3J. 



which the ice halos appear to originate, as my notes show more 

 conclusively than any other notes do. The relation of irides- 

 cence to similar clouds in other places than South Australia 

 is shown by the following reports : — 



Piazzi Smyth, writing from Edinburgh, says of the brilliant 

 iridescent clouds of December 11, 1884 — " Part of the time the 

 iridescent clouds were eclipsed by a heavy cumulo-strata cloud, 

 which showed clearly that the iridescent clouds were cirrus 

 clouds, not as high though as the cirro-stratus haze, acting as a 

 background to show off the beauties of the blue and green 

 iridescent clouds." 



The report from Christiania Observatory, already quoted. 



The continuation of T. W. Backhouse's remarks. He says : — 

 "The colours appear to be related to thin sheets of cloud- 

 haze, which is white, and not coloured at a great distance from 

 the sun." He describes the iridescence as being on a dense pal- 

 lium of cirrus clouds. There are other reports supporting 

 these, but it is needless to quote further. 



The relation of iridescent clouds to states of weather is 

 also indicated by notes that I have been able to make, and 

 by reports of others. Their appearance is most promi- 

 nent in all places during the cool months and on those occa- 

 sions when fine ice-clouds are likely to be forming, or when 

 they have been just disappearing. In Christiania they have 

 been usually observed when mild and dry weather set in after 

 frost. Corroboratory notes come from Prof. A. S. Herschel, T. 

 W. Backhouse, and Piazzi Smyth. Miche Smith, of Madras, 

 states that the appearance of iridescent clouds there is 

 usually noticed before the approach of the monsoons, and these 

 clouds are looked upon as a sure sign of monsoons being near 

 at hand. These periods are the cool periods. 



In reference to the spectrum and absorptions, it is sufficient 

 to say that on all occasions the absorption lines have been in- 

 dicative of dry atmosphere when the sky near the sun has been 

 examined, the blue end of spectrum being relatively brighter, 

 as it generally is when the air is somewhat dry and cold. The 

 tints of the sky are numerous, and each tint has its own par- 

 ticular spectrum, so that a complete account of the tints in the 

 various parts would require a special paper. The only point of 

 value is that the air seems somewhat dry, and that the blue end 

 of the spectrum is relatively brighter, indicating a cold upper 

 stratum. This note only applying to m}^ observations in South 

 Australia. 



^Methods of observation : — I have tried many ways of observ- 

 ing these clouds. They can be viewed by reflection from 

 •substances that absorb part of the light. Thus water contained 

 in a black vessel ; surface of black glass ; coloured solutions. 



