30 On the Vision of Colours. 
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 phe- 
nomenon; when I was surprised to find 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 ob- 
servation 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 per- 
ceptible to me than to others. It was nearly two 
years after that time, when I entered upon an inves- 
tigation of the subject, having procured the assist- 
ance of a friend, who, to his acquaintance with the 
theory of colours, joins a practical knowledge of 
their names and constitutions. I shall now proceed 
to state the facts ascertained under the three follow- 
ing heads: 
J. An account of my own vision. 
II. An account of others whose vision has been 
' found similar to mine. 
III. Observations on the probable cause of our 
anomalous vision. 
I. OF MY OWN VISION. 
It may be proper to observe, that I am short- 
