BARNARD— SELF-LUMINOUS NIGHT HAZE. 225 



this it appears that the phenomenon is really conspicuous when seen 

 under good conditions. It seems to have no preference for any 

 special part of the sky. The motion is always easterly. The den- 

 sity of this haze is not great enough to hide the brighter stars over 

 which it passes. In fact, it does not differ from ordinary haze 

 except in being self-luminous. There does not seem to be any fluc- 

 tuation or pulsation in its light. Under proper conditions it is 

 visible at all hours of the night and in all parts of the sky. Some- 

 times it is very faint and has to be looked for ; at other times it is 

 conspicuous. Often the sky is very pure and dark between sheets 

 or strips of it. It continues luminous as long as it is under observa- 

 tion, which may be for a considerable part of an hour. It is pos- 

 sible that the luminous nights mentioned, where no streaky haze 

 was seen, were due to a thin uniform sheeting of the luminous haze 

 all over the sky. 



To make this paper more complete I have written to Dr. W. J. 

 Humphreys for information as to the name and nature of this haze 

 shown on one of my day photographs. He has kindly supplied me 

 with the following: 



The streaky haze to which you refer is the cirro-stratus cloud. Near 

 the edge, where it is thin, it might better be called cirrus. 



There is no sharp division between cirrus and cirro-stratus. The thin 

 fibrous cirrus often gradually thickens into a more or less continuous cloud 

 veil, or sheet, in which form it is called cirro-stratus. 



Its average altitude in middle latitudes is 8 to lo kilometers. In higher 

 latitudes its altitude is less, say 6 to 8 kilometers; and in equatorial regions 

 roughly ID to I2 kilometers. 



It nearly if not quite always consists of snow crystals, as might be in- 

 ferred from its altitude and consequent low temperatures, and as is known 

 from the halos often seen in it. 



I am also indebted to Professor Eric R. Miller, of the Weather 

 Bureau at Madison, Wisconsin, for valuable data on the frequency 

 of cirrus cloud in Wisconsin. Professor ]\Iiller says : 



You will learn from the data sent you that there is no part of the year 

 when cirro-stratus is absent; that it is fairly evenly distributed throughout 

 the year. 



This information is important as it shows that if the cirro- 

 stratus were naturally self-luminous we should have luminous haze 



PROC. AMER. PHIL. SOC. VOL. LVIII, O, JULY 25, I919. 



