4 EEPOET — 1891. 



In no science, perhaps, does the sober statement of the results which 

 have been achieved appeal so strongly to the imagination, and make so 

 evident the almost boundless powers of the mind of man. By means of 

 its light alone to analyse the chemical nature of a far distant body ; to be 

 able to reason about its present state in relation to the past and future ; 

 to measure within an English mile or less per second the otherwise in- 

 visible motion which it may have towards or from us ; to do more, to 

 make even that which is darkness to our eyes light, and from vibrations 

 which our organs of sight are powerless to perceive to evolve a revelation 

 in which we see mirrored some of the stages through which the stars 

 may pass in their slow evolutional progress — surely the record of such 

 achievements, however poor the form of words in which they may be 

 described, is worthy to be regarded as the scientific epic of the present 

 centni'y. 



I do not purpose to attempt a survey of the progress of spectroscopic 

 astronomy from its birth at Heidelberg in 1859, but to point out what we 

 do know at present, as distinguished from wiiat we do not know, of a few 

 only of its more important problems, giving a prominent place, in 

 accordance with the traditions of this chair, to the work of the last 

 year or two. 



In the spectroscope itself advances have been made by Lord Rayleigh 

 by his discussion of the theory of the instrument, and by Professor Row- 

 land in the constrnction of concave gratings. 



Lord Rayleigh has shown that there is not the necessary connection, 

 sometimes supposed, between dispersion and resolving power, as besides 

 the prism or grating other details of construction and of adjustment of a 

 spectroscope must be taken into account. 



The resolving power of the prismatic spectroscope is proportional to 

 the length of path in the dispersive medium. For the heavy flint glass used 

 in Lord Rayleigh's experiments the thickness necessary to resolve the 

 sodium lines came out 102 cm. If this be taken as a unit, the resolving 

 power of a prism of similar glass will be in the neighbourhood of the sodium 

 lines equal to the number of centimetres of its thickness. In other parts 

 of the spectrum the resolving power Avill vary inversely as the third 

 power of the wave-length, so that it will be eight times as great in the 

 violet as in the red. The resolving power of a spectroscope is therefore 

 proportional to the total thickness of the dispersive material in use, 

 irrespective of the number, the angles, or the setting of the separate 

 prisms into which, for the sake of convenience, it may be distributed. 



The resolving power of a grating depends upon the total number of 

 lines on its surface, and the order of spectrum in use ; about 1,000 lines 

 being necessary to resolve the sodium lines in the first spectrum. 



is it is often of importance in the record of observations to state the 

 efficiency of the spectroscope with which chey were made, Professor 



