RECENT ADVANCES IN SOLAR ASTRONOMY. 25 



and physical investigations have been initiated which give some prom- 

 ise of solving the mysterious problems of the sun's surface-drift, and 

 the periodicity of the spots. We propose in this paper briefly to sum- 

 marize these advances. 



The transit of Venus on December 6. 1882, was widely, and, on the 

 whole, successfully observed. The Americans alone used photography 

 to any great extent, and at the nine different stations (four of them in 

 the southern hemisphere) nearly fifteen hundred photographs were 

 obtained, of which over a thousand are good for measurement. The 

 German heliometer parties were also successful ; and a great body 

 of contact and microtnetric observations and some photographs were 

 obtained by French, English, and Belgian parties. The publication 

 of the photographic and heliometric results is waited for with much 

 interest, but, for some reason, has been greatly delayed. The general 

 impression, however, is that the results will not prove as consistent and 

 accurate as had been hoped, the probable error remaining still pretty 

 large, and indicating that transits can not compete in accuracy with 

 some of the other methods of determining the solar parallax. 



Since 1882 the Washington experiments of Professor Newcomb 

 upon the velocity of light have been completed and published, along 

 with a new and independent determination by Michelson, at Cleveland. 

 The anomalies in Newcomb's earlier observations were traced to their 

 source and removed, and now the results of both observers stand in 

 very close and gratifying accordance. Newcomb's is 299,860 kilo- 

 metres, Michelson's 299,853. 



To go with this in fixing the solar parallax, we have the new 

 determination of the constant of aberration, by Nyren, of Pulkova, 

 based on all the Pulkova observations up to 1883. This value, 

 20"*492, combined with the above velocity of light, and with Clark's 

 value for the earth's equatorial radius (6378*2 kilometres), gives for 

 the solar parallax 8"*794 — almost absolutely accordant with that de- 

 duced from the heliometer observations of Mars, in 1877. The obser- 

 vations of the eclipses of Jupiter's satellites, by Professor Pickering's 

 photometric method, now in progress at Cambridge and Paris, will 

 also give an extremely valuable result when the twelve-year cycle is 

 completed. It has fixed the precise number of seconds required for 

 light to traverse the mean distance between the earth and the sun. 



The most remarkable result which has been arrived at, with refer- 

 ence to the solar radiation since 1882, is the fact, ascertained by Lang- 

 ley, that we do not receive from the sun any of the low-pitched, slowly 

 pulsing rays, such as are emitted from surfaces at or below the tem- 

 perature of boiling water. The solar spectrum appears to be cut off 

 squarely at the lower end, and this cutting off we know can not have 

 been effected in the earth's atmosphere, because we receive from the 

 moon just the very kind of rays that are missing from the solar spec- 

 trum, and that in considerable quantity as compared with the rays of 



