420 ON THE THEORY OF COMPOUND COLOURS. 



observations are confirmed by those of Mr Pole, and by others which I have 

 obtained since. I have hopes of being able to procure a set of colour-blind 

 equations between the colours of the spectrum, which will indicate the missing 

 primary in a more exact manner. 



The experiments which I am going to describe have for their object the 

 determination of the position of the colours of the spectrum upon Newton's 

 diagram, from actual observations of the mixtures of those colours. They were 

 conducted in such a way, that in every observation the judgment of the observer 

 was exercised upon two parts of an illuminated field, one of which was so 

 adjusted as to be chromatically identical with the other, which, during the whole 

 series of observations, remained of one constant intensity of white. In this way 

 the effects of subjective colours were entirely got rid of, and all the observa- 

 tions were of the same kind, and therefore may claim to be equally accurate ; 

 which is not the case when comparisons are made between bright colours of 

 different kinds. 



The chart of the spectrum, deduced from these observations, exhibits the 

 colours arranged very exactly along two sides of a triangle, the extreme red and 

 violet forming doubtful portions of the third side. This result greatly simplifies 

 the theory of colour, if it does not actually point out the three primary colours 

 themselves. 



V. Description of an Instrument for making definite Mixtures of the 



Colours of the Spectrum. 



The experimental method which I have used consists in forming a combi- 

 nation of three colours belonging to different portions of the spectrum, the quantity 

 of each being so adjusted that the mixture shall be white, and equal in intensity 

 to a given white. Fig. 1, Plate VI. p. 444, represents the instrument for 

 making the observations. It consists of two tubes, or long boxes, of deal, of 

 rectangular section, joined together at an angle of about 100. 



The part AK is about five feet long, seven inches broad, and four deep ; 

 KN is about two feet long, five inches broad, and four deep ; BD is a partition 

 parallel to the side of the long box. The whole of the inside of the instrument 

 is painted black, and the only openings are at the end AC, and at E. At the 

 angle there is a lid, which is opened when the optical parts have to be adjusted 

 or cleaned. 



