OPTICAL PHENOMENA. 533 



O'er the earth a cloud, will therein set 

 His triple-coloured bow, whereon to look, 

 And call to mind his covenant." 



It is happily remarked by Mr. Prout, in his Bridgewater treatise, that no pledge could 

 have been more felicitous or satisfactory ; for, in order that the rainbow may appear, the 





Rainbow. 



clouds must be partial, and hence its existence is absolutely incompatible with universal 

 deluge from above. So long, therefore, as " He doth set his bow in the clouds," so long 

 have we full assurance that these clouds must continue to shower down good, and not evil, 

 to the earth. 



When rain is falling, and the sun is on the horizon, the bow appears a complete semi* 

 circle, if the rain-cloud is sufficiently extensive to display it. Its extent diminishes as the 

 solar altitude increases, because the coloured arch is a portion of a circle whose centre is 

 a point in the sky directly opposite to the sun. Above the height of 45 the primary bow 

 is invisible, and hence, in our climate, the rainbow is not seen in summer about the middle 

 of the day. In peculiar positions a complete circle may be beheld, as when the shower 

 is on a mountain, and the spectator in a valley ; or when viewed from the top of a lofty 

 pinnacle, nearly the whole circumference may sometimes be embraced. Ulloa and Bouguer 

 describe circular rainbows, frequently seen on the mountains, which rise above the table- 

 land of Quito. When rain is abundant, there is a secondary bow distinctly seen, produced 

 by a double reflection. This is exterior to the primary one, and the intervening space 

 has been observed to be occupied by an arch of coloured light. The secondary bow 

 differs from the other, in exhibiting the same series of colours in an inverted order. 

 Thus the red is the uppermost colour in the .interior bow, and the violet in the exterior. 

 A ternary bow may exist, but it is so exceedingly faint from the repeated reflections, as 

 to be scarcely ever perceptible. The same lovely spectacle may be seen when the solar 

 splendour falls upon the spray of the cataract and the waves, the shower of an artificial 



