265 



kijer of incandescent clouds produced hv condensation of certain 

 substances having exccj)tiontilly liigli critical teni[)cratiircs, and 

 Skcchi's hypothesis (aftcrwai-ds develoi)C(l by ScHWARZscnn.D and 

 Emden), whicli dispenses witli assuming- ck)ud-formation by supposing 

 the density of the sohii' gases to increase so rapidly with depth 

 near the level called "solai' surface", that within a layer no more 

 than a thousand kilometers thick, their united radiating power in- 

 creases from a veiy low value (in the chromosphere) uj) to that of 

 the black body (in tiie photosphere). 



In 1891, August Schmidt took a new departure by showing that 

 an entirely gaseous body of the dimensions of the sun, in which 

 the density and the racUating power gradually decrease from the 

 center outward — be it even at a slow rate — - must a{)pcar like 

 a cii-cular luminous disk with a sharp edge, as a mere consecpience 

 of ray-curving caused by the radial density gradient. So the circular 

 aspect of the sun is not a sufticient ground for admitting the existence 

 of a real "photosphere", that is, of a layer characterized by some 

 abrupt, or even only rapid change of physical propei-ties. 



Schmiüt's well-known solar theory, however, met with the severe 

 objection that it did not duly consider the effect of absorption and 

 scattering of the light ^). Rays having accomplished such long 

 distances on their spiral paths inside the critical sphere would be 

 almost wholly extinguished before emerging; they could not possibly 

 bring along so much energy from the incandescent core, as would 

 be required in order to account for the brilliancy observed in the 

 marginal parts of the disk. In its original form the optical interpre- 

 tation of the sun's edge cannot be maintained. 



It is also impossible to accept the cloud-theory of the photosphere, 

 because the results of the radiation-measurements made at Maastricht 

 during the annular eclipse of 1912"-) forbid making an absorbing 

 or scattering solar atmosphere responsible for the fall of the sun's 

 brightness from the center toward the limb. Indeed, the absorbing 

 and scattering power of the gases lying outside the photosphere 

 proved to be relativelj- insignificant. The photosphere, therefore, 

 cannot be of such a nature that it would appear like a aniforuily 

 luminous disk if the surrounding gases were absent. On the contrary, 

 it must have in itself the property of appearing much brighter when 

 looked at in the direction of a radius than at an angle with the 



i) R. Emden, Gaskugeln, S. 3S8-394. 

 E. Prlxgsheim, Physik cler Sonne, S. i266— 270. 



~) Proc. Roy. Acad. Amblerdam. XV, 1451, 1913; Asiropliysical Journal 37, 

 p. 225, 1913. 



