1878.] Analysis in connexion with the Spectrum of the Sun. 175 



that the behaviour of the h line was hors ligne, and that the whole 

 spectrum could be reduced to one line, F. 



"1. The Fraunhofer line on the solar spectrum, named h by 

 Angstrom, which is due to the absorption of hydrogen, is not visible 

 in the tubes we employ with low battery and Leyden-jar power ; it 

 may be looked upon, therefore, as an indication of relatively high 

 temperature. As the line in question has been reversed by one of us 

 in the spectrum of the chromosphere, it follows that the chromo- 

 sphere, when cool enough to absorb, is still of a relatively high tem- 

 perature. 



" 2. Under certain conditions of temperature and pressure, the 

 very complicated spectrum of hydrogen is reduced in our instrument 

 to one line in the green, corresponding to F in the solar spectrum." 



As in the case of calcium also, solar observation affords us most 

 precious knowledge. The h line was missing from the protuberances 

 in 1875, as will be shown from the accompanying extract from the 

 Heport of the Eclipse Expedition of that year: — 



" During the first part of the eclipse two strong protuberances close 

 together are noticed ; on the limb towards the end these are partially 

 covered, while a series of protuberances came out at the other edge. 

 The strongest of these protuberances are repeated three times, an 

 effect of course of the prism, and we shall have to decide if possible 

 the wave-lengths corresponding to the images. We expect a priori 

 to find the hydrogen lines represented. We know three photographic 

 hydrogen lines : F, a line near G, and h. F is just at the limit of 

 the photographic part of the spectrum, and we find indeed images of 

 protuberances towards the less refrangible part at the limit of photo- 

 graphic effect. For, as we shall show, a continuous spectrum in the 

 lower parts of the corona has been recorded, and the extent of this 

 continuous spectrum gives us an idea of the part of the spectrum in 

 which each protuberance line is placed. We are justified in assuming, 

 therefore, as a preliminary hypothesis, that the least refrangible line 

 in the protuberance shown on the photograph is due to F, and we 

 shall find support of this view in the other lines. In order to deter- 

 mine the position of the next line the dispersive power of the prism 

 was investigated. The prism was placed on a goniometer table in 

 minimum deviation for F, and the angular distance between F and the 

 hydrogen line near G, i.e., H7, was found, as a mean of several 

 measurements, to be 8'. The goniometer was graduated to 15", and 

 owing to the small dispersive power, and therefore relatively great 

 breadth of the slit, the measurement can only be regarded as a first 

 approximation. Turning now again to our photographs, and cal- 

 culating the angular distance between the first and second ring of 

 protuberances, we find that distance to be 3' 15". We conclude, 

 therefore, that this second ring is due to hydrogen. We, therefore, 



