20 
side, it becomes purple, soon after which they may be lost in the 
narrowness of the line. Thus a single light space may be contrived 
to show with the prism white, yellow, red and blue ; and by mixture, 
orange, green and purple. So that we have primaries and binary 
compounds, but no ternary compounds or browns*. 
Nature having dictated thus much, I presumed to look further, and 
found that there wanted nothing to form browns, except the means 
of bringing the proper proportion of colours together, something like 
the mixing of material colours. It was therefore required to con- 
trive that the yellow, red and blue rays should pass among each 
other, for the production of browns or ternaries. I succeeded in 
accomplishing this, as shall be shown presently. 
Finding that artificial light and dark will, by means of the prism, 
in common daylight, give all the original tints, it was thought 
necessary to show that yellow, red and blue, artificially contrived or 
placed, produce the three or more tints, and the tints so produced 
partake of the colour of the yellow, red, or blue, that produce them ; 
(see Tab. 3. a. h. c.) in which are instanced, first, the yellow, a. The 
white space between the two yellows would produce a proportion of 
white, 1 a proportion of yellow, 2 a proportion of red, and 3 a propor- 
tion of blue. The yellow is pretty good, being produced by the prism 
from yellow. The red being produced by the yellow partakes of it, and 
forms an orange tint. The blue is pale and wants brilliancy, on 
account of the yellow being the palest of colours, therefore a bad 
substitute for black. The penumbra below the blue has a poor 
reddish tint, partaking of the yellow, where violet would take place 
if produced by black, and so of the rest. b. The two reds will be 
seen to give an eflfect to the prismatic tints evidently depending upon 
the red that produces them, and so of the blue. It is however re- 
markable that the red c. 2 is more orange than when produced by 
black, the seven prismatic tints beneath which having among them 
more perfect yellow, red and blue. Thus, black or dark in this 
* The motion of the prism, however, is to be left out of the question at present. 
