On the Vision of Colours. 34 
sighted. Concave glasses of about five inches focus 
suit me best. I can see distin€tly at a proper dis- 
tance; and am seldom hurt by too much or too lit- 
tle light ; nor yet with long application. 
My observations began with the solar spectrum, 
or coloured image of the sun, exhibited ina dark 
- room by means of a glass prism. I found that per- 
sons in general distinguish six kinds of colour in 
the solar image; namely, red, orange, yellow, green, 
blue, and purple. Newton, indeed, divides the pur- 
ple into indigo and violet; but the difference be- 
tween him and others is merely nominal. To me 
it is quite otherwise:—I see only ¢wo or at most 
three distinctions. These I should call yellow and 
blue; or yellow, blue, and purple. My yellow com- 
prehends the red, orange, yellow, and green of others ; 
and my blue and purple coincide with theirs. That 
part of the image which others call red, appears to 
me little more than a shade, or defect of light ; after 
that the orange, yellow, and green seem one colour, 
which descends pretty uniformly from an intense to 
a rare yellow, making what I should call different 
shades of yellow. , The difference between the green 
part and the yeH#sw part is very striking to my eye: 
they seem to be strongly contrasted. That between 
the blue and purple is much less so. The purple 
appears to be blue much darkened and condensed.’ 
In viewing the flame of a candle by night through 
the prism, the appearances are pretty much the same, 
