1875.] the Observation of the Transit of Venus. 75 



ing two dozen each, with the marked corners running along the upper 

 left hand side of the hoxes. Each hox was then legibly marked with a 

 distinguishing letter and the numbers of the plates contained in it thus 



A 



- A sixth box containing marked plates was kept in reserve to be used 



1-24. 



if required, and it was arranged that any plates so used were to be numbered 



at the time of use with their proper number in order of sequence. 



It was also carefully enjoined on the assistants that the utmost care 



was to be taken to preserve the proper order of sequence of the plates 



throughout the operations, but that if, by accident, a plate should be left 



out or any alteration in sequence occur, the officer in charge should be at 



once informed of it and duly record it. Should any of the plates originally 



numbered be broken during any of the operations or put aside from any 



other cause, their places were to be filled up from the marked plates and 



they were to be numbered in their proper order of sequence. 



Arrangements were made for providing four nitrate of silver baths of 



suitable size for sensitising the six-inch plates and a larger one for the Janssen 



plates ; besides these, two small baths and one large one were kept ready in 



reserve in case of one of the other baths getting out of order or becoming 



temporarily unfit for use. The baths used were new and about 45 grains 



to the ounce (10 - 2 per cent.). 



The collodion used was prepared according to a formula given me by 



Colonel Tennant as follows : — 



Cadmium Iodide, s . . 1 gramme. 



Cadmium Bromide, , . . . . 1 ,, 



Ammonium Iodide, 1 ,, 



Pyroxyline, 4 „ 



Ether, , 110 cub. cents. 



Alcohol, 110 „ „ 



This collodion contained a large proportion of pyroxyline and haloid salts 



and was selected because it was found to give more density of the film 



and intensity of image than the ordinary commercial samples. Two pints 



of it were carefully cleared for use during the Transit. 



A reserve supply of a mixture of Thomas' and Rouch's was also used 



for some of the plates. It was arranged that the collodion should only be 



used once, so that each plate might be coated with fresh collodion, thus 



preserving the uniformity of the films and keeping the collodion free from 



impurities. 



An ample supply of developer was also prepared by the following 



formula : — 



Protosulphate of Iron 55 grammes 



Sugar , , , 55 , , 



