Xciv REPORT — 1869. 



to radiant heat (from whence the transition to light is easy), Kirchhoff was 

 jjreceded, though unconsciously, by our own countryman Mr. Balfour Stewart. 

 The inference which Kirchhoff drew from Provost's theorj^ thus extended led 

 him to make a careful comparison of the places of the dark lines of the solar 

 spectrum with those of bright lines produced by the incandescent gas or 

 vapour of known elements ; and the coincidences were in many cases so re- 

 markable as to eataljlish almost to a certaintj' the existence of several of the 

 known elements in the solar atmosphere, producing by their absorbing action 

 the dark lines coinciding with the bright lines observed. Among other 

 elements may be mentioned in particular hydrogen, the spectrum of which, 

 when the gas is traversed by an electric discharge, shows a bright line or band 

 exactly coinciding with the dark line C, and another with the line F. 



Now Mr. Huggins found that several of the stars show in their spectra 

 dark lines coinciding in position with C and F ; and what strengthens the 

 belief that this coincidence, or apparent coincidence, is not merely fortui- 

 tous, but is due to a common cause, is that the two lines are found asso- 

 ciated together, both present or both absent. And Kirchhoff's theory suggests 

 that the common cause is the existence of hydrogen in the atmospheres of 

 the sun and certain stars, and its exercise of an absorbing action on the light 

 emitted from beneath. 



Now by careful and repeated observations with a telescope furnished with 

 a spectroscope of high dispersive power, Mr. Huggins found that the F line, 

 the one selected for observation, in the spectrum of Sirius did not exactly 

 coincide with the corresponding bright line of a hydrogen spark, which latter 

 agrees in position with the solar F, but was a little less refrangible, while 

 preserving the same general appearance. AVhat conclusion, then, are we 

 to draw from the result? Surely it would be most unreasonable to attri- 

 bute the dark lines in the spectra of the sun and of Sirius to distinct causes, 

 and to regard their almost exact coincidence as purely fortuitous, when we 

 have in proper motion a vera causa to account for a minute difference. And 

 if, as Kirchhoff's labours render almost certain, the dark solar lino depends 

 on the existence of hydrogen in the atmosphere of our sun, we are led to 

 infer that that element, with which the chemist working in his laboratory is 

 so familiar, exists and is siibject to the same physical laws in that distant 

 star, so distant, that, judging by the most probable value of its annual paral- 

 lax, light which would go seven times round our earth in one second would 

 take fourteen years to travel from the star. What a grand conception of the 

 unity of plan pervading the universe do such conclusions present to our 

 minds ! 



Assuming, then, that the small difference of refrangibility observed be- 

 tween the solar F and that of Sirius is duo to proper motion, Mr. Huggins 

 concludes from his measures of the minute difference of position that at the 

 time of the observation Sirius was receding from the earth at the rate of 41*4 

 miles per second. A part of this was due to the motion of the earth in its 

 orbit ; and on deducting the orbital velocity of the earth, resolved in the direc- 

 tion of a lino drawn from the star, there remained 29'4 miles per second as 

 the velocity with which Sirius and our sun are mutually receding from each 

 other. Considering the minuteness of the quantity on which the result de- 

 pends, it is satisfactory to find that Mr. Huggins's results as to the motion of 

 Sirius have been confirmed by the observations of Father Secchi made at Rome 

 with a different instrument. 



The determination of radial proper motion in this way is still in its infancy. 

 It is worthy of note that, unlike the detection of transversal proper motion. 



