AND THE THEORY OE LIGHT. 71 



Change of Colour by reversing the motion of 

 the Figure. 



108. One of the most singular and remarkable of the 

 phenomena connected with these experiments is the change 

 of colour which takes place by reversing the motion. 



Suppose we have a card {see plate IV. fig. 8) with con- 

 centric rings cut out of or painted on it, and make it 

 revolve by excentric motion, we shall find that one series 

 of rings is coloured reddish-brown, yellow, or some al- 

 lied colour; another series green, blue, or purple. These 

 rings, when the motion is reversed, change colour. This 

 perplexed me very much at first, for it really indicated 

 something like the polarization of light, A close and re- 

 peated examination of the phenomenon indeed convinced 

 me that it must help to remove much of the mystery from 

 what is called polarized light. But this is too wide a field 

 to enter on at present. 



109. In order, however, to arrive at the cause of this 

 interesting phenomenon I attempted to analyse one of 

 the semi-discs having concentric rings. I therefore made 

 another semi-disc, divided it into seven concentric rings, 

 leaving a small ring at the centre in order to get the disc 

 fixed as usual. II aving drawn the seven concentric rings, 

 I made a new centre, at a distance of half the breadth of 

 one of the rings from the former centre, and drew from it 

 concentric circles so as to meet or touch the other circles, 

 but not to intersect them, as will be seen in plate IV. 

 fig. 9. Each ring was thus divided into two equal parts, 

 by a circle passing as it were diagonally across the ring 

 and forming two triangles, one of which was painted black, 

 so that when in rapid motion the ring composed of these 

 two triangles, a black and a white, might be considered as 

 condensed into a parallelogram divided diagonally, or the 

 white and black parts might be considered as the two faces 



