40 LIGHT SCIENCE FOR LEISURE HOURS. 



is seen all round the interposed object. And it might 

 seem that since the moon is but a globe somewhat 

 larger than our experimental one, and somewhat 

 farther off, we ought to see a similar light all round 

 the black disc of the eclipsing moon. 



But a little consideration will show the fallacy of 

 this reasoning. 



When we hold a globe so as just to hide the sun, 

 we do not throw into shadow those upper regions of air 

 from which the atmospheric glare really comes. But 

 when the moon conceals the sun during total eclipse, 

 she causes an enormous shadow to fall right through 

 the whole depth of the air. This shadow, even at its 

 narrowest, that is, where it reaches the earth, has been 

 in many total eclipses fully one hundred miles wide ; 

 and as the part of the air capable of reflecting solar 

 light to an appreciable extent is shown by the twilight- 

 arch to be but forty or fifty miles high at the outside, 

 we see that, in the case of such eclipses, the moon's 

 shadow in the air is of a vast drum-shaped figure, at 

 least twice as wide as it is high. It is most obvious, 

 then, that to an eye placed at the centre of the vast 

 base of this drum-shaped shadow, no light can possibly 

 come from the air for a wide range all round the place 

 of the eclipsed sun. Imagine a shadow hiding nearly 

 all England and fifty miles high ; then to an eye 

 placed, say at Hereford, the upper surface of the shadow 

 would cover an enormous extent of sky, while the 

 eclipsed sun, at the apparent centre of that surface, 

 would be but as a relatively minute circle. 



