33 



zation discovered. The note was taken at G-awler, and I found 

 that at the very time this appearance took place the plains to 

 the west were receiving a heavy rain. 



It follows from this that there are several forms of apparent 

 iridescence: the halo forms, rainbow fragments, and the true 

 iridescent clouds ; and it is necessary to distinguish between 

 them if we wish to learn much respecting the true iridescent 

 clouds. 



The limits of position of such clouds are, in reference to 

 the sun, evident from the notes. Thus they are seen furthest 

 from the sun both at sunrise and sunset, and closest at noon. 

 They are always within the limits of the pink halo, and this is 

 noticeable in connection with the remarkable change in the 

 dimensions of the pink halo from day to day, and from morn- 

 ing to evening, the pink halo being more extended at morning^ 

 and evening. 



When there are two layers of clouds the iridescence does not 

 appear whilst the sun has a great altitude and whilst the one 

 layer of clouds is between the sun and that layer from which 

 the iridescence comes. But on these occasions the iridescence 

 appears when the sun is lower ; and then, when the clouds that 

 can become iridescent are illumined at the proper angle, the 

 upper clouds are in such a position that their most illumined 

 portion is not in a line drawn from the eye through the irides- 

 cent clouds. At these times the iridescent clouds are most 

 noticeable, owing to the amount of light and the darker back- 

 ground. 



The statements here made are corroborated by other ob- 

 servers — Prof. A. S. Herschel, of Newcastle-on-Tyne. T. W. 

 Backhouse, writing of his observations, says : — " Iridescent 

 clouds have been seen at all times of the day, but they have 

 been most strikingly noticeable near sunrise and sunset. 

 Colours don't appear when the clouds are far from the sun, 

 they being then only white during day. * * * * At about 

 half an hour to sunset iridescent clouds have been seen." 



J. Graves refers to colours in the clouds like rainbow tints, 

 but not regular, and having considerable altitude, at 26 minutes 

 after sunset. 



The reports from Christiania Observatory refer to the great 

 distance of the clouds from the sun at sunset. 



These notes show that in all places where iridescent clouds 

 have been noticed they can be best seen when the sun is below 

 the horizon, or when it is somewhat high in the heavens, the 

 iridescence rarely appearing when the sun is just above the 

 horizon. Their relation to particular cloud forms is indicated 

 in many of the preceding notes. Thus they generally seem to- 

 precede clouds that come up and take the filmy form, from 



