A GREEN SUN AND ASSOCIATED PHENOMENA. 393 



This is particularly interesting on account of its close resemblance to the 

 description so often given of scenes in hill countries just before heavy rain. 



Mr Manley's observations at Ongole, which is the farthest north point at 

 which satisfactory observations seem to have been made, have already been 

 published in Nature, and need not be repeated here. 



The Spectrum. 



The observations on the spectrum were made partly with a direct vision 

 spectroscope, with a lens in front of the slit (Hilger's rain-band spectroscope), 

 but chiefly with my zodiacal light spectroscope, which has a single large dense 

 glass prism and a collimator three feet long. The great length of the collimator 

 permits the use of a very wide slit, which was found to be a great advantage in 

 this case. The only means of recording the positions of the lines is by reference 

 to a reflected scale ; and since all the lenses of the instrument are quartz, the 

 focus of the observing telescope, and consequently of the scale also, has to be 

 changed for different parts of the spectrum. This was found very inconvenient 

 when it was necessary to take a number of readings in different parts of the 

 spectrum in rapid succession. The positions of the bands cannot be considered 

 as strictly accurate, but they cannot be far wrong, as they were fixed by refer- 

 ence to known lines near them, and the scale values for the different parts of 

 the spectrum were obtained by plotting the scale readings for known lines in 

 terms of wave-lengths, and smoothing the curve. The main features of the 

 spectrum taken on the sun when green were — 



(1) A very strong general absorption in the red end. 



(2) A great development of the " rain-band " and of all other lines which are 

 ascribed to the presence of water vapour in the atmosphere, more especially of 

 the group Cj of a, and of the band at W.L. 504. 



The absorption in the red end was of very varying intensity; but when the 

 phenomenon was at its maximum phase it gradually crept up from about B till 

 past C, as the sun sank towards the horizon. On the 12th, when the sun was 

 within a few degrees of the horizon, the absorption was well marked up to 

 W.L. 621 — i.e., to beyond a, while at the violet end the visible spectrum ended 

 at W.L. 428, or just beyond G. 



The lines A and a were never visible, even on the sun, when it was green, 

 and even B could be made out with difficulty from half an hour before sunset 

 onwards ; and before it vanished it grew intensely prominent, with enormously 

 thick bands on the less refrangible side. The band C, on the more refrangible 

 j side of C, became very broad and black, while the fine line between this and C 

 1 remained thin and sharp, and C itself thickened out on the less refrangible side. 

 The rain-band was stronger than I have ever before observed it on the plains ; 



