Notices respecting New Bools. 443 



were observed in the spectrum of hydrogen in the laboratory _ 

 Reproductions' of these historical photographs are given in the 

 'Atlas.' 



In 1864 the nature of Nebulae was still an unread riddle,. 

 Were they composed of fiery mist or were they merely stellar 

 galaxies too remote to be separated into their component stars ? 

 The problem was solved on the evening of August 29, 1864, when 

 for the first time a planetary nebula was examined in the spectro- 

 scope. Three bright lines ! " The answer which had come to us 

 in the light itself read : Not an aggregation of stars, but a luminous 

 gas." Further observations identified one of these lines with the 

 all-pervading hydrogen, but the principal line, although subjected 

 to careful examination, still preserved its incognito as belonging to 

 a substance as yet undiscovered on the Earth. Details are also- 

 given of the spectroscopic examination of a large number of 

 nebula?, one third of which proved to be truly gaseous. 



The determination of the motion of stars in the line of sight 

 by the application of Doppler's principle, suggested itself to 

 Sir William some time in 1862-3, soon after the commencement 

 of his work. He quickly saw how the change of refrangibility 

 might be detected; for as soon as the observations had shown 

 that certain terrestrial substances were present in the stars, the- 

 original wave-lengths of their lines became known and any small 

 want of coincidence of the stellar lines with the same lines pro- 

 duced in the laboratory might be safely interpreted as revealing 

 the velocity of approach or recession between the star and the 

 Earth. In 1868, after many toilsome disappointments, he was 

 able to announce, in a paper published in the Philosophical Trans- 

 actions, the foundation of this new line of research. The method 

 was devised before adequate instrumental power was available for 

 it, and Sir William's observations can be considered as experimental 

 only ; but time has proved how correctly he had forcasted the 

 possibilities and value of the method which has since become so- 

 fruitful in other directions. It gives the power of distinguishing 

 not a few double stars, the components of which are too close 

 to be separated in the telescope, by revealing the difference of 

 their motions in the line of sight as they revolve about their 

 common centre of gravity. The motion of the gases constituting 

 sun-spots has just been determined by the same method, but 

 perhaps most remarkable of all is the recently published Memoir 

 from the Cape Observatory on the spectroscopic determination of 

 the value of the Solar Parallax from the observed changes during 

 the course of the year in the radial velocity of the Earth, relative 

 to a large number of stars. 



New or variable stars did not escape Sir William's attention, and 

 iu 1866 he was the first to discover the compound nature of their 

 spectrum in his observations of T Corona?, whilst his work on 

 Nova Auriga? in 1892 was of extreme importance. 



The spectra of Comets are very fully dealt with. Here again it 

 was Sir William Huggins who first in 1868 showed that in addition, 



