NEGATIVE MAKING 55 



coarse, and this indistinctness increases with 

 the magnification. It is simply impossible to 

 focus delicate structures possessing much 

 detail upon so rough a surface. Recourse 

 must be had to something else and several 

 present themselves to our seeking. One is to 

 cement a thin microscopic cover glass with 

 Canada balsam to the ground surface of the 

 screen which clears up that amount of space, 

 permitting very accurate focusing within its 

 limits by means of a focusing glass. If this 

 be placed as is usually done in the center 

 of the screen, only that portion of the object 

 visible through this disc can be focused, and 

 very often this will not answer at all. A 

 remedy may be found in the employment of 

 several glasses cemented to different portions 

 of the screen, but this is often as bad or worse 

 than the disease: it is but a makeshift at best 

 and we will seek some other means of over- 

 coming the difficulty. If a sheet of thin plate- 

 glass or a gelatine dry plate treated in the 

 manner suggested by myself many years ago, 

 be substituted for the ground-glass, either 

 will make a screen, the surface of which is 

 suitable for the most accurate focusing with 

 highest powers, though differing from each 

 other and from the ground-glass in many par- 



