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XII. 



REGISTRATION OF STAR-TRANSITS BY PHOTOGRAPHY. 



By SIR HOWARD GRUBB, F.R.S., Vice-President of the Royal 



Dublin Society. 



[Eead, November 17 ; Keceived for Publication, November 20, 1903 ; 

 Published, February 10, 1904.] 



Notwithstanding the great assistance that photography has 

 proved in many branches of astronomical work, very few serious 

 attempts have been made to register transits by its means, though 

 at first sight it would seem to be eminently fitted for this particular 

 work. 



It seems comparatively an easy problem to cause the star- 

 image passing across a photographic plate to form its own register 

 by some such means as instantaneously obscuring the image at 

 stated intervals, thus forming a broken instead of a continuous line, 

 the breaks corresponding to certain seconds of the standard clock. 



Such an arrangement has been tried with fair success by 

 Mr. W. E. Wilson, f.r.s., amongst others ; but at or near the 

 Equator, where the apparent movement of the star is greatest, and 

 when observations are of the most importance, it is only the larger 

 stars that can be thus treated. Smaller stars are not sufficiently 

 brilliant to impress their trail even on the most sensitive plates, 

 and photographers will understand that it is not desirable to use 

 highly sensitive plates for this purpose, as the higher the sensitive- 

 ness, the coarser the granulation of the film ; and when delicate 

 microscopical measures have afterwards to be taken of the positions 

 of the star-images, these highly sensitive but coarsely-grained 

 plates are unsuitable. 



In order, therefore, to provide an efficient photographic transit 

 instrument, it is necessary to cause the plate to travel with the 

 star-image, and register the seconds by some other means ; but 

 here a difficulty arises, for the star-images pass across the plate at 

 speeds varying with the angle of declination north or south of the 

 Equator. 



