46 THE CHEMISTRY OF THE SUN. [CHAP. 



inquiry whether some or all of the fixed lines observed by Wollaston 

 and Fraunhofer, may not have their origin in our own atmosphere. 

 Experiments made on lofty mountains or the cars of balloons on the one 

 hand, and on the other with reflected beams which have been made 

 to traverse several miles of additional air near the surface, would 

 decide this point. The absorptive effect of the sun's atmosphere, 

 and possibly also of the medium surrounding it (whatever it be), 

 which resists the motion of comets, cannot be thus eliminated.' 

 Herschel's Astronomy, p. 212, note. See also his Essay on Light, 

 Encyclopedia Metropolitana, art. 505. The object of the experiment 

 now described is to show a method of elimination which applies, at 

 least, to the sun's atmosphere." 



To test this view, then, Prof. Forbes determined to take 

 advantage of the opportunity afforded him by the eclipse 

 to examine the spectrum of the edge of the sun without any 

 interference from the central rays, for, he argued, if the absorp- 

 tion is due to the solar atmosphere, it ought to be more marked 

 at the edge of the sun, from which the light has to pass through 

 a thicker stratum of atmosphere than at the centre, from 

 which the rays proceed vertically and through a minimum of 

 atmosphere. That there is a general darkening of the sun's 

 limb is a well-known fact that can be observed at any time 

 by examining the sun through a dark glass or a fog. Prof. 

 Forbes was probably aware of this, and expected to find the 

 darkening accompanied by an increased selective absorption. 



The result of the observation was that as the eclipse 

 progressed, and the proportion of lateral to central light 

 consequently increased, no change whatever was observed in 

 the number, position, or thickness of the lines, and from this 

 observation Prof. Forbes concluded that 



" This result proves decisively that the sun's atmosphere has 

 nothing to do with the production of this singular phenomenon." 



His conclusion, then, was at variance witli that held by his 

 predecessors, and we now know that Prof. Forbes's conclusion 



