36 PHYSICAL SCIENCE BK. i 



liquid at rest. Then we are in the habit of noting 

 how the moon obstructs the sun, and by the inter- 

 position of her body hides his, which is so much larger, 

 sometimes partially, if it so fall out that she only 

 encounter a portion of his orb, sometimes completely. 

 2 The latter is called a total eclipse : it quite shuts 

 out the light and shows us the stars ; it occurs when 

 the centre of the two bodies lies in the same straight 

 line. Now, just as the image of both sun and moon 

 can be seen on earth, so it is in the case of mock 

 suns in the atmosphere. The still air is so compact 

 and yet clear that it can receive the sun's likeness. 

 Other clouds receive it, but let it go if they are 

 either in motion, or thin, or black. The moving 

 clouds disperse it, the rare let it slip, the black and 

 impure do not take the impress of it, just as on 

 earth soiled objects do not reflect an image. 



XIII 



1 MOCK suns are wont to be formed in pairs and on 

 the same principle. There is nothing, in fact, to 

 prevent the formation of as many as there are clouds 

 suitable for exhibiting an image of the sun. Some 

 writers are inclined to hold that when two such 

 phantoms are visible, one arises directly from the 

 sun, the other from his image. For, to use an 

 illustration from common experience, when several 

 mirrors are so arranged that one is in sight of the 

 other, all reflect the same image ; but only one is 

 directly from the original, the rest are reflections 



2 of images. The nature of the object presented to 

 the mirror makes no difference in the effect. What- 

 ever it sees it reproduces. So, up on high there, if 



