[From the Transactions of tiie Royal Society of Edinburgh, Vol. XXI. Part II.] 



VII. Experiments on Colour, as perceived by the Eye, with remarks on Colour- 

 blindness. Communicated by Dr Gregory. 



THE object of the following communication is to describe a method by 

 which every variety of visible colour may be exhibited to the eye in such a 

 form as to admit of accurate comparison ; to shew how experiments so made 

 may be registered numerically ; and to deduce from these numerical results 

 certain laws of vision. 



The different tints are produced by means of a combination of discs of paper, 

 painted with the pigments commonly used in the arts, and arranged round an 

 axis, so that a sector of any required angular magnitude of each colour may be 

 exposed. When this system of discs is set in rapid rotation, the sectors of 

 the different colours become indistinguishable, and the whole appears of one uni- 

 form tint. The resultant tints of two different combinations of colours may be 

 compared by using a second set of discs of a smaller size, and placing these over 

 the centre of the first set, so as to leave the outer portion of the larger discs 

 exposed. The resultant tint of the first combination will then appear in a ring 

 round that of the second, and may be very carefully compared with it. 



The form in which the experiment is most manageable is that of the com- 

 mon top. An axis, of which the lower extremity is conical, carries a circular 

 plate, which serves as a support for the discs of coloured paper. The circumfer- 

 ence of this plate is divided into 100 equal parts, for the purpose of ascertaining 

 the proportions of the different colours which form the combination. When the 

 discs have been properly arranged, the upper part of the axis is screwed down, 

 so as to prevent any alteration in the proportions of the colours. 



The instrument used in the first series of experiments (at Cambridge, in 

 November, 1854) was constructed by myself, with coloured papers procured from 



