CO On Remarkable Atmospheric Phenomena in Ceylon. 



that as the veil of darkness approaches it must tend to assume 

 a moro vertical position until it reaches the critical angle 



Fig. 2. 



when total internal reflection ceases and the veil suddenly dis- 

 appears. Its apparent tilting over on the spectator I imagine 

 to be merely an illusion produced by the rapid approach and 

 rising of the dark veil without any gradual disappearance 

 which can be watched and estimated. It will be evident that 

 it is the illumination of the innumerable particles floating in 

 the atmosphere, and capable of reflecting light, that causes this 

 aerial shadow to be visible by contrast. 



Another atmospheric phenomenon visible in Ceylon, of 

 great interest but not so striking in character as the above, 

 admits of an equally simple explanation. Not unfrequently in 

 the mountain districts broad beams apparently of bluish light 

 may be seen extending from the zenith downwards, converging 

 and narrowing as they approach the horizon. This ray-like 

 appearance is very similar to that seen before sunrise ; only the 

 point from which the rays proceed is opposite the sun : the 

 rays themselves are very broad and blue in colour ; and the 

 spaces between them have the ordinary illumination of the rest 

 of the sky. If we suppose in this instance that the lower 

 strata of air are colder than the upper (a condition of the 

 atmosphere which not unfrequently occurs in a tropical moun- 

 tain district like that of Ceylon, where large currents of heated 

 air sweeping up a valley cross another valley nearly at right 

 angles and at a considerable elevation above it), the refraction 

 spoken of in the case of Adam's Peak will be downwards instead 

 of upwards. If, too, the observer be below, the veil of darkness 

 will appear to him like a very elongated triangle apex downwards, 

 or broad ray, through which the blue sky beyond may be seen 

 free from the palish illumination of the atmosphere, whilst on 

 either side the ordinary illuminated sky will be seen. If now 

 we suppose several isolated masses of cloud to partially obscure 

 the sun, as was the case when I witnessed the phenomenon, 



