22 MR. J. SMITH ON THE ORIGIN OF COLOUR 



the united effect of the two lights as a unit. It is only by 

 casting a shadow that the light on the screen is known 

 to be physically compound, and it is by the shadow being 

 coloured that the intellect discovers, as in the wafer expe- 

 riment, that there must be vibrations or intervals in the 

 motion of both luminous waves or there could be no 

 colour. Besides, this white light must have a certain 

 intensity or the shadow could not be coloured, for as soon 

 as the daylight begins to lessen, the shadow begins to get 

 grey, even although the screen may appear quite white 

 when the candle is extinguished. 



36. But this is not all the experiment. Let us light 

 another candle. The united effect on the screen of the 

 daylight and the two candles will still appear white and 

 as originating from one source, but there will be now two 

 shadows of any opaque body; properly speaking, three, 

 for the daylight also produces a shadow; but we are only 

 speaking at present of the shadows caused by the candles. 

 Make the one shadow partly to overlie the other shadow, 

 then the part where the shadows overlie will be of a much 

 deeper tinge of blue than the other parts. This deeper 

 shadow, when estimated by the eye, appears of more than 

 double the intensity of the others. We need not attempt 

 to count beats or pulsations of the ether at present; but 

 supposing the candle to give the greater shadow, let us 

 imagine the effective pulsation of the light of each candle 

 to be double that of daylight. On the screen the number 

 of pulsations would be increased, and consequently also 

 on the retina ; and on this supposition there would be five 

 pulsations where before there was only one, and still the 

 eye would estimate the whole five only as one. The pul- 

 sations would be one of daylight plus four of candlelight, 

 two for each candle. On the darkest shadow there would 

 be one pulsation of light, namely, one of daylight ; on the 

 shadows not overlying there would be three pulsations of 



