Q4< J. Waterhouse — Photography in connection with [No. 2, 



X. Photography in connection with the Observation of the Transit of Venus 



at Boorhee, December 9th (Civil), 1874. — By Captain J. Wateeiiouse, 

 Assistant Surveyor General of India. 



(Eeceived July 30th ;^Eead August 4th, 1875.) 



In December last I communicated to the Society a brief account 

 of tbe proposed arrangements for observing the Transit of Venus at Roor- 

 kee, drawn up by Capt. W. M. Campbell, R. E., and although the popular 

 interest in the subject has now somewhat worn off, a description of the opera- 

 tions connected with the application of photography to the observation in 

 India of this very important astronomical event may not be without 

 interest to the members of the Society, and as a record of experience gained, 

 be useful on a future occasion. 



Object of Photographic Observations. — Without entering into the 

 consideration of the astronomical problems involved, it may be briefly 

 stated that the object in view in making photographic observa- 

 tions of the Transit of Venus was to obtain a series of images show- 

 ing, with the utmost attainable accuracy, the exact relative positions 

 of the planet and the sun at carefully noted times during the pro- 

 gress of the Transit at the different stations of observation ; so that 

 by combining these photographs, the path of the planet across the solar 

 disc might be accurately determined and the solar parallax be estimated by 

 comparing the paths thus deduced for different stations. It was further 

 proposed to endeavour to secure a graphic time-record of the exact 

 moments at which the internal contacts of the planet and the limb of 

 the sun took place, by means of an arrangement enabling a large 

 number of photographic pictures to be taken on a single plate at 

 intervals of a second or so just about the time of contact. It was antici- 

 pated that results of the highest possible value and reliability would be 

 obtained if photographs sufficiently exact to allow of minute micrometrical 

 measurement could be secured, as such photographs would form a permanent 

 and indisputable record, entirely free from the errors and imperfections in- 

 separable from personal observation, and have the further advantage that they 

 might be examined at leisure and, if necessary, carefully compared by seve- 

 ral independent examiners. How far these anticipations have been fulfilled 

 still remains to be seen ; but as several hundred photographs have been ob- 

 tained in various parts of the world by different photographic processes and 

 with dissimilar instruments, sufficient data will probably have been gained 

 to test the value of photography for observations of so delicate a nature 

 and, if this is satisfactorily proved, to show by what methods it may most 

 successfully be applied. 



