34 PHYSICAL SCIENCE BK. i 



of the bow, but none of its curve. They lie in 

 a straight line. They are formed near the sun, 

 as a rule, in a moist cloud that has begun to break 

 up. Thus, they have the same colour as is found 

 in the rainbow, but there is a difference in the 

 shape, due to the corresponding difference in the 

 clouds over which they stretch. 



THERE is a similar variety of colours in Halos. 

 But there is this difference in the various pheno- 

 mena : Halos are formed at any point in the sky, 

 wherever there is a heavenly body ; rainbows are 

 not found except opposite the sun ; streaks, only 

 in the neighbourhood of the sun. I may express 

 their difference in another way : Bisect a halo and 

 you have a rainbow ; make it a straight line and 

 you have a streak. In all three there is the same 

 multiplicity of colours, the scale running from dark 

 blue to orange. Streaks, then, are found only close 

 to the sun. Rainbows are all either solar or lunar. 

 Halos are seen with all the heavenly bodies. 



XI 



1 ANOTHER kind of streak is visible when thin rays 

 of bright light equidistant from one another are shot 

 out through narrow apertures in the clouds. These, 

 too, are a prognostication of rain. How am I to 

 express myself here ? What shall I call them ? 

 Images of the sun ? The chroniclers call them 

 merely suns, and have put on record that they have 



2 been seen in twos and threes. The Greeks call 



