44 MR. J. SMITH ON THE ORIGIN OF COLOUR 



68. From this aud the foregoing observations I conclude, 

 that the value of a colour is known only when both the 

 active and the passive or negative elements are known, 

 and that there is no colour without the negative as well as 

 the passive element. They are coordinate elements of 

 colour. 



A similar observation beautifully illustrative of the 

 above may be made almost every day when the sun is 

 shining. I see every morning on my window blinds^ when 

 the weather is favourable, coloured shadows such as I have 

 been describing. Let us subject them to examination. 



Observation, B. 



Before a sheet of white paper used as a screen, hold a 

 small opaque body, as a pencil, so that its shadow may fall 

 on the paper. There will be seen two shadows ; one blue 

 or purple, depending on the state of the atmosphere, or at 

 times on the distance of the object which casts the shadow 

 on the screen — the other, red, ochre or yellow. These 

 are the same colours which were seen in the candle obser- 

 vation or experiment. There is evidently here a partial 

 analysis of the illuminating medium, the separation of 

 it into two distinct waves. But the question arises, Are 

 these two distinct waves red and blue? I say they are 

 not. But this is a problem not so easily solved. We 

 must, therefore, resort to our former observations for 

 assistance, if they can afford us any. 



The state of the problem is this. We have two shadows; 

 one, blue or purple — the other, red, ochre or yellow. 

 Whence come these colours ? At the time these shadows 

 are seen, some of the rays from the lower part of the 

 sun are still intercepted ;• there is, consequently, a great 

 penumbra in the direction of the ray, although not easily 

 detected but by such an experiment. The rays within 

 this penumbra are direct ; the rays without it are reflected. 



