136 report — 1859. 



addition of dilute nitric acid it may be made to work ; but all additions of 

 acetate of soda, carbonate of soda, or acetic acid, are quite inefficacious for 

 correcting a bath that does not work satisfactorily. In order to obtain the 

 extreme point of sensitiveness, the best plan on the whole is to make a new 

 bath ; the silver being, as is well known, easily recoverable from its solutions 

 and in part, by evaporation and crystallization, as nitrate. 



Collodion. — The condition of the collodion is also an all-important point, 

 and it appears to be very capricious in its properties. It is preferable not 

 to make the collodion oneself, but to use that prepared by makers of repute ; 

 I usually employ Thomas's or Hardwich's collodion, both of which I have 

 found to be very uniform in quality. 



It is desirable to sensitize frequently new batches of collodion, and to 

 determine by experiment from time to time the gradual development and 

 decline of their sensitiveness. 



Collodion should not be sensitized until after it has stood for, at least, a 

 week after it has been purchased, and it must then be carefully poured into 

 the mixing vessel without disturbing the sediment which always is present. 

 It must be agitated occasionally for some hours after mixing with the sensi- 

 tizer, before it is set aside to rest and deposit the new sediment which forms. 

 After standing for a week, it should be carefully decanted for use, to the 

 extent of three-fourths, into a perfectly clean glass vessel. 



The glass mixing vessels should invariably, previous to use a second time, 

 be washed out, first with a mixture of equal parts of ether and alcohol, and 

 then with water and pieces of blotting-paper, well shaken up, so as to reduce 

 the paper to pulp; and finally, rinsed out with distilled water, and suspended 

 in a warm place, mouth downwards, to drain and dry thoroughly. 



Iodide of cadmium appears, on the whole, to be the best sensitizer for 

 collodion to be used in celestial photography : collodion, prepared with this 

 salt, is not very active when first mixed ; hence it differs from collodion 

 prepared with iodide of potassium and iodide of ammonium in this respect, 

 but it gradually acquires a degree of sensitiveness unsurpassed, if equalled, 

 by collodion rendered active with the latter salts, used either alone or mixed 

 with other salts. Collodion, mixed with iodide of potassium, acquires, it is 

 true, great sensitiveness soon after it is prepared, but in a few days it loses 

 in this respect, is moreover continually changing, and is seldom available in 

 celestial photography after standing a month or six weeks; whereas cadmium 

 collodion will retain its qualities for several months. As fresh mixed collo- 

 dion is certain to produce both white and dark specks in the photograph, as 

 large or larger than the details visible in the picture with a magnifier, it will 

 be seen that a collodion which can be kept for a long time to deposit, 

 without losing in sensitiveness, must be the most valuable; moreover, in 

 collodion mixed with the alkaline iodides there is always an evolution of free 

 iodine which soon impairs the sensitiveness of the nitrate of silver bath by 

 rendering it acid ; and for these reasons I generally give the preference to 

 cadmium collodion. 



Sometimes collodion exhibits a reticulated structure after the photograph 

 has dried, which materially militates against the beauty of the picture, and 

 prevents its being highly magnified ; it occasionally happens that this defect 

 cannot be cured, in which case the collodion should be rejected. I have 

 generally found, however, that this "craping" may be obviated if the collodion 

 be diluted, more or less, with a mixture of two parts of ether and one part 

 of alcohol when it is being sensitized, care being taken to add as much of 

 the solution of iodide in relation to the diluting liquids as would have to be 

 added to an equal volume of collodion. 



