Mr Dalton on the Vision of Colours. 89 



&c. excited by the same object may be very different in them- 

 selves, without our being aware of it ; and that we may never- 

 theless converse intelligibly concerning such objects, as if we 

 were certain the impressions made by them on our minds were 

 exactly similar. All, indeed, that is required for this purpose, 

 is, that the same object should uniformly make the same im- 

 pression on each mind ; and that objects which appear different 

 to one should be equally so to others. It will, however, scarce- 

 ly be supposed, that any two objects, which are every day be- 

 fore us, should appear hardly distinguishable to one person, 

 and very different to another, without the circumstance imme- 

 diately suggesting a difference in their faculties of vision ; yet 

 such is the fact, not only with regard to myself, but to many 

 others also, as will appear in the following account. 



I was always of opinion, though I might not often mention 

 it, that several colours were injudiciously named. The term 

 pink, in reference to the flower of that name, seemed proper 

 enough ; but when the term red was substituted for pink, I 

 thought it highly improper ; it should have been blue, in my 

 apprehension, as pink and blue appear to me very nearly 

 allied ; whilst pink and red have scarcely any relation. 



In the course of my application to the sciences, that of optics 

 necessarily claimed attention ; and I became pretty well ac- 

 quainted with the theory of light and colours before I was 

 apprized of any peculiarity in my vision. I had not, however, 

 attended much to the practical discrimination of colours, owing, 

 in some degree, to what I conceived to be a perplexity in their 

 nomenclature. Since the year 1790, the occasional study of 

 botany obliged me to attend more to colours than before. 

 With respect to colours that were white, yellow, or green, I 

 readily assented to tlie appropriate term. Blue, purple, pink, 

 and crimson appeared rather less distinguishable ; being accord- 

 ing to my idea, all referable to blue. I have often seriously 

 asked a person whether a flower was blue or pink, but was 

 generally considered to be in jest. Notwithstanding this, I was 

 never convinced of a peculiarity in my vision, till I accidentally 

 observed the colour of the flower of the Geranium xonaie by 

 candle-light, in the autumn of 1792. The Bower was pink. 

 but it appeared to me almost an exact sky-blue by day ; in 



