ON ODB KNOWLEDGE OF SPECTEUM ANALYSIS. 307 



between aluminium-points, in addition to the usual blue fluorescence 

 exhibited a reddish colour, extending not nearly so far into the cry- 

 stal; this reddish fluorescence was ascertained to be produced by the 

 rays of extreme refrangibility. Prof. Stokes found that as the distance 

 between the electrodes was diminished the reddish fluorescence appeai-ed 

 to increase. Similar experiments with this crystal lead him to the con- 

 clusion that the proportion of rays of extremely high refrangibility is 

 decidedly greater for the spark at the contact-breaker than for the secondary 

 discharge. 



Arc-discharge and lines of blue negative light. — When the electrodes are 

 made to nearly touch, and the spark passes with little noise, a new set of 

 strong lines make their appearance in the invisible region of moderate 

 refrangibility. Although in this mode of discharge the jar has not much 

 influence, the lines in question are better seen when it is suppressed 

 altogether. Under these circumstances the visible discharge is very in- 

 significant, but a very considerable effect is produced in the invisible 

 region. 



From the publication of the papers just referred to until the year 

 1874, we find nothing recorded which materially adds to our know- 

 ledge of the more refrangible i*ays of the ultra-violet spectrum. About 

 this time M. J. L. Soret constructed a spectroscope provided with a 

 fluorescent eyepiece.^ The modification introduced consists essentially 

 in placing a transparent and fluorescent substance at the focus of 

 the object-glass, and, in order to view the spectrum to advantage, 

 having an eyepiece inclined to the axis of the telescope. By means 

 of this spectroscope (still further slightly modified) M. Soret ex- 

 amined the solar spectrum at dififerent altitudes,^ He found that the 

 intetisity of the ultra-violet spectrum is notably greater at high eleva- 

 tions than at the sea-level, but that the spectrum does not extend 

 further. Similar observations were made by Janssen in India : he re- 

 marked that at a great elevation it was possible to distinguish by direct 

 vision ultra-violet rays, which, with the same instrument, could not 

 be distinguished at the level of the sea. M. Soret draws the inference 

 that it is the sun's atmosphere, and not that of the earth, which absorbs 

 the solar rays of a smaller wave-length, a conclusion which is, he says, 

 already admitted by some savants, and which is confirmed by the fact 

 that the light emitted by the edge of the sun exerts a less energetic 

 chemical action than that emanating from the centre. Our atmosphere 

 exerts a twofold absorbing action : the one, due to the vapour of water 

 and to gaseous substances, is elective and gives rise to atmospheric 

 bands ; the other is continuous, and acts with an increasing energy the 

 more refrangible the rays. The latter is pi'obably due to solid or liquid 

 particles in suspension in the air, for on the sky becoming clouded the 

 ultra-violet rays lose much of their intensity, and when the sun is near 

 the horizon they disappear altogether. 



By the publication by M. Soret in 1878, of the results of further in- 

 vestigations, our knowledge of the extremely refrangible rays was some- 

 what advanced. He examined many of the substances previously 

 investigated by Professors Stokes and Miller, and also a considerable 

 number of other bodies. As, however, in most cases the substances 

 employed had not been specially prepared, the conclusions to be dz'awn 



' ArcJdveg des sc. j'hi/x. et tiat, 1874, t. 49. 

 ^ Zoc. cit, t. Ivii. 1876. 

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