1875.] the Observation of the Transit of Venus. 79 



Janssen plates ready when required, without hurry ; and so only two plates 

 were taken between the Janssens instead of four, as had been arranged. 



The result of the day's work was 109 six-inch plates taken, but of 

 these two failed entirely, so that only 107 can be counted. These are all 

 fairly clean and free from fog or stains but in many of the plates the images 

 are not so sharp as could have been desired. Though the day was fine and 

 cloudless, there was a good deal of haze and I think the want of sharpness is 

 chiefly due to this and other atmospheric conditions, as the same faults 

 were observed for two or three days after the Transit. 



Of the Janssen plates there were five which also were, for the most 

 part, clean, good plates, fairly well defined though not perfectly sharp. 



Several of the photographs shew marked irradiation round the planet, 

 and a want of sharpness which may be partly due to the atmosphere of the 

 planet, as the limb of the sun is very much sharper. On some of the pic- 

 tures distinct streamers are visible round the limb of the planet and pro- 

 ceeding from it. I have not seen anything of the kind mentioned as being 

 observed by other parties, and, as the appearance is not visible on all the 

 negatives, it is no doubt a form of photographic irradiation ; but, if not, 

 a comparison of the Roorkee negatives with those taken at other places 

 may throw light on the cause of it. 



None of the plates were varnished, as it was considered undesirable to 

 varnish plates intended for future measurement, and also to obviate any 

 chance of the varnished films cracking when removed to England, as is often 

 the case with negatives taken in this country. 



"With the exception of the want of sharpness of some of the plates, the 

 operations may be considered quite successful as far as the mere photography 

 is concei'ned. The arrangements described above and the programme of 

 operations answered admirably and I cannot suggest any improvement. 

 Whether the photographs are sufficiently sharp and perfect in other respects 

 to answer the purpose intended still remains to be seen. 



General Remarks. — During the course of the preparations a good deal 

 of time had to be devoted to putting some of the instruments into proper 

 working order, in which work I was much assisted by Captain Campbell, who 

 had charge of the operations with the great 36 in. theodolite. Thus for some 

 time, the Janssen plates were found to be fogged and so indistinct as to be 

 almost useless. This was due, partly to reflection of light from the polished 

 surface of the wood-work of the slide and the brass-work of the under sur- 

 face of the exposing disc, which was partially obviated by covering with dead 

 black varnish all the surfaces capable of reflecting light on to the sensitive 

 plate, and partly to the ruby -red glass fixed in the revolving disc not being 

 perfectly impervious to the actinic rays, but this defect was overcome by 

 substituting a piece of thick ruby-glass for the thin, light-coloured piece 

 originally supplied. Even with these precautions, some white light found 

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