&04 Mr. G. J. Stoney on the Solar Eclipse, August 1868. 



itself into a multitude of bright lines, the brightest being coin- 

 cident with the faintest of Fraunhofer's lines. If this should 

 prove to be the case, and if the observer could train himself to 

 distinguish in the hurry and under such novel circumstances* 

 the lines of the different gases, it would even be possible to as- 

 certain how high in the sun's atmosphere each reaches, by using 

 a curved slit, and noting the moment at which each set of lines 

 is obliterated by the advancing moon. This would be a deter- 

 mination of exceeding interest. The observations should com- 

 mence immediately after the beginning of totality and be kept 

 up to the end of it, as it is only from situations close to the sun's 

 disk that the brightest lines can come. 



Directly outside the photosphere there lies a stratum of the 

 sun's atmosphere which is still hotter than the photosphere ; and 

 on the outer boundary of this hot region there appears to be a 

 shell of excessively faint cloud, part of which is to be seen in 

 Mr. De la Rue's photographs of the eclipse of 1860. It pro- 

 bably extends the whole way round the sun. It is therefore 

 very desirable that this faint shell, which seems to lie at a dis- 

 tance of 8 or 10 seconds of space from the edge of the sun's 

 disk, should be observed, both from a central station and from 

 stations close to the northern and southern limits of totality, so 

 as to ascertain whether, as we have reason to presume, it is con- 

 tinuous round the disk. For this purpose telescopes of moderate 

 power, and any way mounted, would suffice. 



It is not likely that there will be many spots; but if any 

 should present themselves upon the edge of the disk within a 

 week of the eclipse, they should be observed for some days before 

 and after, with a view to learning whether they are related to 

 ascending clouds in the way pointed out in the memoir. These 

 observations could probably be best made at Kew ; but to guard 

 against bad weather it might be advisable to associate with Kew 

 some other observatory. Possibly there may be records in ex- 

 istence for instituting this inquiry in regard to the eclipse of 

 1842, 1851, or 1860. It is perhaps not impossible that the 

 heavy mass of cloud marked gg l in Mr. De la Rue's diagram of 

 the eclipse of 1860 may have been the source of a cyclone which 

 occasioned the neighbouring group of spots. If so, the cloud 

 must have afterwards drifted a good deal towards the pole (which 

 is the direction in which the probable course of the solar trade- 

 wind would have carried it), and also come through the rotation 

 of the sun into a position in which it was much foreshortened. 



* One of the most novel of these circumstances is that the spectrum of 

 each gas will present itself with inverted intensity, the brightest lines ob- 

 tained by artificial means being in general those which are dimmest in the 

 corona, and vice versa. 



