Photography of the Skies 



which Mr. Galton introduced in human portrait- 

 ure. That image, for all its minuteness, may bear 

 a most informing superscription. 



In the northwestern sky one may observe the 

 constellation of the Charioteer to most advan- 

 tage in April or May. At Harvard Observatory, 

 in 1889, it was remarked that a spectrum from 

 a star in that constellation, Beta Aurigag, varied 

 from night to night in a singular manner. The 

 cause was found to be that the light comes, not 

 from a single star, but from a pair of stars, period- 

 ically eclipsing each other, and having a period 

 of revolution of slightly less than four days. In 

 determining the rate of motion of these stars as 

 one hundred and fifty miles a second, their dis- 

 tance from each other as 8,000,000 miles, and 

 their combined mass as two and three-tenth 

 times that of the sun, Professor Pickering regards 

 the prism as multiplying the magnifying power 

 of the telescope about five thousand times. To 

 a telescope such a double star appears as but a 

 single point of light; in a spectroscope each com- 

 ponent star reveals its own spectrum. When the 

 star is approaching the earth its spectral lines are 

 shifted to the violet end of the scale; when the 

 star is receding from the earth, its lines are dis- 

 placed to the red end of the scale. In the case of 

 Beta Aurigae the change in the spectrum is so 

 rapid that it is sometimes perceptible in quickly 

 successive photographs, and becomes very 

 marked in the course of an evening: 



" Plate XVII illustrates this phenomenon. In 

 91 



