have some scratches made by 

 a diamond in the direction of the diagonals of the plate, and 

 these scratches should be carefully focussed with the focussing 

 eye-piece before any actual focussing of the microscopic image 

 is attempted. 



An alternative method, and one which may be adopted 

 where only the ground-glass screen is provided with the camera, 

 is to cement on to the centre of the screen with Canada balsam 

 an ordinary microscopic cover-glass. The Canada balsam has 

 the effect of making this small area perfectly clear, effectually 

 eliminating the ground-surface, and the image can therefore at 

 the same time be observed over the whole area of the screen. 

 For fine focussing purposes this central clear area will often be 

 found sufficient, but it is open to the objection that the clear 

 area is very limited, and consequently, should the part to be 

 focussed lie at any other spot than the centre, it cannot satis- 

 factorily be observed. 



A substitute for the ground-glass surface may be made by 

 coating a piece of patent or thin plate-glass with a matt varnish 

 made as follows : 



Sandarac 1 gin. 



Mastic . . . . . 0-25 gm. 



Ether ..... 6 c.c. 



Benzole . . . . from 2 to 4 c.c. 

 The finer the surface required, the greater the degree of dilution 

 of this solution that is, more benzole should be added so that 

 a finer film is deposited on the glass. This will give a very fine 

 focussing-screen, but it is open to the objection that the matt 

 surface is very liable to abrasion, and in course of work, 



