154 SOME RECENT ASTRONOMICAL EVENTS. 



After Sir William Huggins's first experiments in 1868 and those of 

 Professor Vogel at Potsdam in 1871 the work was taken up at Green- 

 wich and pursued for thirteen years. Those early results were but 

 rough, however, and we owe to the introduction of astronomical 

 photography the present advances in this as in so many other lines. 

 The introduction of photography, and with it the first results of great 

 value for accuracy, date from the observations of Vogel at Potsdam 

 in 1887. Not long after this Professor Keeler, then director of Alle- 

 gheny Observatory, obtained his famous spectrograph^ proof that the 

 rings of Saturn consist of small bodies revolving about the planet in 

 obedience to Keplar's laws and are not continuous rigid sheets of 

 matter as they appear to be in a telescopic view. 



The most celebrated instrument used for these line of sight researches 

 is that known as the Mills spectrograph of the Lick Observatory, with 

 which Professor Campbell, the present director, has made and is still 

 continuing his noted line of sight determinations for all the brighter 

 stars of the northern hemisphere. An illustration of this instrument 

 attached to the 36-inch equatorial is here given (PI. I). The reader 

 may see in the illustration what care is used to avoid temperature dis- 

 turbances. With the Mills spectrograph the accuracy of Professor 

 Campbell's determinations has become very great, so that the probable 

 error of a determination from a single photograph may be, for stars 

 having favorable spectra, far within a single kilometer per second. 



While most of the stars observed have line of sight velocities not 

 exceeding 10 kilometers per second, certain of them give evidence of a 

 far greater rapidity of motion, amounting in the case of C Ilercidis to 

 no less than 70 kilometers per second (or nearly 45 miles). Still more 

 interesting are the variable velocities reported in numerous cases. 

 From evidence of this kind Professor Campbell has concluded, for 

 instance, that the Pole Star is not single as it appears in the tele- 

 scope, but consists of a system of no less than three bodies revolving 

 about in mutually influenced orbits. 



It has become possible with the spectroscope not only to prove that 

 several stars exist where only one is seen with the most powerful tele- 

 scopes, but to determine the time of revolution of such a spectroscopic 

 pair in its orbit, and even with considerable certainty to determine the 

 form and size of this orbit and its inclination to the plane of the eclip- 

 tic, although, as 1 have said, the separate stars are so close and their 

 orbit so circumscribed as never to be seen. 



Line of sight determinations have now become one of the most 

 important features of astrophysical study. A new telescope is to be 

 devoted to this purpose at the Cape of Good Hope Observatory. The 

 Astrophysical Observatory at Potsdam has very recently obtained a 

 new stellar spectrograph of the most approved construction. The 

 Lick Observatory is establishing a branch observatory in South Amer- 



