AND THE THEORY OF LIGHT. 45 



as is seen by the direction of the shadow. The shadow 

 produced by the rays within the penumbra is illuminated by 

 the light without the penumbra, and it is, consequently, 

 blue or purple. The shadow cast by the light without the 

 penumbra is illuminated by the stronger and more direct 

 light which is within it, and is red or yellow. This is pre- 

 cisely similar to the fire and candle experiment; but the 

 origin of the two rays is not so easily discovered. Terres- 

 trial reflection has often much to do with the colour of the 

 red ray. 



I have no doubt that these remarks will be met by the 

 counter assertion, that the light of the atmosphere is blue, 

 and that the other, or the red light, is refracted light, red 

 and blue united making white. 



In the present state of the science this is quite a legiti- 

 mate objection. Let us, however, test the truth of the 

 usual explanation by the following 



Observation, C. 



Suppose we ask the question, Do the two rays, red and 

 blue, which are obtained in the above experiment, make 

 white ? If they do, if we unite them again, the resultant 

 should be white. Let us then unite them. It is done as 

 follows. 



In place of one pencil, or opaque body, hold up two. 

 There will now be four shadows, each object producing 

 two. Let us then cause the red shadow of the one to 

 overlie the blue of the other. These two shadows should 

 by uniting produce white ; but it will be found that the 

 compound shadow is not white, it is green. 



But why has white not been produced by the combination 

 of these two shadows in the manner described ? Because, 

 although the two lights have been united, they are each 

 compounded with shadow. There are two waves of light 

 and two shadows, and they are each, as it were, insulated 



