476 Scientific Intelligence. 



5. The Aurora Line of the Night Sky. — A green line of 

 unknown origin corresponding to the wave length 5578, in the 

 spectrum of the aurora, has been reported by several observers as 

 present in the sky on ordinary nights, and in comparatively low 

 latitudes. Various questions suggested by this occurrence have 

 been systematically investigated by Lord Rayleigh. If the phe- 

 nomenon is directly connected with the polar aurora it might be 

 expected that a gradation of intensity between the usually very 

 faint effect and a bright auroral display would be observed and 

 that this would become more pronounced at higher latitudes. A 

 series of spectral photographs was made at Terling (near Lon- 

 don), every night from Feb. 26 to July 3. From this systematic 

 series estimates of the intensities of the line were prepared and 

 comparisons made with the amount of magnetic disturbance and 

 the transit of spots over the sun's central meridian. No obvious 

 connection between the intensity of this green line and the ter- 

 restrial or solar phenomena was found. 



Comparisons between photographs made in the neighborhood 

 of Newcastle with those taken near London, showed that the 

 intensity appeared to be greater at the more southerly station, 

 which would indicate that the cause must be different from that 

 of the polar aurora. This conclusion is further supported by the 

 fact that the aurora line is visually observable by California 

 observers, 15° further south, while Rayleigh has been unable to 

 see it at all under ordinary conditions, 



Some authors have identified this aurora line with krypton 

 5770, but the more recent measurements clearly prove that it does 

 not originate with this element. Rayleigh also tested the sugges- 

 tion that the green line might be the fluorescent spectrum of 

 ozone excited by the ultra-violet light of negative oxygen bands 

 but nothing corresponding to it was indicated by his experi- 

 ments.— Proc. Boy. Soc. 100, 367, 1922. f. e. b. 



6. The Color of the Sea. — The early theories of the color of 

 the sky assigned its blue color to the scattering of light by par- 

 ticles of water or dust in the air. It is now known from the work 

 of, Cabannes and of Rayleigh on the scattering of light by dust 

 free gases, and from the measurements on the light from the 

 atmosphere, that the sky owes its colors to diffraction by the mole- 

 cules of the air itself. 



In regard to. the color of large masses of clear water consid- 

 erable divergence of opinion has existed. The late Lord Ray- 

 leigh was inclined to the view that the blue color of the deep sea 

 was simply the blue of the sky seen by reflection. Judged by 

 the literature of the subject, the trend of opinion appears to have 

 been that in so far as there is any real effect apart from 

 reflected skylight, the color is to be explained by absorption in 

 the water-the return of light from the depths of the liquid being 

 due to matter suspended in it. 



