AND THE THEORY OF LIGHT, 43 



of liglitj which I am endeavouring to establish, rests — 

 at least so far as they relate to the primary colours, blue, 

 green and red — I shall for the purpose of recapitulation 

 describe two other experiments which can be made at any 

 time, and which, to my mind, are beautifully illustrative 

 of the whole theory. 



Observation, A. 



In a room with a good bright fire, the flame of which is 

 so strong as to produce a distinct shadow on the ceiling of 

 any object such as the gaselier, suppose we light a candle 

 or a jet of gas, then the candle or gas will also form a 

 shadow of the same object, but the two shadows will be 

 coloured, the one differently from the other. The shadow 

 formed by the jet of gas will be brownish-red, for the flame 

 of the fire which illuminates it is red ; the shadow cast by 

 the fire will be blue, for the light of the gas which illu- 

 minates it is white or nearly white. 



Now if we withdraw one of the lights, say the jet of gas 

 which is easily managed, the ceiling will still be white, as far 

 as the eye can judge, but the shadow is grey because it is no 

 longer illuminated with the gas-light ; and the same thing 

 happens to the shadow from the gas, it also becomes grey 

 when the light of the fire gets dull : thus demonstrating, 

 not that the one light compared to the other is as brown 

 to blue, as we were told in the experiment with the can- 

 dle and daylight by Dr. Young, but that each coloured 

 shadow is some fraction of the unit of force agitating the 

 retina ; but that fraction of the unit of force agitating the 

 retina cannot be calculated until we know the value of the 

 unit and the value of each shadow. For that each of the 

 shadows has a value, independently of the illuminating 

 power exerted on it by the other's light, may be seen by 

 examining each of them separately. It will be observed 

 that neither of them is absolute darkness, or a complete 

 negation of force. 



