THE BOOK OF THE LANTERN. Ill 



say two grains to the ounce of water, and is afterwards 

 strengthened by citric acid and silver. 



A far greater importance is attached to the next method 

 under disscussion, by which the very finest results can be 

 obtained ; but it requires, at every stage of the process, 

 such great care that few in these days care to take it up. 

 Still it has a commercial importance, and is known as the 

 Albumen process. Here is a sketch of the operations in- 

 volved in it. 



After the plate of glass has been rendered chemically 

 clean it is coated with a film of albumen from fresh eggs, 

 to which has been added some iodide and bromide of 

 potassium. The plate is then inverted on its pneumatic 

 holder, and revolved by means of a vertical cord attached 

 to the bottom of that holder, so that by centrifugal force 

 some of the albumenous coating is scattered, leaving the 

 thinnest possible film on the glass. The plate is then 

 dried, as yet insensitive to light. Next it is immersed 

 in an acid silver bath for about three minutes, and after 

 washing with several changes of water, a preservative 

 consisting of a saturated solution of gallic acid, is applied 

 to it. Drying by gentle heat completes the manufacture 

 of this form of plate. The development is brought about 

 by a saturated solution of gallic acid, to which has been 

 added a few drops of silver nitrate. 



COLLODIO-BROMIDE PROCESS. 



We will now give our attention to the beautiful 

 Collodio-bromide process, a brief description of which is 

 only necessary, for the collodion emulsion for the im- 



