43 MR. J. SMITH ON THE ORIGIN OF COLOUR 



inspection of this phenomenon will show us that there is a 

 penumbra cast by the object on the eye from the sub- 

 stance interposed between the light and the eye, and that 

 it is this penumbra which is coloured. Is not the red of 

 the atmosphere produced in the same manner ? Is it not 

 the light of the sun, reflected from the atmosphere at 

 intervals with the penumbra of the earth cast on the same 

 atmosphere ? In the one case we can detect the penum- 

 bra; in the other we can only infer its existence. But the 

 inference is perfectly legitimate. 



65. I may be told that to produce this parallelism in rays, 

 refraction is necessary. True, refraction is necessary; but 

 not for the purpose of decomposing a compound hetero- 

 geneous substance into its constituents parts, but rather 

 for the purpose of compounding an active with an inert 

 or passive sensation. 



66. But it will be asked, Is not this what the wave theory 

 teaches ? If it is, it teaches it in a very different way. 

 The wave theory assumes different lengths of waves ; and 

 these different lengths must be owing to the compound 

 nature of light, or to some affections in matter not under- 

 stood, and only hypothetically assumed, 



6y. These observations on red are satisfactory only so 

 far as the phenomena are seen in connection with shadow; 

 but combining them with others, they may assist in 

 establishing the elements of the science of light; and 

 it has been considered necessary to refer to them, because 

 the proofs derived from experiment would not be under- 

 stood nor acquiesced in, unless accompanied by preliminary 

 remarks illustrative of the method of investigation, and 

 showing that the experiments were not made accidentally 

 or at random, but for the purpose of developing the laws 

 on which the optical phenomena everywhere surrounding 

 us depend. 



Having thus gone over the arguments on which the theory 



