48 ONTHECONSTRLTCTIONANDIJSEOF 



prep.ired the materials for the rnllorlioii 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. ^\'hen 

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

 graphy, wher(^ the original picture is taken at the focus of an object glass or mirror 

 —imperfections invisible to the naked eye assume an importance Avhich c-auses 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 liglit, and again submitting it to the action of water, 

 and dipping it back into the nitrate of silver bath before developing. Tlu' cjuantity 

 of nitrate of silver necessary to dev(^lopment when pyrogalli<- 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 2j x 'ij, 

 inches are k(^pt in nitric acid and water until wanti'd. They are tlien washed under 

 a tap, benig well rublx^d witli the fing(>rs, which ha\c 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 ilainiel. L'liey 

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

 plate box. This system of cknuiing with collodion was suggested by INlnjor llussel, 

 to whose skilful experiments photography is indebted for tlie taniun })rocess. 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 tiie night's work, a dozen or so, being thus prepared, one 

 of them is tak(m, and by movement through the air is freed from fibres of cotton. 

 It is then coated with filtered collodion 1)eing held near the damp sink. The coated 

 plate, when .sufficiently dry, is immersed in a 40 grain nitrate of sil\ cr bath, acidified 

 with nitric acid until it reddens litmus papcn'. The exact amount of acid in tlie bath 

 makes in this "Washed Plate Process" Imt little dift'ercnce. 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 greasiiiess, 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 tlie teh^scope, and deposited on the sliding plateholder whicli has 

 been set to the direction and rate of the moon, while the plate was in the ]:)ath. 

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

 dnlum. 



The method by which exposure without causing tremor is accomplish(>d, 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 



