STELLAR PKOTOGRAPHY. 



183 



meridian, would form an image on the western edge of the plate, gradually travers- 

 ing it along a horizontal line. The velocity would be proportional to the focal 

 length, and in this case would be 0.16 in. per minute. If then the plate was moving 

 with the same velocity in the same direction, the star would evidently form a cir- 

 cular dot upon it. A panoramic photograph was thus taken, and the region covered 

 was only limited by the length of the plate. It was proposed to expose in this way 



of plates, one follow 



and thus photograph zones several hours 



in length. On December 6, 1882, the first photograph was taken by this method. 

 On one plate 462 stars were counted where only 55 occur in the Uranometria 

 Argentina. The faintest stars were probably of about the ninth magnitude. The 



images were large and distorted, owing to defects of the lens and to imperfect adjust- 

 ment of the apparatus. Otherwise, much fainter stars would have been pho 

 graphed. For zones in other declinations a conical motion could bo given to the 



by means of a series of curved tracks, or by other devices. On February 21 



o- 



1883, a photograph of Orion was obtained by Mr. W. II. Pickering with a small 

 Voigtliinder camera and without clockwork. Although the aperture was only 1.6 in. 

 and the focus 5.2 in., trails w r ere obtained of stars as faint as the fifth magnitude. 

 Still better results were obtained with a Voigtliinder No. 4 lens (Series B), having an 

 aperture of 2 inches and a focal length of 7 inches. 



Attaching clockwork and giving an exposure of thirty minutes, a photograph 

 was obtained of all the stars down to the eighth magnitude in a region about 15° 

 square. Plans were prepared for photographing a large part of the sky in this way. 

 The lens was mounted equatorially with a long rod attached to the declination axis. 

 A ribbon of brass was fastened to the end of the rod, and by clockwork was wound 

 uniformly around a drum. A second series of photographs w r as undertaken with 

 this same instrument. Six portions of the plate could be brought in turn to the 

 focus of the lens, and three exposures were to be made on each part of the plate. 

 No clockwork was to be used, and the different regions taken on the same part of 

 the plate were to be distinguished by the length of the trail s. This was accomj dished 

 by giving to the various regions a single exposure, a long exposure followed 



short one, or a short exposure followed by a long one. In order to direct the tele- 

 scope quickly to the desired portion of the sky, notches were made in both decli- 

 nation and right ascension circles at intervals of 15°. To reduce the results to a 

 scale of stellar magnitudes, trails were taken with a series of apertures, whose area 



formed a measure of the light transmitted. Another method which is also applicable 

 when clockwork is used, consists in attaching a prism of very small angle to the 



