'.'IS OPTICS. 



For if the rays, after being separated in this 

 manner by the prism, are made to fall upon a con- 

 vex lens G, Fig. 2., they will converge at H, form- 

 ing a spot of white light as at first. The same may 

 also be shown by fixing pieces of cloth of all the 

 seven colours, on the rim of a wheel, and whirling 

 it round with great velocity ; they will appear white. 

 White, therefore, is the mixture of all the colours, 

 as black is the want or deprivation of colour. 



Though seven different colours are distinguish- 

 able in the prismatic spectrum, yet all these, as 

 well as all other broken colours, can be composed 

 by mixing together pigments of three colours only, 

 viz. red, blue, and yellow ; and, perhaps, in nature, 

 there may be only those three, although as yet no 

 experiments have been able to reduce the spectrum 

 to three only : the orange being produced by the 

 mixture of red and yellow; green, by mixing blue 

 and yellow j and violet, or purple, from blue and 

 red. 



The instance of the separation of the primary 

 colours of light which seems most remarkable, is 

 that of the rainbow. It is formed, in general, by 

 the reflection of the rays of the sun's light from 

 the drops of falling rain, though frequently it ap- 

 pears among the waves of the sea, whose heads or 

 tops are blown by the wind into small drops, and 

 it is sometimes seen on the ground, when the sun 

 shines on a very thick dew. Cascades and foun- 

 tains, whose waters are in their fall divided into 

 drops, exhibit rainbows to a spectator, if properly 

 situated during the time of the sun's shining ; and 

 water blown violently from the mouth of an ob- 

 server, whose back is turned towards the sun, 

 never fails to produce the same phenomenon. 



