TWILIGHT PHENOMENA. 205 



how can they warm them? They nearly all vanish into 

 space. (See Fig. 42.) 



Finally, there is not a phenomenon, even to that of dawn 

 and twilight, which cannot, on these principles, be very fully 

 and clearly explained. I have indicated, in a preceding 

 paragraph, the rainbow-glories of colour noticeable on the 

 line of demarcation between the illuminated and the darkened 

 hemispheres. They are wanting where the rays of light strike 

 vertically or nearly vertically. It is this circumstance which 

 explains why, in the intertropical regions, the crepuscular 

 phenomena are nearly null; why the sun, so to speak, sets 

 and rises abruptly, like a taper which we extinguish or re- 

 kindle. These iris-gleams increase, on the other hand, in 

 intensity, in proportion as we recede from tropical regions: 

 the red touches the horizon, while the violet blends with 

 the azure of the sky \ between these two extremes, which 

 are always very clearly marked, are arranged in less per- 

 ceptible fashion, and in the order of refraction, the other 

 colours of the rainbow. 



What time and labour does it not require for the mind to 

 disengage, to free itself from the fetters and incumbrances of 

 sensorial appearances, the illusions of the senses, and to rise 

 sufficiently high to seize at a glance all the dynamics of the 

 world ! 



It is this faculty, however, which distinguishes the intellect 

 from the imagination. 



