216 Perspective Aerial Light and Shade. [1826. 



before others, but was observed upon the whole for about half 

 an hour. 



At first the phenomenon seemed inexplicable, but after a 

 little consideration, was referred (and as it appeared from 

 after observations correctly) to an effect of aerial perspective. 

 The rays which seemed to originate from a common centre on 

 the east, were really only the intervals between long shadows 

 caused by the occurrence of clouds far to the west, and were 

 in fact passing to the place from whence they seemed to 

 originate, and the circumstances of the case seem to have been 

 as follow: the atmosphere contained a slight haze, which 

 allowed the sun's beams to pass forward with but little inter- 

 ruption, but was yet in sufficient quantity to reflect a consider- 

 able portion of light to the eye. The sun was just setting; 

 clouds, very far to the west and out of sight from the place 

 where the observer stood, stopped the light wherever they 

 interfered, and cast immense horizontal or nearly horizontal 

 shadows along the sky, parallel to each other, and over the 

 head of the observer. The difference between these shadows 

 and the intervening illuminated parts, could not be observed 

 over head or on the right or left hand, i. e. perpendicular to 

 their direction, because of the want of sufficient depth, as it 

 were, in the parts thus circumstanced, to make them visible ; 

 but as they receded from the observer in the direction from 

 the sun, they became fore-shortened, and then, from the greater 

 depth of mass, and consequently greater number of particles 

 looked at, became visible. This is at least one reason why 

 they were so visible towards the east; but another is the pro- 

 bable existence of more haze in that direction than towards 

 the west, or to the right or left of the observer's situation : 

 the rays could not be seen between the sun and the observer, 

 though the sun was out of sight, and consequently the general 

 light; it may be supposed, not too great ; which seems to imply 

 that less haze existed in that direction ; and its presence was 

 fully proved towards the east by the dull red colour which the 

 moon assumed upon rising a short time after the appearance 

 had ceased. The convergence of the rays to one spot, and 

 that opposite the sun, was merely an effect of perspective, 

 and requires no explanation here. (See Plate I. fig. 6.) 



Although the appearance on this evening was exceedingly 



