AND THE THEORY OF LIGHT. 73 



that is that one disc would produce only one set of co- 

 lours, the other disc the other ; and when the motion was 

 changed that each would represent the colours of the 

 other. I was delighted with the appearance as well as 

 the success of this experiment when seen by gaslight. 

 The same eflfect is produced in good sunlight. 



111. It may be mentioned here that all these experi- 

 ments were originally made by gaslight. I may also add 

 that it would be useless to describe the colours of any of 

 these figures, as they vary so much, not only from the 

 nature of the atmospheric light, but according to the spot 

 which is made the centre of the motion. For instance, 

 when^^. 9, plate IV., already described, was made to re- 

 volve excentrically in daylight, the sun shining white 

 through a haze, and the ground being completely covered 

 with snow, there appeared two series of the most beau- 

 tiful greens and purples imaginable. 



112. It is to be remarked that though the amount of 

 light and shade on these two semi-discs (plate V. figs, i 

 and 2) is the same, still, when they are put in motion, the 

 colours are difi"erent. The ratio of light to shade would 

 thus appear to be no more the cause of difference of colour 

 than the refrangibilities of the rays, or the lengths of 

 waves. Other circumstances must be taken into con- 

 sideration, such as whether the vibration is one belonging 

 to an ascending or a descending series, to an increasing 

 or a decreasing force. But what is this, if it is not the 

 ratio of light and shade? I shall return to this subject 

 again under the head of perpendicular motion. 



Now if we consider the diagonal of a ring, as dividing 

 it into two triangles, one being white, the other black; 

 and if we suppose the white triangle to represent a beam 

 of light and each revolution of the machine to correspond 

 to a vibration of light, the angular point being the com- 

 mencement of the vibration, and the third side or base 



SER. III. VOL. I. L 



