160 PHOTOMICROGRAPHY 



first formed and give a deep blue solution. Copper 

 sulphate or copper acetate solutions by themselves only 

 absorb the red. 



The transmissions of the " M " gelatine screens are 

 given on page 174. 



Cells or Troughs. Cells to hold solutions are sold in 

 various forms. The Kingsford trough, for instance, 

 consists of two glass discs held together by a brass rim 

 drawn round them by screws. India-rubber is used to 



,, ... . 





Fig. 49 



TROUGHS FOR LIGHT FILTERS 



make the joints tight and keep the glass sides at the 

 requisite distance apart. Troughs made by Ley hold's 

 process, entirely of glass, are supplied by Messrs. Zeiss, 

 who also make a globe about 9 cm. in diameter which 

 acts as a trough and condenser combined. It is very easy 

 to make serviceable cells in the following manner : 

 depressions are sunk in two pieces of wood of the size of 

 the glass to be used, 2" or 3" square is all that is necessary ; 

 into these the glasses are placed, and the two mounts are 

 screwed tightly together, a piece of india-rubber sheet, 

 say y thick, cut V-shaped, or as part of a circle, being 



