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

 Blindness. By James Clerk Maxwell, B.A., Trinity College, Cambridge. 

 Communicated by Dr Gregory. (With a Plate.) 



(Read 19th March 1855.) 



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 show how experiments so made may be regis- 

 tered 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 uniform 

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

 pared 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 common 

 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 circumference of 

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

 portions 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 No- 

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

 D. R. Hay. The experiments made in the present year were with the im- 

 proved top made by Mr J. M. Bryson, Edinburgh, and coloured papers prepared 

 by Mr T. Purdie, with the unmixed pigments used in the arts. A number of 

 Mr Bryson's tops, with Mr Purdie's coloured papers has been prepared, so as to 

 afford different observers the means of testing and comparing results independ- 

 ently obtained. 



VOL. XXI. PART II. 4 E 



