The first communication which Dalton ever made to the Literary and Philosophical Society of Manchester was a paper read on 31 October 1794, entitled, ‘Extraordinary Facts relating to the Vision of Colours: with Observations by Mr John Dalton’.l Dalton had discovered that he saw colours differently from other people and his paper was the first serious study on colour deficiency to be published. So great was the stir produced by Dalton’s defect that for a long time this visual infirmity was known, to his amusement, as ‘Daltonism’. I trust my use of the term on this occasion will not be regarded as a mark of disrespect for the great man.
What is Daltonism? Rather than give a textbook answer in terms of wavelength discrimination and colour mixture curves, I would prefer to approach the problem as Dalton had to do, unaware at first that his colour vision differed from that of most other people and knowing very much less than we now know about the visual processes. This may, indeed, be of some help to others who are colour defective, for it must be a strange experience when you discover for the first time that your world of colour is very different from your neighbour’s.
Dalton first became convinced of the peculiarity of his vision at the age of twenty-six and as a result of observations on a pink geranium. This is what he reported:
The flower was pink, but it appeared to me almost an exact sky-blue by day; in candle-light, however, it was astonishingly changed, not having then any blue in it, but being what I called red, a colour which forms a striking contrast to blue. Not then doubting but that the change of colour would be equal to all, I requested some of my friends to observe the phenomenon; when I was surprised to find that they all agreed, that the colour was not materially different from what it was by day-light, except my brother, who saw it in the same light as myself. This observation clearly proved, that my vision was not like that of other persons; and, at the same time, that the difference between day-light and candle-light, on some colours, was indefinitely more perceptible to me than to others.
This passage could serve as a text for the remainder of my paper and I certainly want to comment on it in some detail. First, we have to remind ourselves that Dalton had to use the same vocabulary of colour names as everyone else; he had no other. Yet the range of colours that he could see was much restricted and he was almost certainly quite unaware of what redness really looked like. We can only speculate on why he chose to describe the appearance of the geranium in candlelight as red. Perhaps it appeared dark brown to him and since he was very insensitive to red wavelengths, he would tend to regard redness and darkness as synonymous.
We have to look at Dalton’s observation in the context of modern studies on the colour-rendering properties of light sources. You will know that lighting engineers have paid much attention during the last ten or twenty years to the development of fluorescent lamps. In the early days these lamps tended to make our faces and our food look too green, partly because of the mercury lines and partly owing to the lack of a good red phosphor. Now the lamps have been improved and their colour rendering performance in comparison with daylight can be rated in terms of a colour-rendering index.