1879.] 



On the Photographic Spectra of Stars. 



21 



lenses of quartz. It is so arranged in the telescope that the image of 

 the star can be brought with certainty upon any part of the slit, and 

 can be kept there during the photographic exposures, by a method of 

 continuous supervision and control. The slit is provided with two 

 shutters, through one-half of which a solar or other spectrum may be 

 taken on the same plate for comparison and the determination of the 

 lines in position in the spectrum. 



The photographs were examined and the lines measured by means 

 of a micrometer attached to a microscope of low power. These 

 measures were reduced to wave-lengths by the help of solar and 

 terrestrial spectra, use being made of M. Cornu's map of the ultra- 

 violet part of the spectrum, and of M. Mascart's determination of the 

 wave-lengths of the lines of cadmium. 



' Photographs have been obtained of the stars Sirius, Vega, 

 ex. Cygni, « Virginis, rj Ursee Majoris, ex. Aquilae, Arcturus, /3 Pegasi, 

 Betelgeux, Capella, « Herculis, and a. Pegasi. Also of the planets 

 Jupiter, Yenus, and Mars, and of the different parts of the moon. 



The spectra of Sirius, Vega, a. Cygni, « Virginis, rj Ursee Majoris, 

 ex. Aquilse, and Arcturus are laid down in a map on the scale of 

 M. Cornu's map of the ultra-violet part of the solar spectrum. 



The stellar spectra extend from about G to O in the ultra-violet. 

 The stars of the white class are arranged so as to exhibit, first, the 

 remarkable spectrum typical of this class, and then some of the 

 modifications it appears to undergo. 



The typical spectrum of this region of this class consists of twelve 

 strong lines winged at the edges. The continuous spectrum extends 

 in the photographs beyond S, but no lines are seen more refrangible 

 than the twelfth line at 3699. Two of these lines agree in position 

 with the hydrogen line (7) 4340, and the other line at li. The third 

 line agrees with Hi. The remaining nine form a group, in which the 

 distance between any two adjacent lines is less as the refrangibility 

 increases, suggesting that they are connected with each other, and 

 represent, probably, one substance. 



A very suggestive modification presents itself in the difference of 

 character of the line H 2 . In these stars this line is either absent or very 

 thin as compared with its appearance in the solar spectrum. In the 

 spectrum of Arcturus, which belongs to the solar type, this line exceeds 

 in breadth and intensity its condition in the solar spectrum. The white 

 stars may, therefore, be arranged in a series in which the line H 2 passes 

 through different stages of thickness, at the same time that the typical 

 lines become narrower and more defined, and other finer lines present 

 themselves in increasing numbers. Arcturus seems to present a 

 spectrum on the other side of that of the sun in the order of change 

 from the white- star group. 



The spectra of the planets were taken on the plan suggested by the 



