1883.] 



on the Ultra- Violet Spectra of the Elements. 



249 



altitude, and making the observation from an elevated station (the 

 Riffelberg), the solar spectrum only reaches to wave-length 2932, 

 that is, only a very trifle beyond U. We must therefore suppose that 

 the absorbent substance, whatever it bo, is not in our atmosphere. 

 The same reason will lead us to reject the notion that the absorption 

 can be due to matter in planetary space, for it is not easy to suppose 

 that the gases which pervade that space in extreme tenuity can differ 

 much from those in our atmosphere, because the earth in its annual 



Fig. 2. 



rO M N M NN 



fU CO CO 0) -f^-f^ 



'g o tu 00 ^^ 



r\) o Nj to o 



ro 10 10 



01 01 0) 



U 00 G) 



o (n O 



ro 10 ro 



o ^ ^ 

 0) CD *^ 



10 u o 

 10 o ^ 



N 



ro 



H u> 



Ci) CO 0) u 



N U fi U\ 



CD G> .p> CO 



01 O O O 



N CD <0O 10 t^ 



N ro 0) ;b o — 



Mo MCJl NO 



z Sri 



No. 1. Sulai- spectrum. 



No. 2. Candle flame. 



No. 3. Lime light. 



No. 4. Carbon disulphide and hydrogen flame. 



No. 5. Magnesium flame. 



course must pick them up whatever they are, and they must then 

 diffuse into our atmosphere, and we must in time have them in a more 

 condensed state in our atmosphere than in planetary space. The 

 absorbent is therefore probably neither in our atmosphere nor in 

 planetary space, and we must look for it in the solar atmosphere. 

 When we notice how much of the radiation of our terrestrial elements 

 is of shorter wave-length than the solar line U, we might almost fancy 

 that the blotting out of the sun's light beyond that point is simply 

 due to an increase in the number and breadth of the Fraunhofer lines. 

 Indeed we have frequently observed the strong magnesium line, wave- 

 length 2852, expanded so that the dark absorption band in its middle 

 reached quite up to U on one side (see photograph No. 1) and equally 

 far on the other side, and this, together with such expansion of the 

 strong iron lines beyond as we have occasionally observed, would go a 

 long way towards completely hiding all light above U. But such expan- 

 sions of iron and magnesium lines, high in the scale of refrangibility, 

 do not occur without a considerable expansion of the lines of the same 

 elements lower in the scale, expansions far exceeding what we actually 

 observe in the Fraunhofer lines. Moreover the Fraunhofer lines, though 



