18 ON THE CONSTRUCTION AND USE OF 



prepared the materials for the collodion process, then recently invented by Scott 

 Archer. We produced in 1856 many photographs under a power of 700 diameters, 

 by the means described in the next section. 



At first the usual processes for portrait photography were applied to taking the 

 Moon. But it was soon found necessary to abandon these and adopt others. When 

 a collodion negative has to be enlarged and this is always the case in lunar photo- 

 graphy, where the original picture is taken at the focus of an object glass or mirror 

 imperfections invisible to the naked eye assume an importance which causes the 

 rejection of many otherwise excellent pictures. Some of these imperfections are 

 pinholes, coarseness of granulation in the reduced silver, liability to stains and mark- 

 ings, spots produced by dust. 



These were all avoided by washing off the free nitrate of silver from the sensitive 

 plate, before exposing it to the light, and again submitting it to the action of water, 

 and dipping it back into the nitrate of silver bath before developing. The quantity 

 of nitrate of silver necessary to development when pyrogallic acid is used, is how- 

 ever better procured by mixing a small quantity of a standard solution of that salt 

 with the acid. 



The operation of taking a lunar negative is as follows. The glass plates 2f x 3-j 

 inches are kept in nitric acid and water until wanted. They are then washed under 

 a tap, being well rubbed with the fingers, which have of course been properly cleaned. 

 They are wiped with a towel kept for the purpose. Next a few drops of iodized 

 collodion are poured on each side, and spread with a piece of cotton flannel. They 

 are then polished with a large piece of this flannel, and deposited in a close dry 

 plate box. This system of cleaning with collodion was suggested by Major Eussel, 

 to whose skilful experiments photography is indebted for the tannin process. It 

 certainly is most effective, the drying pyroxyline removing every injurious impurity. 

 There is never any trouble from dirty plates. 



The stock of plates for the night's work, a dozen or so, being thus prepared, one 

 of them is taken, and by movement through the air is freed from fibres of cotton. 

 It is then coated with filtered collodion being held near the damp sink. The coated 

 plate, when sufficiently dry, is immersed in a 40 grain nitrate of silver bath, acidified 

 with nitric acid until it reddens litmus paper. The exact amount of acid in the batli 

 makes in this " Washed Plate Process" but little difference. When the iodide and 

 bromide of silver are thoroughly formed the plate is removed, drained for a moment, 

 and then held under the tap till all greasiness, as it is called, disappears. Both front 

 and back receive the current in turn. 



It is then exposed, being carried on a little wooden stand, Fig. 43, covered with 

 filtering paper to the telescope, and deposited on the sliding plateholder which has 

 been set to the direction and rate of the moon, while the plate was in the bath. 

 The time of exposure is ascertained by counting the beats of a half-second pen- 

 dulum. 



The method by which exposure without causing tremor is accomplished, is as 

 follows : A yellow glass slides through the eyepiece-holder, Fig. 33, just in front 

 of the sensitive plate, and is put in before the plate. The yellow-colored moon is 

 centred on the collodion film, and the clepsydra and slide are set in motion, the 



