RECENT ADVANCES IN SOLAR ASTRONOMY. 29 



In 1883 Egoroff, a Russian physicist, succeeded in showing that 

 the great A and B groups of the solar spectrum are due to the oxygen 

 in our atmosphere. 



Cornu, by a very ingenious arrangement, in which he makes a small 

 image of the sun, four or five millimetres in diameter, oscillate across 

 the slit of a powerfid spectroscope three or four times a second, has 

 succeeded in bringing out conspicuously, and at a glance, the differ- 

 ence between the true solar and the telluric lines in the spectrum. 

 The solar lines oscillate slightly as the eastern and western, the ad- 

 vancing and receding, limbs of the sun come alternately to the slit, 

 while the telluric lines stand fast. 



Mr. Lockyer has called in question the existence of the so-called 

 " reversing layer " of the chromosphere, being disposed to hold that 

 certain of the lines which we identify as belonging to the spectrum of 

 any given substance, say iron, are due to absorption in upper and cooler 

 regions of the solar atmosphere, while others are produced low down. 

 In support of this idea, he adduces the observation that, at the Egyp- 

 tian eclipse of 1882, certain of the so-called " iron-lines," between b 

 and F, were much longer, though no brighter, than other " iron-lines " 

 close by them, and remained much longer visible as the moon ad- 

 vanced to cover the chromosphere. There is not room to discuss the 

 matter here. Those who believe that the Fraunhofer lines are mainly 

 produced by that portion of the solar atmosphere which bathes and 

 sustains the photospheric clouds, or lies immediately above them, 

 would not quarrel with the idea that the upper regions also co-operate 

 to a certain extent ; but we see no proof from observation, as yet, 

 that lines which are produced by the absorption of the upper regions 

 of the solar atmosphere are not also found in the lower. The question 

 undoubtedly is interesting and important : Does each region of the 

 solar atmosphere have its own spectrum, peculiar and distinct from 

 those of other regions above and below ; all of them co-operating by 

 simple summation to form the spectrum as we see it — or, on the other 

 hand, as has been usually admitted, does the spectrum of the lowest 

 stratum contain everything, while the spectra of the higher regions 

 differ from it merely by defect ? Eclipse observations may possibly 

 decide it. Of course, if Mr. Lockyer is right, the fact would be a very 

 effective argument for the theory of "compound elements," which 

 theory, notwithstanding the failure of its " basic-line " defense, seems 

 to be decidedly gaining ground in scientific opinion. 



As to the bright-line spectrum of the chromosphere, no great dis- 

 coveries have been made ; a number of lines, probably fifteen or 

 twenty, have been added by the writer to the two hundred and sev- 

 enty-three long ago catalogued as constantly or occasionally appearing. 

 Most of the new lines are in the violet and ultra-violet. Not one of 

 them is below C. 



Trouvelot has observed (or thinks he has) dark prominences — i. e., 



