132 EECORD OF SCIENCE FOR 1886. 



sulphide of carbon prism, kept at au even temperature by running 

 water. The measures were made with a tiue glass pointer. 



Gornu's device for distinguisMng the telluric lines in the solar spec- 

 trum. — At the meeting of tho Eoyal Astronomical Society, on June 11, 

 1886, M. Cornu gave a descrii^tion of an iugenious method he has de- 

 vised for distinguishing between those lines of the solar spectrum which 

 are atmospheric and those which are due to solar absorption. The east 

 and west equatorial limbs of the sun are alternately thrown on the slit 

 of the spectroscope by means of an oscillating mirror. As one limb of 

 the sun is approaching us and the other receding, there is a real differ- 

 ence of wave-length in the same radiation as obtained from the two 

 limbs, and consequently the solar lines appear to oscillate while the 

 atmospheric lines remain perfectly stationary. "It is as if you shook 

 the spectrum ; and if a line were a solar one it moved, if a terrestrial 

 one it remained steady." 



The absorption spectrum of oxygen. — About three years ago M. Egoroff 

 was able to show that the great groups A and B in the solar spectrum 

 were due to the absorption of oxygen. More recently the a band was 

 also found to be due to the same gas. M. Janssen, studying the ab- 

 sorption of oxygen, has now discovered that, under certain conditions, 

 the gas yields another spectrum, composed no longer of lines easily 

 separated, but of shaded bands, which can only be resolved with great 

 difiQcnlty. This system of bands appears for moderate pressures much 

 later than the spectrum of lines, but it shows itself very quickl}" with in- 

 crease of the density ; the two systems are so different that it is possible 

 to obtain either the first without the second, or vice versa. M. Janssen 

 was at first unable to explain how it was that these bands were not 

 visible in the solar spectrum when they were easily obtained by passing 

 light through thicknesses of oxygen far less than the sun's light has to 

 traverse before reaching us. Bat further experiments showed that 

 these bands did not develop in proportion to the thickness of the stratum 

 of oxygen producing them, multiplied by its density, but in proportion 

 to the thickness multiplied by the square of the density. The density 

 of our atmosphere being small as compared with some of the pressures 

 at which M. Janssen worked, the non-appearance of these bands amongst 

 the telluric lines of the solar spectrum is readily explained. 



M. Janssen is continuing his experiments at Meudon, and is building 

 tubes which can be loaded with 1,000 atmospheres of hydrogen, oxy- 

 gen, or carbonic acid. In this last case the real density of the gas will 

 be superior to the density of water. (Nature.) 



For a thorough and authoritative review of recent advances in our 

 knowledge of the sun the reader should consult Professor Young's ar- 

 ticle which appeared in the Popular Science Monthly for November, 

 1886 (30:24-33), and also his "Ten Years' Progress in Astronomy," in 

 vol. 5 of the Transactions of the New York Academy of Sciences. 



