DRIFTING LIGHT-WAVES. 



THE method of measuring the motion of very swiftly 

 travelling bodies by noting changes in the light-waves which 

 reach us from them one of the most remarkable methods 

 of observation ever yet devised by man has recently been 

 placed upon its trial, so to speak ; with results exceedingly 

 satisfactory to the students of science who had accepted the 

 facts established by it The method will not be unfamiliar 

 to many of my readers. The principle involved was first 

 noted by M. Doppler, but not in a form which promised 

 any useful results. The method actually applied appears 

 to have occurred simultaneously to several persons, as well 

 theorists as observers. Thus Secchi claimed in March, 

 1868, to have applied it, though unsuccessfully; Huggins 

 in April, 1868, described his successful use of the method. 

 I myself, wholly unaware that either of these observers was 

 endeavouring to measure celestial motions by its means, 

 described the method, in words which I shall presently 

 quote, in the number of Fraser's Magazine for January, 

 1868, two months before the earliest enunciation of its 

 nature by the physicists just named. 



It will be well briefly to describe the principle of this 

 interesting method, before considering the attack to which it 

 has been recently subjected, and its triumphant acquittal 

 from defects charged against it. This brief description will 

 not only be useful to those readers who chance not to 

 be acquainted with the method, but may serve to remove 

 objections which suggest themselves, I notice, to many who 



