32 



SCIENCE. 



[N. S. Vol. II. No. 28. 



spectroscope, utilizing that teclinicality 

 called Doppler's principle. This means 

 that the spectra from opposite sides of the 

 Sun (the east side coming toward the Earth, 

 and the west receding from us) are opti- 

 callj' brought alongside each other; then 

 careful measurement of the amount of di- 

 vergence of a given line in the two spectra 

 forms the basis for calculating the rapidity 

 of rotation. M. Duuer, of Lund, Sweden, 

 carrying this research still farther, into 

 high solar latitudes, finds for the equatorial 

 regions a period of sidereal rotation equal 

 to 25''. 46, in close correspondence with the 

 determinations of Carrington and Spoerer 

 from the spots alone; while the slowing 

 down as the poles are approached is re- 

 markably verified; for his results give, for 

 the rotation period at latitude 75°, no less 

 than 38''. 54. M. Duner's observations 

 were made near the time of minimum spots, 

 and it would be interesting to repeat the 

 determination near the epoch of maximum 

 spottedness. 



The Sun's axis is inclined 83° to the 

 plane of the Earth's orbit; and if prolonged 

 northward to the celestial sphere, the axis 

 would intersect it near the third magnitude 

 star o Draconis, so that in March the Sun's 

 north pole is turned farthest from the 

 Eai"th, in Sej)tember it is inclined 7° to- 

 ward us. Spectroscopic study of the sun- 

 spots shows that their inferior brilliance is 

 due in part to a greater selective absorption 

 than obtains in the photosphere generally. 

 Continuous and systematic records of the 

 solar spots are now kept at Greenwich (in 

 connection with Dehra Dun, India), at 

 Potsdam near Berlin, at Chicago, and else- 

 where. Exceedingly fine photographs of 

 sun-spots and the solar surface have been 

 obtained at Potsdam (Himmel unci Erde, ii., 

 1890, 24). 



Also at Meudon, Paris, M. Janssen has 

 had extraordinarj^ success in photographing 

 the Sun's surface in detail, and the granula- 



tion is very sharply defined in his originals. 

 In viewing the Sun with a telescope this 

 granulation can be satisfactorily seen with 

 a magnifying power of about 400 or 500, 

 under good atmospheric conditions. 



While the 42 years' faithful work of 

 Schwabe, as revised by "Wolf and collated 

 with other and scattering results, gives an 

 average sun-spot period of 11 5 years, there 

 are great irregularities. During the latter 

 half of the 17th century, the ordinarj' pro- 

 gress of the spot cycle appears to have in- 

 termitted; the intervals between maxima 

 have varied from 8 to 15i years, and be- 

 tween minima from 9 to 14 years. True in- 

 terpretation of this indicates with an ap- 

 proach to certainty that the cause of the 

 periodicity does not lie in planetary or any 

 exterior agencj^, but that it is seated in the 

 Sun itself. 



The solar prominences, or hydrogen 

 flames, are drawn in full sun-light, by 

 means of a spectroscope adjusted delicately 

 on the edge of the Sun, this instrument 

 reducing the sky-glare, without dispersing 

 very much the light of the prominence it- 

 self. This method has now been iu common 

 use more than a quarter centurj\ But by 

 means of the spectro-heliograph devised by 

 Professor Hale, of the University of Chi- 

 cago, the hindering efiects of our atmos- 

 phere are in greater part evaded; and he is 

 enabled to secure on a single plate (with 

 single exposure) not only the photosphere 

 and sun-spots, but the chromosphere and 

 protuberances. Also the same instrument 

 (which utilizes monochromatic light, or 

 light of a single color onlj^) has demon- 

 strated that the faculse, which to the eye 

 are ordinarily seen only near the Sun's 

 limb, actually extend all the waj^ across the 

 disk of the Sun, in approximately the re- 

 gions of greatest spot-frequeucj-. Profes- 

 sor Hale's progressive methods of solar re- 

 search will soon give us large accumulations 

 of facular observations, from which the 



