552 Prof. R. W. Wood on the Production of 



wave-lengths very near those which are absorbed by the 

 vapour ; consequently the light most strongly refracted, if it 

 could be sorted out and examined with the spectroscope, 

 would resemble very closely the light emitted by the vapours. 

 Julius shows how this sorting out of the more refrangible 

 rays may account for the bright-line spectrum usually attri- 

 buted to the reversing layer, these rays moving in curved 

 paths in the solar atmosphere, thus reaching us after the 

 photosphere has been hidden by the moon. 



For the reproduction of this phenomenon in the laboratory 

 it is necessary to form an atmosphere of metallic vapour in 

 which the refractive index changes rapidly from layer to 

 layer. This I succeeded in accomplishing by allowing the 

 flame of a Bunsen-burner, fed with metallic sodium, to play 

 against the under side of a white plaster plate. On looking 

 along the surface of the plate, it was seen that a dark space 

 existed between the flame and the cold surface, resembling 

 somewhat the dark space surrounding the cathode of a 

 Crookes's tube. It seemed highly probable that, inasmuch 

 as the temperature of the flame was lowered to such a degree 

 by contact with the plate, the density of the sodium vapour 

 would increase very rapidly from the surface of the plate 

 downwards. The change may of course be abrupt instead 

 of progressive, though I am inclined to favour the latter 

 supposition. In either case the action will be practically the 

 same, the case being similar to the transition from a curved 

 ray to a broken-line ray, as the change of the index of the 

 medium becomes less gradual. Having covered the under 

 surface of the plaster plate with a non-homogeneous layer of 

 sodium vapour, a spot at the edge of the flame was illuminated 

 with sunlight concentrated by. a large mirror. This spot 

 radiated white light in every direction and corresponded to 

 the incandescent photosphere of the sun. A telescope, pro- 

 vided with an objective direct-vision prism, was directed 

 towards the white spot and moved into such a position that, 

 owing to the reduction in the width of the source of light by 

 foreshortening, the Fraunhofer lines appeared in the spec- 

 trum (fig. 1). This represents the stage of an eclipse when 

 only the thin crescent of the sun is visible. The sodium- 

 flame appeared superposed on the spectrum of course. On 

 moving the spectroscope until it was well inside of the plane 

 of the illuminated surface and feeding the flame with fresh 

 sodium, the solar spectrum vanished, and there suddenly 

 blazed out two narrow bright yellow lines almost exactly in 

 the place of the sodium lines. Fig. 2 shows the inverted 

 sodium-flame, the faint continuous spectrum and the " flash/' 



