264 EXPERIMENTS ON THE PERCEPTION OF COLOUR, 



following experiments relate to the combinations of six well-defined colours only, 

 and I shall describe them the more minutely, as I hope to induce those who 

 have good eyes to subject them to the same trial of skill in distinguishing 

 tints. 



The method of performing the experiments is described in the Transactions 

 of the Royal Society of Edinburgh, Vol. xxi. Part 2. The colour- top or teetotum 

 which I used may be had of Mr J. M. Bryson, Edinburgh, or it may be easily 

 extemporized. Any rotatory apparatus which will keep a disc revolving steadily 

 and rapidly in a good light, without noise or disturbance, and can be easily 

 stopped and shifted, will do as well as the contrivance of the spinning-top. 



The essential part of the experiment consists in placing several discs of 

 coloured paper of the same size, and slit along a radius, over one another, so 

 that a portion of each is seen, the rest being covered by the other discs. By 

 sliding the discs over each other the proportion of each colour may be varied, 

 and by means of divisions on a circle on which the discs lie, the proportion of 

 each colour may be read off. My circle was divided into 100 parts. 



On the top of this set of discs is placed a smaller set of concentric discs, 

 so that when the whole is in motion round the centre, the colour resulting from 

 the mixture of colours of the small discs is seen in the middle of that arising 

 from the larger discs. It is the object of the experimenter to shift the colours 

 till the outer and inner tints appear exactly the same, and then to read off the 

 proportions. 



It is easy to deduce from the theory of three primary colours what must 

 be the number of discs exposed at one time, and how much of each colour must 

 appear. 



Every colour placed on either circle consists of a certain proportion of each 

 of the primaries, and in order that the outer and inner circles may have precisely 

 the same resultant colour in every respect, there must be the same amount of 

 each of the primary colours in the outer and inner circles. Thus we have as 

 many conditions to fulfil as there are primary colours ; and besides these we 

 have two more, because the whole number of divisions in either the outer or 

 the inner circle is 100, so that if there are three primary colours there will be 

 five conditions to fulfil, and this will require five discs to be disposable, and 

 these must be arranged so that three are matched against two, or four against one. 

 If we take six different colours, we may leave out any one of the six, and 

 so form six different combinations of five colours. It is plain that these six 



