NATURAL PHILOSOPHY. 235 



attributable, and almost every instance of success has been attained by those 

 who, adopting one formula, have adjusted their silver bath and developer in 

 accordance ; and, therefore, when the impressions fail to be successful, have 

 only these three items to examine or replace. The silver bath varies in its 

 strength from twenty to sixty grains per ounce of water, and it is on the 

 proper reaction between the collodionized plate and the nitrate of silver in 

 solution all success depends. The most approved formula for the silver bath 

 for the negative and dry processes I believe to be the following: 



Nitrate of silver, that has been fused and probably containing a portion 



of nitrite, 1 ounce. 



Dissolve in 4 oz. water, to which must be added iodide of potassium, 20 grains. 



This mixture must be well agitated at intervals for one hour. By this 

 means the strong solution of nitrate of silver dissolves a portion of the pre- 

 cipitated iodide, and lets fall another portion on the addition of water to 

 make up fourteen fluid ounces. The solution is then filtered and is ready 

 for use. The collodion plates used will present an even primrose-colored 

 surface, without strice or other markings, and the developed impressions will 

 be clear and free from any irregularity of coating. The collodion process 

 negative has, in itself, many advantages. Its great advantage is exquisite 

 sensibility, inasmuch as under favorable circumstances instantaneous impres- 

 sions may be obtained ; at the same time the delicacy of definition is all that 

 can be desired, and if not equal to albumen, it is the fault in the preparation 

 of the pyroxyline from which the collodion is made. It is facile in manipu- 

 lation and speedy in its perfected results. Its disadvantage is the necessity 

 of obtaining the impressions in the camera in the first few minutes from the 

 excitation of the plate ; it is therefore impossible to work far away from the 

 dark closet or the tent in which the plates are prepared and developed. It 

 is this that has prompted experimenters to devise some plan by which the 

 sensitiveness of the plate might be indefinitely prolonged, and I lay before you 

 the various plans proposed, in the order of their publication, as correctly as 

 my means will allow. 



If we excite a collodion plate in the usual way, and leave it in the dark 

 slide until dry, we shall find that the nitrate of silver film, as it concentrates 

 by evaporation, dissolves out the iodide from the collodion surface, and 

 eventually crystallizes in minute stellar groups, completely destroying the 

 utility of the surface as a photographic medium. The prevention of this 

 result was the first problem to be solved if the plates were to be preserved 

 any length of time after preparation. At first various plans were tried to 

 prevent the evaporation altogether, as, for instance, laying a second glass 

 plate directly upon the collodionized surface. Thus Messrs. Spiller and 

 Crooks attempted the same object by steeping the prepared plate in a strong 

 solution of a deliquescent nitrate. At first zinc, and subsequently magnesia, 

 were employed. At a later period, Messrs. Shadbolt and Lyte partially 

 washed off the excess of nitrate of silver from the plate, and poured over it a 

 solution of honey, and I do not think there is, even now, a better process 

 extant. 



