28 KEPORT 1891. 



and fourth spectra of a grating with 14,438 lines to the inch. The mar- 

 Tellous accuracy attainable in his hands on a suitable star is shown by 

 observations on three nights of the star Arcturus, the largest divergence 

 of his measures being not greater than six-tenths of a mile per second, 

 while the mean of the three nights' work agreed with the mean of five 

 photographic determinations of the same star at Potsdam to within one- 

 tenth of an English mile. These are determinations of the motions of a 

 sun so stupendously remote that even the method of parallax practically 

 fails to fathom the depth of intervening space, and by means of light- 

 waves which have been, according to Elkin's nominal parallax, nearly 

 200 years upon their journey. 



Mr. Keeler with his magnificent means has accomplished a task 

 which I attempted in vain in 1874, with the comparatively poor appli- 

 ances at my disposal, of measuring the motions in the line of sight of 

 some of the planetary nebulas. As the stars have considerable motions 

 in space it was to be expected that nebulae should possess similar motions, 

 for tbe stellar motions must have belonged to the nebnlse out of which 

 they have been evolved. My instrumental means, limiting my power of 

 detection to motions greater than twenty-five miles per second, were in- 

 sufficient. Mr. Keeler has found in the examination of ten nebulae 

 motions varying from two miles to twenty-seven miles, with one excep- 

 tional motion of nearly forty miles. 



For the nebula of Orion, Mr. Keeler finds a motion of recession of 

 about ten miles a second. Now this motion agrees closely with what it 

 should appear to have from the drift of the solar system itself, so far as 

 it has been possible at present to ascertain the probable velocity of the 

 sun in space. This grand nebula, of vast extent and of extreme tenuity, 

 is probably more nearly at rest relatively to the stars of our system 

 than any other celestial object we know ; still it would seem more likely 

 that even here we have some motion, small though it may be, than that 

 the motions of the matter of which it is formed were so absolutely 

 balanced as to leave this nebula in the unique position of absolute immo- 

 bility in the midst of whirling and drifting suns and systems of suns. 



The spectroscopic method of determining celestial motions in the 

 line of sight has recently become fruitful in a new but not altogether un- 

 foreseen direction, for it has, so to speak, given us a separating power 

 far beyond that of any telescope the glass-maker and the optician could 

 construct, and so enabled us to penetrate into mysteries hidden in 

 stars apparently single, and altogether unsuspected of being binary 

 systems. The spectroscope has not simply added to the list of the 

 known binary stars, but has given to us for the first time a knowledge 

 of a new class of stellar systems, in which the components are in some 

 cases of nearly equal magnitude, and in close proximity, and are re- 

 volving with velocities greatly exceeding the planetary velocities of our 

 system. 



