another, with the same appearance as the above ; and the proper 
motion of the prism or paper will produce tlie green, orange, indigo, 
and violet, if the jjajjer is not too broad. We therefore find three 
original primitive tints may be formed distinctly on white paper*. 
Rich lights, or whites, and full or more perfect darkness opposing 
each other, produce the more brilliant tints. The rich light of the 
sun dispelling the darkness of the morning, or passing away in the 
evening, produces more apparent colour than at noon when the 
whole atmosphere is illuminated ; and a small candle in a large room, 
surrounded as it were by much darkness, gives as great brilliancy 
in the prism as the sun itself: and again, the prismatic rays col- 
lected from the sun require a quantitj^ of shade to show them most 
brilliantly: — thus they were generally refracted into a dark room. 
Next to this we might consider convexity and concavity ; and it will 
be found that they add to the brilliancy, as they allow a greater 
strength of light and shade. Convexity and concavity amount, by 
undulation, as it were, to nearly tiie same thing as light and shade 
(by which artists have their imitative power) ; and any light with 
shade will produce prismatic tints ; thus, as it were, reducing the ])ro- 
duction of tints to light and shade, or white and black. We shall 
find the means of illustrating this in every necessary example. Hence 
it is found, that as the appearance of colour depends upon a certain 
opposition of light and darkness, so much depends upon the pro- 
portion of each for the colour they may produce, and the degree or 
strength of it. I therefore look at the middle of a piece of dense 
smooth black paper with the prism, and perceive no colour ; if I 
produce a small dot, or ray of light, or streak of white, immediately 
certain prismatic tints are produced, according to the proportion, from 
* Dr. WoUaston, however, as an original in philosophy, examined a narrow ray of 
light, :ind found what he considered as the four primitives ; and Dr. Young, a broader 
ray, by which he determines seven primitives, inchiding white. I think it will be seen 
that the breadth of the ray will naturally vary the appearance, as ill a broad ray, or a ray 
of a certain proportion, they ellow and blue do not mix as in a n.irrow one, unless by turn- 
ing llie prism. 
D 
