Barker.l ^^^ [April 1, 



"The prism was always placed with its edges horizontal when the tele- 

 scope was in the meridian. The spectrum then extended north and south. 

 If clock-work was attached, a line of light would be formed, too narrow 

 to show the lines of the spectrum satisfactorily. The usual method (»f re- 

 moving this difficulty is the employment of a cylindrical lens to widen the 

 spectrum; but if the clock-work is disconnected, the motion of the star 

 will produce the same effect. Unless the star is very bright, the motion 

 will, however, be so great that the spectrum will be too faint. It Is only 

 necessary to vary the rate of the clock in order to give any desired width 

 to the spectrum. A width of about one millimeter is needed to show the 

 fainter lines. This distance would be traversed by an equatorial star in 

 about twelve seconds. The longest time that it is ordinarily convenient to 

 expose a plate is about an hour. If then the clock is made to gain or lose 

 twelve seconds an hour, it will have the rate best suited for the spectra of 

 the faintest stars. A mean lime clock loses about ten seconds an hour. It 

 is only necessary to substitute a mean time clock for the sidereal clock to 

 produce the required rate. It was found more convenient, however, to 

 have an auxiliary clock whose rate could be altered at will by inserting 

 stops of various lengths under the bob of the pendulum. One of these 

 made it gain twelve seconds in about five minutes, the other produced the 

 same gain in an hour. The velocity of the image upon the plate when the 

 clock is detached could thus be reduced thirty or three hundred and sixty 

 times. This corresponds to a difference of 3.7 and 6.1 magnitudes respec- 

 tively. Since the spectrum of a star of the second magnitude could be 

 taken without clock-work, stars of the sixth and eighth magnitudes re- 

 spectively could be photographed equally well with the arrangement de- 

 scribed above." 



The work already undertaken in this direction developed so rapidly that 

 the Bache appropriation soon proved entirely inadequate to carry it fur- 

 ther. Whereupon early in 1886, Mrs. Henry Draper, who from the first 

 had taken a great interest in this work as a continuation of that so auspi- 

 ciously begun by Dr. Draper himself, generously came forward and agreed 

 to place at Prof. Pickering's disposal, not only the excellent eleven 

 inch photographic telescope which Dr. Draper had so successfully used 

 in his spectrum researches, but also a sufficient sum of money to enable 

 the experiments already suggested to be fairly tried. In consequence. 

 Prof. Pickering decided to continue the investigation along three more or 

 less independent lines : First, he purposed to make a general survey of 

 stellar spectra, each spectra being photographed with an exposure of not 

 less than five minutes. These photographs exhibit in general the spectra 

 of all stars brighter than the sixth magnitude, with sufficient distinctness 

 for measurement. Second, he desired to undertake a determination of the 

 spectra of the fainter stars, each photograph of this set receiving an expo- 

 sure of an hour. All stars not fainter than the ninth magnitude, and in- 

 cluded in a region ten degrees square, are represented upon a single plate. 

 The work in both these directions has been done thus far with the Bache 



