100 JAMKS CLKItK MAXWKU. 



These discs are placed on a top or teetotum, which 

 is spun rapidly. The axis of the top passes through 

 the centre of the discs, and the quantity of each 

 colour exposed is measured by graduations on the 

 rim of the top, which is divided into 100 parts. 

 When the top is spun sntHciently rapidly, the 

 impressions due to each colour separately follow cadi 

 other in quick succession at each point of the retina, 

 and are blended together; the strength of the im- 

 pression due to each colour is, as can he shown 

 experimentally, the same as when the three kinds of 

 light in the same relative proportions enter the 

 eye simultaneously. These relative proportions are 

 measured by the areas of the various discs which 

 are exposed. Two sets of discs of ditlerent radius 

 are used; the largest discs are put on first, then the 

 smaller, so that the centre portion of the top shows 

 the colour arising from the mixture of those of the 

 smaller discs; the outer portion, that of the larger 

 discs/' 



In experimenting, six dis<;s of each si/e are used, 

 black, white, red, green, yellow and blue. It is found 

 by experiment that a match can be arranged between 

 any five of these. Thus three of the larger discs are 

 placed on the top say black, yellow and blue and 

 two of the smaller discs, red and green, are placed 

 above these. Then it is found that it is possible so 

 to adjust the amount exposed of each disc that the two 

 parts of the top appear when it is spun to be of the 

 same tint In one series of experiments the chromatic 

 effect of 40-8 parts of black, 2<M of yellow, and 24'1 

 of blue was found to be the same as that of <M*G of 



