AND THE THEORY OF LIGHT. 19 



Experiment III., or Candle Experiment. 



Light a candle in daylight, and observe the shadows it 

 casts on the window blind or on a sheet of white paper : 

 they are sometimes blue, at other times purple. The 

 spot on which the shadow is cast, both before and after 

 the candle is lighted, will be, to all appearance, white, — 

 at least the eye can detect no colour j but after the 

 candle is lighted, the spot on which the shadow falls 

 is seen to be coloured. What part does the shadow 

 perform here ? Does it merely remove the light of the 

 candle from the spot, and enable us to contrast the light 

 of the sun with the light of the candle? It does more. 

 It shows in this case also that colour is connected with 

 shadow, 



33. The received philosophical interpretation of this expe- 

 riment is to the following effect. I quote from Dr. Young's 

 works : — " If from the light of a candle we take away some 

 of the abundant yellow light, and leave or substitute a por- 

 tion actually white, the effect is nearly the same as if we 

 took away the yellow light from white, and substituted 

 the indigo which would be left ; and we observe, accord- 

 ingly, that in comparison with the light of a candle the 

 common daylight appears of a purplish hue." Let us 

 look for a moment at this explanation, and see what we 

 can make of it. He says. If we take away some of the 

 abundant yellow light from the light of a candle, and leave 

 or substitute a portion actually white; but he does not 

 tell us how' to perform this very nice experiment. True, 

 he puts it in our power to leave or substitute a portion 

 actually white ; but this only makes the experiment more 

 mysterious and more difficult to perform. It would appear 

 as if the remarks of Dr. Young presupposed some power 

 of working with a compound beam of light as a painter 

 works with his colours on his pallet. He adds — suppo- 



