PROGEESS IN ASTKONOMY. 133 



Another application of the method of long exposures has been the 

 discovery of minor planets by the trails impressed by their motion 

 among the stars on the photographic plates on which the images of 

 both are impressed. 



A complete spectroscopic survey of the stars b}" means of photogra- 

 phy was commenced in 18<S6 at Harvard College, as a memorial to 

 Draper, who died while he was lal)oring diligently and successfully in 

 securing advances in astrophysical inquiries. To carry on this work 

 at Harvard, Professor Pickering wisel}- reverted to the method first 

 employed by Fraunhofer and utilized by Respighi and another in 1871, 

 of placing prisms in front of the object glass. 



In the photographing of stellar spectra 1)V means of objective prisms 

 the driving clock of the telescope must not go exactly at sidereal rate, 

 but at certain speeds, depending on the l)rightness and position of the 

 star under examination. 



This is necessary because the image of the spectrum of a star on 

 the photograph is only a thin line in which it is impossible to see the 

 spectral lines. The spectrum nuist be broadened, and this is accom- 

 plished by making the star image ''trail" to a certain degree on the 

 plate. This trailing is accomplished by means of the clock, the rate 

 of which is made to vary. In this way the trail of a spectrum of a 

 stai on the photographic plate is always obtained of the same width, 

 while the density of the image is made fairly constant by increasing 

 the rate for bright stars and decreasing it for fainter ones. In this 

 way spectra of the brighter stars, rivaling in perfection and detail 

 those obtained of the spectrum of the sun itself thirty .vears ago, have 

 been obtained. Such photographs have rendered a minute chemical 

 classification of the stars possible. 



One of the most interesting applications of photography to spec- 

 trum analysis during the latter part of the century has been the utili- 

 zation by Messrs. Deslandres and Hale of a suggestion made by 

 Janssen, that by employing photography images of the sun and its 

 surroundings can be obtained in light on one wave length. In this 

 way we can study the distribution of any one of the chemical con- 

 stitiuMits of the sun separately and note its behavior, not onh" on the 

 sun itself, but in the atmosphere which enfolds the disk. 



It is strange that, in spite of the suggestion of Faye and others after 

 him, one of the great advantages of the employment of photography 

 in astronomical work, namely, the abolition of ''personal equation,"" 

 has so far been almost entirely neglected. What '* personal equation " 

 is can be perhaps illustrated by considering an observer who is observ- 

 ing the transit of a star over the wires in a transit instrument. 



His object is to note the exact time, to a fraction of a second, when 

 a star passes each wire, and this is done b}' listening to the beats of a 



