AND THE THEORY OF LIGHT. 69 



the figure a spiral sort of form, and at the same time drew 

 a number of rings, so arranged that when put in motion 

 the light might gradually increase in proportion to the 

 shade from the inner part of the ring to its circumference, 

 or vice versa. Some point in the inner circumference of 

 the ring was made the centre of motion as o, plate V. 

 fig. 3. Although the figure was of a spiral form before 

 being put in motion, like every other figure moving hori- 

 zontally, the disc when in motion seemed composed of 

 concentric rings. This experiment gave me nearly all I 

 desiderated in regard to the production of colour. The 

 arithmetical law involved, however, still eluded research. 

 The investigation became difficult, for the smallest change 

 of the centre and the least alteration in the motion created 

 a new obstacle which could not be overcome but by enu- 

 merating the elements, and these were unknown, for we 

 have no terms to designate the variety and the shade of 

 a colour. 



104. This form afforded a bright yellow, and I hence 

 concluded that a regular spiral would give less yellow and 

 more green. I therefore cut out a spiral of the form shown 

 in plate V. fig. 4, having its centre of revolution in 0. It 

 is so simple that one feels some surprise at the beautiful 

 figure produced by its motion. 



One of these discs was made for the purpose of pro- 

 ducing yellow, the other to produce green. In a good 

 light they both exhibit very remarkable phenomena. The 

 effect produced by fig. 4 plate V. when in motion is shown 

 on a reduced scale on plate III. The discs themselves 

 were, in general, about six inches in diameter. 



Half Discs. 



105. It will be observed that half discs are often used 

 in the experiments. I had not experimented long before 

 I found that it was requisite to take much light away, or 



