394 PROFESSOR C. MICHIE SMITH ON OBSERVATIONS ON 



and even with the dispersion produced by a single prism, at least eight lines 

 could be measured in it, while many more were visible. 



The low-sun band was not very conspicuous ; but this was partly due to 

 contrast with the very strong rain-band. The line W.L. 568 at the more 

 refrangible side of the low-sun band was very well marked, and the band itself 

 seemed to consist of a series of equidistant lines. The apparently much 

 stronger absorption in the red than in the blue end was a very marked feature, 

 which became still more conspicuous when a photograph of the blue end was 

 examined. 



A photograph was taken on the evening of the 23rd September, when the 

 sun was very green, and the visible spectrum extended between W.L. 645 and 

 W.L. 410. Half of the slit was exposed for twenty seconds, and the other half 

 for thirty-three seconds. Both gave good photographs, extending from about 

 F to a distance beyond H, twice as great as the distance between F and H. 

 This is very nearly the same length as I obtain under similar circumstances in 

 ordinary weather, though not nearly so long as I obtained last summer at an 

 altitude of 6000 feet. The lines were sharp and well defined, and contained no 

 bands that were specially prominent, though some seemed darker than usual. 

 I have, unfortunately, no means at present of making an accurate determination 

 of these lines. The atmospheric band C was very strongly marked, and was 

 decidedly more conspicuous than C itself. It was thick and dark even at an 

 altitude of 60° or 70° in the middle of the day, and formed then, next to the 

 general absorption, the most characteristic feature of the spectrum. The 

 contrast of the thin line between C and C with these lines was most interesting. 



Since the passing away of the abnormal conditions, I have made careful 

 observations of the sunset spectrum with the same apparatus, and I find that 

 ordinarily A and a are clearly visible, as well as B, though at times they are 

 strongly marked, and a good deal of shading is observable between them. C 

 is much thinner, and the rain-band is less prominent than the low-sun band, 

 which, however, does not now have the appearance of a number of fine lines. 

 The nearest approach to the green sun spectrum was observed recently during 

 a severe thunderstorm, which was accompanied by a fall of about 1^ inch of 

 rain. 



A very similar though less intense spectrum can be observed almost any 

 evening by taking advantage of the passage of a small thin cloud over the sun's 

 disc. If a lens is used in front of the slit of the spectroscope, the absorption 

 due to the cloud will be seen as a band in the middle of the bright spectrum 

 from the unclouded part of the sun ; and, owing to the strong contrast, the 

 details of the absorption will be well seen, just as in the case of a spectrum of 

 a sunset. 



