Since then, 1. A solar ray may be re solved into several differently coloured rays ; 2. Since their colours are immutable either by reflection or refraction, and therefore probably not generated in those operations ; and 3. Since from the mix ture of those coloured rays solar light may be formed, it seems an indisputable conclusion that the differently coloured rays do exist in solar light previously to any separation that takes place in expe riments.
White is compounded of all the pri mary colours, mixed in their due propor tions, for if a solar ray be separated by the prism into its component parts, and at a proper distance a lens be so placed as to collect the diverging coloured rays again into a focus, a paper placed_ per pendicularly to the rays in this point will exhibit whiteness.
The same conclusion may be drawn from the experiment of mixing together paints of the same colours as the parts of the spectrum, and in the same proportion ; the mixture will be white, though not of a resplendent whiteness, because the colours mixed are' less bright than the primary ones ; this may likewise be prov ed, by fixing pieces of cloth of all the different colours on the rim of a wheel, and whirling it round with great velocity, it will appear to be white- Though seven different colours are distinguishable in the prismatic spectrum, yet, upon exa mining the matter with more accuracy, we shall see that there are, in fact, only three original colours, red, blue, and yellow : for the orange being situated be tween the red and yellow, is only the mixture of these two : the green, in like Manner, arises from the blue and yellow ; and the violet from the blue and red.
As the colour of a body, therefore, pro ceeds from a certain combination of the primary rays which it reflects ; the com bination of rays flowing from any point of an object will, when collected by a glass, exhibit the same compound colour in the corresponding point-of the image. Hence appears the reason why the images formed by glasses have the co lours of the objects which they represent.
The instance of the separation of the primary colours of light which seems most remarkable, is that of the RAIN BOW. It is formed, in general, by the re flection of the rays of the sun's light from the drops of falling rain, though frequently it appears among the waves of the sea, whose heads, or tops, are blown by the wind into small drops, and it is sornetimes seen on the ground, when the sun shines oh a very thick dew. Cas cades and Muntains, whose waters are in their fall divided into drops, exhibit rainbows to a spectator, if properly si tuated during the time of the sun's shining; and water blown violently from the mouth of an observer, whose back is turned towards the sun, will, with care, produce the same phenomenon. See