360 Proceedings of Societies. 



•37 Vermilion, | f 

 + -27 Ultramarine, = + . 72Black 

 + •34 Emerald green, ) « 

 By experiments on various individuals, it was found (1.) that a good 

 eye could be depended upon within two of these divisions, or hundredths 

 at most, and that by repetition of experiments, the average result might 

 be made much more accurate. 



(2.) That the difference of the results of experiments on different indi- 

 viduals was insensible, provided the light used remained the same. 



(3.) That when different kinds of light were used, or when the resultant 

 tints were examined with coloured glasses, the results were totally changed. 

 It follows from this that the cause of the equality of the resulting tints 

 is not a true optical identity of the light received by the eye, but must be 

 sought for in the constitution of the sense of sight. The materials for 

 this inquiry are to be found in the equations of colour, of which the above 

 is an example, and these are to be viewed in the light of Young's theory 

 of a threefold sensation of colour. 



The first consequence of this theory is, that between any/o«r colours 

 an equation can be found, and this is confirmed by experiment. 



The second is, that from two equations containing different colours a 

 third may be obtained by the ordinary rules, tmd that this also will agree 

 with experiment. This also was found to be true by experiments at 

 Cambridge, which include every combination of five colours. 



A graphical method was then described, by which, atter fixing arbi- 

 trarily the positions of three standard colours, that of any other colour 

 could be obtained by experiments in which it was made to form a neutral 

 grey along with two of the standard colours. In the diagram so formed, 

 the position of any compound tint is the centre of gravity of the colours 

 of which it is composed, their masses being determined from the equation, 

 and the resultant mass of colour being the sum of the component masses. 

 The colour-equations represent the fact that the same tint may be pro- 

 duced by two different combinations. This diagram is similar to those 

 which have been given by Meyer, Hay, and Professor J. D. Forbes, as 

 the results of mixing colours. It is identical with that proposed by Young, 

 and figured in his Lectures on Natural Philosophy . The original con- 

 ception, however, seems to be due to Newton, who gives the complete 

 theory, with an indication of a construction in his Optics. 



The success of this method depends entirely on the truth of the suppo- 

 sition that there are three elements of colour as seen by the eye, every 

 ray of the spectrum being capable of exciting all three sensations, though 

 in different proportions. It is at present impossible to define the colours 

 appropriate to these sensations, as they cannot be excited separately. 

 But it appears probable that the phenomena of colour-blindness are due 

 to the absence of one of these elementary sensations, and, if so, a compa- 

 rison of colour-blind with ordinary vision will show the relation of the 

 absent sensation to those with which we are familiar. 



A method was then described, by which one observation by a colour- 

 blind eye was made to determine a certain point representing the absent 

 sensation, which thus appears to be a red approaching to crimson. The 

 results of this hypothesis were calculated in the form of " equations of 

 colour-blindness" between colours which seem to defective eyes identical. 

 These equations were compared with those previously determined from 

 the testimony of two colour-blind, but accurate observers, and found to 

 agree with remarkable precision, rarely differing by more than 0*02 in 

 any colour. The effect of red and green glasses on the colour-blind was 

 then described, and a pair of spectacles having one eye red and the other 

 green was proposed as an assistance to them in detecting doubtful colours. 



