70 Proceedings of the Royal Society 
any season of the year, or to any hour of the day. It is no doubt 
due to the diffraction or inflection of light acting under rather 
unusual circumstances, and is the most notable example of the 
kind to be seen by the naked eye, without any artificial arrange- 
ment. I well recollect M. Necker showing me this beautiful 
appearance in the course of our tour of 1832, and I have often 
observed it since. The remarkable circumstance is, so far as I 
recollect, the absence of prismatic colours, which might, however, 
be anticipated from the infinite variety of dimension of the objects 
diffracting the light. 
The third of these occasional memoirs by M. Necker, having for 
its subject certain “ diverging rays which are seen long after sun- 
set,” appeared in the Annales de Chimie et de Physique for Febru- 
ary and March 1839. It was communicated, I believe, by Arago’s 
request. This paper excited little notice at the time, and is now 
perhaps nearly forgotten. Yet, though somewhat diffuse in com- 
position, it contains observations and speculations worthy of record. 
It contains ample and specific descriptions of the second coloration 
of Mont Blanc, and the exact intervals after sunset at Geneva of 
the various appearances of illumination presented by the Alps, 
which have been more vaguely described by several writers. But 
the more interesting and original part of the paper refers to the 
production of divergent beams streaking the calm western sky, at 
a period about 45 minutes after the sun’s disappearance. These, 
no doubt, are most usually caused by detached clouds intercepting 
the sunlight, and throwing their dusky shadows athwart the vapor- 
ous sky. When such is the cause, M. Necker remarked that bad 
weather usually followed within a short period.* But he also 
observed that some of these crepuscular phenomena had a more fixed 
character, and did not indicate a change of weather; moreover, that 
they recurred (he thought) as often as the sun set in the same 
position, — that is, every spring and autumn, especially on certain 
days of February and October, at Geneva. 
Hence he began to entertain the idea that the dark rays were 
shadows of distant mountains lying westward from the spectator, 
^ This is the foundation of the popular phrase applied to the appearance 
of “the sun drawing- water.” See Herschel’s Astronomy (Lardner’s Encyc.), 
p. 31. 
