60 MR. J. SMITH ON THE ORIGIN OF COLOUR 



ciation of colour, but for delicacy of taste in regard to 

 harmony of tints, could at first see little or no colour. 



The experiment, however, was successful ; blue was ob- 

 tained by the motion of the slip of white card. 



90. The theory I now considered to be established. The 

 only desideratum was to acquire some knowledge of details, 

 some knowledge of the ratio which the shade, or rather 

 penumbra, ought to have to the light for any given colour. 

 This I knew could only be obtained by experiment, and I 

 never doubted but that some approximation would be 

 made to it. I did not, however, anticipate the difficulty of 

 the calculation, nor yet the splendid results at which I 

 have arrived in the production of colour by the motion of 

 a white ray. The experiment is essentially only a repe- 

 tition of the wafer experiment, the card experiment, and 

 the candle experiment ; it consists in the casting of unre- 

 fracted light on a shadow. Of course the shadow in this 

 case is the darkness over which the white ray moves, and 

 is only cast on the eye as in the wafer experiment, but it 

 is virtually not less a shadow than if it were first cast on 

 a piece of paper. This will presently be made more ap- 

 parent. 



9 1 . After this experiment, being otherwise much engaged, 

 I rested for some time quite satisfied with the result, but on 

 repeating it afterwards I began to think that I had made 

 some great mistake, that there had been an optical illusion, 

 for I could now only obtain purple in place of blue. This 

 last experiment was made in bright sunshine and with a 

 blue sky. In making the same experiment in a dark dull 

 day, during slight rain, again the same purple appeared. I 

 had neglected to note the state of the atmosphere in the first 

 experiment. When, however, in a few days the sky began 

 to clear, and white fleecy clouds to make their appearance, 

 the same blue was obtained as at first. The paper, at 

 least the small shp I used, on a dull day or with a bright 



