The advance of photography : its history and modern applications (1911)

ASTRONOMICAL PHOTOGRAPHY 311 Laboratory Work. — A good illustration of how the work in the laboratory may be made to supplement that in the observatory is to be found in the work of Prof. Fowler on the comparison of the spectrum of o Ceti with that of titanium oxide. Prof. Fowler has carefully examined the spectrum of that oxide, and by comparison with a spectrum of o Ceti obtained by Slipher, has come to the conclusion that nearly all the characteristic flutings observed in the spectra of the Anturian type of stars are produced by the absorption due to titanium oxide. This kind of work would have been next to impossible were it not for the help of photography. Attempts have been made to solve other important problems by the help of photography. Thus, Dr Zencker hoped to be able to trace the path of shooting stars by means of it. Unfortunately, these were found to give out too little light to produce, while they lasted, an impression on the photographic plates then in use. With the much more rapid plates now in use, the trails of meteors are sometimes recorded. Professor Turner has worked out at some length the question as to the information which can be obtained from such trails.1 Transit of Venus. — The transit of Venus affords a grand problem for photography. In determining the distance of heavenly bodies, the diameter of the earth's orbit is taken as a base line ; therefore the knowledge of the exact length of this base is assumed. Now, this amount has only hitherto been determined by approximation, and is in round numbers one hundred and eighty millions of miles. Many efforts have been made to determine this distance more accurately. It is, however, a problem of great difficulty. Let it be conceived that there are at two opposite points of the earth, a and b (fig. 130), two 1 See " Monthly Notices," R. A. 8., vol. lxvii. No. 9, pp. 562 et seq.