( 143 ) 



paths of those beams by the system of surfaces of discontinuity. 

 The foregoing inferences really imply the whole of our inter- 

 pretation of the results, thus far obtained with the spectroheliograph. 

 This we shall show by amply discussing some of their main features. 



The broad dark bands, desi.s;nated by Hale and Et-lerman as H^ 

 and K^ , are not absorption bands, but dispersion bands. Real absorp- 

 tion by the solar calcium vapour we hold to be restricted to the 

 central dark lines i/, and K^. The bright bands ƒ/, and K^, predo- 

 minating in the spectrum of the "flocculi" and attributed by Hale 

 and Ellerman to strongly radiating calcium vapour, result in our 

 theory from the fact, that with beams of light the wavelength of 

 which is very near to that of the central absorption lines, the 

 divergence may be diminished or even changed to convergence by 

 the tubular structure. Indeed, such rays deviate more strongly than 

 those standing farther from the absorption lines ; and as soon as they 

 undergo more than one incurvation, they have a chance of reaching the 

 Earth with increased intensity. This chance improves in proportion 

 as the index of refraction departs from unity, be it in a positive or 

 in a negative sense'). We conclude from it, that the brightness of the 

 calcium flocculi must, as a rule, increase as the monochromatic light 

 in which the Sun is photographed approaches the true absorption line. 



This consequence of our theory exactly corresponds to one of the 

 chief peculiarities, which immediately struck Hale and Ellerman on 

 inspecting sets of photographs taken at short intervals of time with 

 the second slit in different positions within the H and K bands. In 

 order to account for the same fact, those investigators are obliged, 

 by their working hypothesis, to suppose that in higher regions of 

 the Sun's atmosphere the calcium vapour radiates more strongly than 

 in lower levels. This cannot be called a very satisfactory inference; 

 and less so, as the supposition is added that the incandescent vapour 

 is rising from much deeper layers and, therefore, considerably 

 expanding — a process during which, according to our physical 

 notions, the temperature must fall. Here we meet with a serious 

 difficulty ; Hale and Ellejl\n try to get rid of it by means of the 

 rather vague assumption, that some electrical or chemical effect may 

 be responsible for the bright radiation emitted by this calcium layer, 

 which is intermediate between two absorbing layers"). 



3) In the experimental investigation on dispefsion bands, before mentioned, this 

 brightening in the middle of the dark bands has been distinctly observed. Gf. also: 

 Proc. Roy. Acad. Amst. Vol. IV, p. 596. 



2) Hale and Ellerman, Astrophysical Journal XIX, p. 44, 



