. 274 



hypothesis suggested in 1909 ^). A spot was supposed to be a region 

 where, from a central minimum outward, the optical density 

 increases with a gradually decreasing gradient. If sun-spots are solar 

 vortices, such conditions are very lilicly to obtain. It was then argued 

 that, when a similar structure is traversed by the light from an 

 extensive source radiating, as the photosphere does, with intensities 

 decreasing from the center toward the limb, refraction must exactly 

 produce the characteristic optical features observed in a spot: an 

 umbra surrounded by a penumbra. Taking anomalous dispersion 

 effects into consideration, one is led by the same argument to an 

 explanation of the principal properties of the spot-spectrum. Lately 

 we succeeded in realizing, in the laboratory, the formation of a 

 typical "sun-spot" by refraction of light in a whirling mass of gas, 

 and could witness several phenomena, rather closely resembling the 

 appearances produced by the real solar objects. A description of 

 those experiments, together with a discussion of their possible bearing 

 on several spot-problems (e.g. on the apparent effect of the earth 

 on the formation and growth of sun-spots) must be deferred to a 

 separate paper. 



We now only wish to emphasize the fact that the above con- 

 ception of sun-spots naturally fits in with our dioptrical explanation 

 of the photosphere. The levels where vortex-motion should occur so 

 as to produce the appearance of a spot, will be found somewhere 

 between spheres corresponding to PF and QQ of our Fig. 2. The 

 conditions in a spot need not differ very much from those obtaining 

 in the surrounding regions. Their chief characteristics are: 1. the 

 rotary motion, which determines a magnetic field and a systematic 

 arrangement of density gradients (which need not be steeper than the 

 average irregular gradients otherwise present in the same levels), 

 and 2. the differences of temperature and of composition connected 

 witli the special form of circulation. 



Suminary. 



Various views concerning the nature of the photosphere are 

 criticized, and a new dioptrical interpretation of several photospheric 

 phenomena is proposed. 



1) Proc. Roy. Acad. Anist. 12, 273, 1909; Physik. Zeitschr. 11, 62, 1910. 



