1886. J NEW YORK ACADEMY OF SCIENCES. 243 



the interior of the spot are wholly gaseous, and tends to dis- 

 prove the idea that they are mostly of the nature of smoke or 

 steam. We mention also, in passing, another thing which has 

 been sliown by our large instrument at Princeton — that tlie 

 apparently bulbous, finger-tip-like terminations of the penum- 

 bral filaments are often, under the best circumstances of vision, 

 resolved into fine, bright, sharp-pointed hooks which look like 

 the tips of curling flames. 



The Solar Spectrnm. 



In 1877, Dr. Henry Draper, of New York, by a series of most 

 laborious, time-consuming, and expensive researches, discovered 

 the presence of oxygen in the sun, evidenced in his photographs, 

 not by fine dark lines, as in the case of elements previously recog- 

 nized, but by bright, hazy bands. It is difficult to assign any 

 reason why this gas should behave so peculiarly and so differently 

 from others, and for this reason many high authorities are indis- 

 posed to accept the discovery. But the evidence of the photo- 

 graphs seems fairly to outweigh any such purely negative 

 theoretical objections. 



Other advances have been made in the study of the spectrum, 

 due mainly to the great improvements in spectroscopic appara- 

 tus. Until recently it has not been easy to decide with certainty 

 as to some lines in the spectrum whether they were of solar or 

 telluric origin; the great bands known as A and B for instance. 

 It was only in 1883 that the Russian Egoroff succeeeled in prov- 

 ing that these are produced by the oxygen in the earth's atmo- 

 sphere. In his experiments on a scale previously unknown, the 

 light was transmitted through tubes more than sixty feet in 

 length, closed at the end with transparent plates, and filled with 

 condensed gas. 



It was quite early pointed out that the sun's rotation ought to 

 produce a shift in the position of lines in the spectrum according 

 as the light is derived from the advancing or receding edge of 

 the solar disc, and Zollner thought he could perceive it. The 

 earliest meastcres, however, were, I believe, those obtained inde- 

 pendently by Vogel and the writer, in 1876. In the great bisul- 

 phide of carbon spectroscope of Thollon, the displacement becomes 

 easy of observation; and very recently Cornu, by taking advantage 

 of it, and by an extremely ingenious arrangement for making a 

 small image of the sun to oscillate across the spectroscope slit 

 two or three times a second, has been able to discriminate at a 

 glance between the telluric and solar lines; the former stand 

 firm and fast, while the latter seem to wave back and forth. 



In this connection also should be mentioned the great map of 

 the solar spectrum, for which Thollon received the Lalande 



