224 On some Recent Spectroscopic Researches. [April, 



Some further considerations and speculations relating to cometary 

 matter, and the phenomena wliich comets exhibit, will be found in 

 a paper presented by the writer to the Koyal Society.* 



The invisibility on ordinary occasions of the bright prominences 

 which appear round the sun at the time of a total solar echpse is 

 not due to the great splendour of the solar disc, for the sun could be 

 easily eclipsed artificially by suitable diaphragms, l)ut to the imper- 

 fect transparency of our atmosphere, which, on this account, near 

 the sun is much brighter than the prominences which lie beyond it. 

 When, however, the great natural diaphragm, the moon, cuts off 

 the sun's rays before they reach the earth's atmosphere, the screen 

 of illuminated air no longer exists, and the fainter phenomena 

 beyond are seen. 



In order to render the prominences visible without an eclipse, it 

 is necessary to reduce by some means the light from the air in a 

 much greater proportion than the light of the prominences. It is 

 possible to do so, because while the solar light of the atmosphere 

 consists of all colours of the spectrum, the hght of the prominences 

 is made up of three refrangibilities only. Any method, therefore, 

 which would spread out the colours, or would produce absorption 

 on certain j^arts of the si^ectrum, would diminish the brightness of 

 the air relatively to that of the prominences, and might render 

 them visible. 



Though to Mr. Lockyer is due the first published statement of 

 the possibility of rendering the prominences visible by the spectro- 

 scope, the same idea had also occm'red, quite independently, to the 

 writer, and to Mr. Stone, F.K.S. The writer tried the method on 

 several occasions without success, in consequence of not knowing in 

 what part of the spectrum the lines of the prominences would be 

 found. When their places had been determined approximately by 

 the observations in India of the solar eclipse, with the same instru- 

 ment, he found the bright hnes at the first moment of looking for 

 them. Nearly three years ago it occurred to the writer that the 

 form and appearance of the objects might be seen by means of 

 coloured glasses and other absorptive media, by which the parts of 

 the spectrum in which the bright lines occur could be isolated, and 

 the light of all other refrangibilities extinguished by absorption. 



By this method, combined with a modified form of spectroscope, 

 he has found that it is possible to see the outlines of these objects. 



In this connection it may be well to refer to an observation 

 made on the spectrum of a solar spot. The sun's surface, when 

 seen under favourable conditions, consists of aggregations of bright 

 bodies, between which are minute spaces, more or less dark, which 

 are kno^\^l as the " pores." Such is the normal state of things, but 



* 'TMiil. Trniis.; ISCS, jip, oGO-C-l. 



