210 STELLAR PHOTOGRAPHY. 



Four exposures of five minutes each were made, setting the telescope at —10', 0°, 

 +10°, and +20^ The first of the two stops was used in each case, so that each 

 spectrum had a width of about a milhmetre. As the deviation of the prism is 

 about d\ the centre of the region photographed at the first exposure had a decli- 

 nation of —19°, and extended from —24' to — 14°. The four exposures covered a 

 region forty minutes of time in width, and extending from — 24° to +16°. Thirty-six 

 plates are required to complete this series, taken at intervals of forty minutes 

 sidereal time, and beginning at 0'' 20™. Thirty-six more plates are taken at inter- 

 vals of forty minutes, beginning at 0'' 0"', the declinations being diminished by five 

 degrees. They cover the region from — 29° to +11°, and stars near the corners of 

 the plates in the first series are near the centres of the second series of plates. 

 The region from +11° to +56° is similarly covered by seventy-two plates, arranged 

 as before, in two series. The settings in declination for the first of these are 30°, 

 40', 50°, and 60°, and of the second series 25°, 35°, 45°, and 55°. The length of 

 each exposure is five minutes. Finally, the northern stars are included in thirty- 

 six plates arranged in two series. Tlie first of these contain three exposures, set- 

 ting at declinations of 70°, 80°, and 90°, and at intervals in right ascension of one 

 hour and twenty minutes. The right ascension of the centre of the first plate of 

 this series is 0'' 40'". For the second series of eighteen plates, the declinations are 

 diminished by 5°. In right ascension, they lie midway between those of the other 

 series, the first being at 0'' 0". 



Photographs were also taken of the spectra of the fainter stars in certain regions. 

 The auxiliary clock was set so that it should gain about ten seconds in an hour, and 

 a single exposure of about an hour was made upon each plate. The work of 

 photographing the entire sky by this process proved to be too large to be under- 

 taken by the aid of the Bache Fund. Fortunately, Mrs. Henry Drapei', as a 

 memorial to her husband, has made provision for continuing this investigation at 

 the Observatory of Harvard College. The results will therefore be described more 

 fully elsewhere. 



Bright Stars in the Pleiades. 



As an example of some of the results to be derived from stellar photographs 

 such as have been described above, the following examination has been made of 

 several photographs of the Pleiades. The relative brightness of the principal 

 components of this group has been determined from four photographs, Nos. 209, 

 248, 327, and 361. The first of these plates was taken on December 15, 1885. It 



