AND THE THEORY OF LIGHT. 75 



duced, they were not so defined as to afford any basis for a 

 unit of measure. The experiments only proved the neces- 

 sity for assuming a negative element in colour. 



The experiments by excentric motion introduced a wider 

 range of phenomena^ and showed how this wider range 

 was obtained : but the colorific power of a molecule of 

 light is to me still as immeasurable as its weight is im- 

 ponderable. 



The spiral figures showed perhaps still more distinctly 

 than the excentric how one ray modifies another, and how 

 gradation of tint is produced. 



115. When treating of the colours produced by re- 

 versing the motion, I have shown how an increasing 

 series of rays produces one species of colour and a de- 

 creasing series another. 



If then we are still left without a mathematical element 

 for expressing the value of a ray of light, we are not in a 

 worse condition mathematically — at least as far as colour 

 is concerned — ■ than we have ever been. For never since 

 the science of light has been systematically taught has the 

 value of a ray been expressed mathematically. All such 

 calculations are mere empiricism, and of no practical 

 utility. 



116. If we take a piece of Bristol board, and draw on it 

 a spiral figure in china-ink or lamp-black, or draw on it 

 several concentric black circles, and make the disc revolve 

 with excentric motion, the black circles or spiral in gas- 

 light will appear a brilliant red. On reversing the motion 

 they will appear green or blue, the effect depending much 

 on the light.* {See plate IV. fig. 9.) 



* To prove that there are, as it were, two discs in motion, and each 

 different in colour from the other when the motion is excentric, let us 

 make a semi-disc of about six inches diameter, and draw on it several 

 concentric rings. Make a few of the lines darker than others to give 

 variety. Let o be the centre, and x the centre of motion. The two discs 

 will have for radius x a and x b, and the two series of rings will be dis- 



