[From the Transactions of the Royal Scottish Society of Arts, Vol. iv. Part in.] 



VI. On the Theory of Colours in relation to Colour- Blindness. 

 A letter to Dr G. Wilson. 



DEAR SIR, As you seemed to think that the results which I have obtained 

 in the theory of colours might be of service to you, I have endeavoured to 

 arrange them for you in a more convenient form than that in which I first 

 obtained them. I must premise, that the first distinct statement of the theory 

 of colour which I adopt, is to be found in Young's Lectures on Natural Philo- 

 sophy (p. 345, Kelland's Edition) ; and the most philosophical enquiry into it 

 which I have seen is that of Helmholtz, which may be found in the Annals of 

 Philosophy for 1852. 



It is well known that a ray of light, from any source, may be divided by 

 means of a prism into a number of rays of different refrangibility, forming a 

 series called a spectrum. The intensity of the light is different at different 

 points of this spectrum ; and the law of intensity for different refrangibilities 

 differs according to the nature of the incident light. In Sir John F. W. 

 Herschel's Treatise on Light, diagrams will be found, each of which represents 

 completely, by means of a curve, the law of the intensity and refrangibility of 

 a beam of solar light after passing through various coloured media. 



I have mentioned this mode of defining and registering a beam of light, 

 because it is the perfect expression of what a beam of light is in itself, con- 

 sidered with respect to all its properties as ascertained by the most refined 

 instruments. When a beam of light falls on the human eye, certain sensations 

 are produced, from which the possessor of that organ judges of the colour and 

 intensity of the light. Now, though every one experiences these sensations, and 

 though they are the foundation of all the phenomena of sight, yet, on account 

 of their absolute simplicity, they are incapable of analysis, and can never become 

 in themselves objects of thought. If we attempt to discover them, we must 



