PHYSICS NINETEENTH CENT.-SPECTROSCOPY. 501 



eclipses occurred to Janssen's mind. This was the same that Lockyer 

 was working out ; and Janssen appears to have applied it successfully 

 on the day following the eclipse. From this time until the 4th of 

 September Janssen continued daily his observations of the red promi- 

 nences. These observations give results comparable to such as might 

 have been attained during an eclipse of seventeen days' duration. The 

 enormous rapidity with which is effected the movements of the masses 

 of gas constituting the red prominences was remarked by Janssen, and 

 this has been confirmed by other observers. Mr. Norman Lockyer 

 not only discovered independently the method of spectroscopically 

 viewing the red prominences at all times, but he has also been one of 

 the most assiduous and successful spectroscopists. In a communica- 

 tion to the Royal Society, dated November, 1868, Mr. Lockyer an- 

 nounces that the prominences are merely local aggregations of a gaseous 

 stratum which completely envelopes the sun. He suggests for this layer 

 of gas the term chromosphere, in allusion to its coloured light, while 

 he calls the inner and most luminous layer the photosphere. The 

 light from the photosphere is white or colourless, and this would of 

 itself give a continuous spectrum, but for the presence of a third gaseous 

 stratum, between the photosphere and the chromosphere. This layer is 

 comparatively thin, and it contains those elements which by absorption 

 of certain rays give rise to the Fraunhofer dark lines. It has, there- 

 fore, been called the reversing layer. In the sun there is, first and 

 deepest, the photosphere ; then the reversing layer ; outside of the 

 reversing layer is the chromosphere ; and external to all is the more 

 diffused corona. Mr. Lockyer observed that the spectrum of the chro- 

 mosphere is modified in its lower regions, just as the spectrum of hy- 

 drogen is known by the experiments of Pliicker, Hitorff, and Frank- 

 land to be modified by increased pressure and temperature. The 

 spectroscope, therefore, not only reveals the chemical constitution of 

 the sun, but indicates something of the conditions of temperature and 

 pressure under which the elements probably exist there. Even the 

 motion of masses of glowing gas have their spectroscopic indications. 

 The rapid changes which occur in the forms of the red prominences 

 imply movements of enormous velocity, a velocity which is comparable 

 with that of light. The effect of these rapid movements is to increase . 

 or diminish the periodicity-rate (or wave-length) of the emergent rays. 

 Their refrangibility is affected accordingly, and a slight displacement 

 of the lines is observed towards the violet end of the spectrum if the 

 gas is ascending, and towards the red end if descending. These results 

 are confirmed by a recent observation, in which the spectra of the parts 

 of the margin of the solar disc situated at each extremity of its equa- 

 torial diameter were viewed in juxtaposition, when a displacement of 

 the Fraunhofer lines was distinctly perceptible. The reason of this is 

 that the one part of the sun's surface is approaching towards, the other 

 receding from, the spectator. 



