142 KXI'I'IUMENTS ON COLOUR, AS PKKCKIVED BY THE EYE. 



of three elements of colour, by Professor Grassmann, is translated in the Phil. 

 Mag., April, 1854. 



References to authors on colour-blindness are given in Dr G. Wilson's papers 

 on that subject. A valuable Letter of Sir J. F. W. Herechel to Dalton on his 

 peculiarity of vision, is to be found in the Life of Dalton by Dr Henry. 



I had intended to describe some experiments on the propriety of the method 

 of mixing colours by rotation, which might serve as an extension of Mr Swan's 

 experiments on instantaneous impressions on the eye. These, together with the 

 explanation of some phenomena which seem to be at variance with the theory of 

 vision here adopted, must be deferred for the present. On some future occasion, 

 I hope to be able to connect these simple experiments on the colours of pigments 

 with others in which the pure hues of the spectrum are used. I have already 

 constructed a model of apparatus for this purpose, and the results obtained are 

 sufficiently remarkable to encourage perseverance. 



NOTE I. 

 On different Methods of Exhibiting the Mixtures of Colours. 



(l) Mechanical Mixture of Coloured Powder*. 



By grinding coloured powders together, the differently-coloured particles may 

 be so intermingled that the eye cannot distinguish the colours of the separate 

 powders, but receives the impression of a uniform tint, depending on the nature 

 and proportions of the pigments used. In this way, Newton mixed the powders 

 of orpiment, purple, bise, and viride cms, so as to form a gray, which, in sun- 

 light, resembled white paper in the shade. (Newton's Ojtticks, Book I. Part n., 

 Exp. xv.) This method of mixture, besides being adopted by all painters, has 

 been employed by optical writers as a means of obtaining numerical results. 

 The specimens of such mixtures given by D. R. Hay in his works on Colour, 

 and the experiments of Professor J. D. Forbes on the same subject, shew the 

 importance of the method as a means of classifying colours. There are two 

 objections, however, to this method of exhibiting colours to the eye. When 

 two powders of unequal fineness are mixed, the particles of the finer powder 

 cover over those of the coarser, so as to produce more than their due effect 

 in influencing the resultant tint. For instance, a small quantity of lamp-black, 



