ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 135 



hands, I take up the plate by the edge, being now very careful not to touch 

 the surface with the hand, and wipe it, first with one cloth, then thoroughly 

 dry with the second, and lastly, rub both surfaces at the same time with the 

 dry wash-leather. I afterwards breathe on each side of the plate, to ascertain 

 whether it is clean, wipe off the condensed moisture and place the plate in a 

 grooved box, with the best surface turned to face a marked end of the box, 

 so as to know on which side to pour the collodion. Proceeding in the above- 

 described manner, I have never any failure attributable to a dirty plate, and can 

 feel certain of obtaining four or five good pictures of the moon out of about 

 seven plates generally used. I am usually, however, provided with one or two 

 dozen cleaned plates, for it is desirable to have a sufficient reserve, and 

 experience has proved that plates so cleaned may be used even after a week, 

 if the box containing them be kept in a dry room. 



The Bath. — It is of the utmost importance that the nitrate of silver bath 

 should be in the most sensitive condition ; the rapidity of the process appears 

 to depend in a great measure on its not being in the slightest degree acid, 

 but as nearly neutral as possible. It is almost needless to add that, for 

 such a refined application of photography as that under consideration, the 

 solution should be kept in glass in preference to gutta percha. The vessel 

 must be carefully covered, to exclude dust, and, from time to time, the 

 solution should be filtered through pure filtering paper (Swedish paper). 

 The nitrate of silver used in the preparation of the bath is invariably fused 

 in my own laboratory, in quantities never exceeding a drachm at one time, 

 the requisite heat being gradually applied, and care being taken not to raise 

 the temperature higher than is necessary to effect the fusion. 



The solution I employ is the ordinary one of thirty grains of nitrate of 

 silver to the ounce of water, with a quarter of a grain of iodide of potassium. 

 In the preparation of a bath, after the mixing of the nitrate of silver, dissolved 

 in a small portion of the water, with the solution of iodide of potassium, it 

 is customary to add the remaining chief bulk of water, which causes 

 an immediate precipitation of iodide of silver, and then to filter the liquid 

 after the lapse of half an hour. It is, however, advisable to agitate the 

 solution from time to time, during several hours before it is filtered ; for 

 unless this be done, the bath does not become thoroughly saturated with 

 iodide of silver, and has a tendency for some time to dissolve a portion of 

 the iodide of silver which first forms in collodion immersed in it. 



I avoid adding alcohol or acetic acid to the bath, for these substances 

 impair its sensitiveness. As, after use for a certain time, the bath becomes 

 charged with more or less alcohol and ether, and their products of oxidation, 

 its properties become changed, and a picture cannot be taken with it with 

 sufficient rapidity ; when I find this to occur, I discard the bath and make a 

 fresh one. The bath, in its most sensitive state, usually exhibits a very 

 feeble alkaline reaction with reddened litmus paper, and if it be found to 

 have a tendency to fog, it is corrected in this way : — A single drop of pure 

 nitric acid is taken on the point of a glass rod, and mixed with a drachm of 

 distilled water; with this diluted acid (1 to 60) I moisten the point of the 

 glass rod and stir it about well in the bath, which contains about fourteen 

 fluid ounces of solution, and make a trial. If it still fogs, the acidification 

 is repeated ; and thus, after several trials, the fault is corrected. It is better 

 to proceed in this manner than to rely on litmus papers as a test for neu- 

 trality ; the object being to retain the bath in as sensitive a state as possible, 

 the test by light is the only one to be ultimately depended on. 



Moist hydrated oxide of silver may be used to bring back a bath, which 

 has become acid by use, to a neutral state, and by the subsequent careful 



