1875.] the Observation of the Transit of Venus. 73 



As soon as arrangements were sufficiently advanced, preliminary drills 

 were commenced with the ohject of finding out the best mode of working - , 

 in the event of dry plates being used, and after a few trials, it was arranged 

 that instead of developing every twelfth dry plate, as proposed by the 

 English observers, every fifth plate should be prepared by the wet process 

 and developed at once to ascertain if all the adjustments were correct, the 

 necessary alterations in the exposure of the plates being arranged by trials 

 beforehand. 



From some cause all the dry plates prepared at Roorkee were covered 

 with spots, some transparent, others opaque and comet-like, and as it was 

 impossible to trace the cause of these spots or to avoid them, even with the 

 most careful precautions, trials were made, about the 17th November, to ascer- 

 tain if the ordinary wet process could be used instead and, after working a few 

 days, it was found that there was no difficulty in keeping a regular supply of 

 plates every two minutes by the use of four sensitising baths. The superior con- 

 venience of working by the wet-plate system and the great saving of time and 

 trouble that would be gained became so manifest that it was definitely de- 

 cided to adopt it and thenceforth the wet plate drills were carried on daily 

 between the hours of 7 and 12, during which the Transit would take place ; 

 as a rule in the early morning and forenoon, alternately, sometimes 

 twice during the same day. Particular attention was given to practising 

 the mounting of the Janssen slide by signal and again unmounting it and 

 resuming the ordinary plates in the interval. 



Although the use of dry plates was said to possess the great advantage of 

 enabling irradiation to be much diminished by the use of albumen in the re- 

 sensitizer and also in reducing the shrinkage of the film to a mininum ; as well 

 as great convenience in preparing and developing the plates at leisure free from 

 excitement or hurry, and in facilitating the working of a large number of plates 

 with a small staff of assistants, the substitution of the wet process had many 

 advantages in avoidingthe very tedious operations of preparing and developing 

 so large a number of plates, which alone would have taken up about two days 

 before and after the Transit, and more particularly in enabling the state of the 

 work to be seen throughout the Transit and any necessary alterations to be 

 carried out immediately. The manipulations of the wet process were per- 

 fectly familiar to all my assistants and by a division of labour they were 

 able to carry on the work with ease and without the slightest confusion. 



By giving the films a substratum I hoped to avoid any shrinkage of 

 the collodion in drying and by placing pieces of wet red blotting paper behind 

 the plates to lessen the tendency to irradiation. 



My programme of operations having been drawn up and approved by 

 Colonel Tennant, the first rehearsal took place on the 28th November with 

 tolerable success, and several points were noticed as requiring modification. 



