820 Dr. Megh Nad Saha on 



diminished concentration, not only (m, p), but also (m, d) 

 orbits are produced. As these absorbing atoms occur only 

 at large heights in regions of low concentration, they are 

 not in sufficient number to produce a weakening or! the 

 corresponding regions of the continuous spectrum by ab- 

 sorption. 



The occasiona-1 reversals of He-lines in the penumbra of 

 sun-spots seem to be an effect of diminished concentration 

 without a corresponding loss in temperature. The temper- 

 ature of the penumbra is intermediate between those of the 

 spot and the undisturbed photosphere, and may be supposed 

 to lie between b500° K. and 7000° K. We have no direct 

 observational result which can give us some idea of the 

 pressure, but Evershed * has found that gases are flowing 

 through the penumbra radially outwards, parallel to the 

 surface of the sun, with velocities ranging from rather small 

 values to about three or four kilometres per second. As the 

 mass-motion of a quantity of gas is always attended with a 

 fall in pressure, the penumbral regions certainly possess 

 lower pressure than the undisturbed photosphere. The 

 physical conditions in the penumbra are therefore favourable 

 to the production of a larger percentage of He-atoms with 

 the (2, />) orbits. Probably this accounts for the occasional 

 weak reversal of He-lines over the penumbra. 



The Flash Spectrum. 



From paper A and the foregoing part of the present 

 paper, it will be seen that the Fraunhofer spectrum can 

 mainly be regarded as function of a single physical condition, 

 viz., the temperature. The flash spectrum is, on the other 

 hand, a function of temperature and concentration, a low 

 concentration favouring a greater percentage of ionization, 

 and consequently a relative intensification of the enhanced 

 lines. Special attention is called to the word " relative " 

 when terrestrial sources of limited extent are considered, for 

 though the percentage of ionized atoms becomes greater, 

 the absolute number of particles becomes less, and hence all 

 lines, as a rule, become fainter. But the lines of neutral 

 atoms would become much fainter than the enhanced lines. 



All this was substantially stated in paper A, but at the 

 time of writing this paper I was acquainted with no experi- 

 mental work which could be cited in support of the view. 

 I have since come across some works which support my case. 



* Evershed, Astrophysical Journal, vol. xxv. (1909). Hale, he. ciU 

 vol. xxviii. (1910). 



