NEWTON'S PRISMATIC SPECTRUM. 43 



glow of colours greens, yellows, blues, and reds as they change their position. So, 

 too, the beautiful display which is seen on the breast of the wild dove, and still more 

 beautifully in the feathers of the humming-bird. 



142. The cause of these colours, and of the colours of light generally, was first dis- 

 covered bv Sir ISAAC NEWTON, who, in his investigations, followed the second of the 

 methods here mentioned, the production of colour by refraction. Of late years, the at- 

 tention of philosophers has been much turned to the third method, that of interference ; 

 this, as will be presently seen, possesses, in particular cases, very great advantages over 

 that by refraction (140), or the first one, by absorption (139). 



143. NEWTON'S methods of investigating the production of colour by refraction may 

 be briefly described, as follows: Let a beam of light coming from the sun (s,Jig. Ill) 

 pass through a circular hole (a b) in the shutter of a dark room, and fall upon a piece 

 of glass cut into the form of a triangular prism, c d e, placed as in the figure. On the 

 farther side of the prism let there be a sheet of white card-board, M N, or some other 

 such screen, to receive the rays. The light, oil its entrance into the room through the 

 hole in the shutter, is white, and were the prism not interposed, it would advance for- 

 ward on a straight line, and, falling on the screen at z, would there give a circular 

 white image of the sun. The interposition of the prism disturbs this white light from 

 its rectilinear course, and, refracting it, bends it into a new path. The disturbance pro- 

 duced bv the prism is of two kinds : first, the ray is moved out of the rectilinear posi- 

 tion in which it would have gone ; and, second, it no longer gives rise to a white circu- 

 lar image, but to an elongated and highly-coloured image, which goes under the name 

 of the solar spectrum. Of the intensity and beauty of these colours it is impossible to 

 give any description by words. An imperfect representation of their position is given 

 in the frontispiece. In the coloured figure thus received on the card-board, NEWTON de- 

 tected seven different tints, red, orange, yellow, green, blue, indigo, violet Of these 

 colours, the red is uniformly nearest to the point (z) to which the ray would have gone 

 had the prism not intervened ; the violet is most distant. And as the production of 

 the colours depends upon the refracting action of the prism, NEWTON designated the 

 red as the least refrangible, because it was least removed from its natural course ; and 

 the violet as the most refrangible, because it was the most removed. 



144. Now if these seven coloured rays be collected together again by any appropri- 

 ate arrangement, so as all to fall on one common point, that point will be of a brilliant 

 white, as was the point z. If two of them, as the yellow and blue, in like manner be 

 directed together, they will give rise to a green ; or the red and bine to a purple ; or 

 the yellow and red to an orange ; showing, therefore, that while different coloured rays 

 thus give rise to compound tints, all the rays in the spectrum converged together pro- 

 duce white light, the same as that which originally came through the aperture a b, in 

 the shutter. 



145. NEWTON, therefore, w T as correct in the explanation which he gave of these phe- 

 nomena. That the white light of the sun consists of seven differently-coloured rays, 

 some ot which are more, and some less, refrangible bv glass. That, consequently, when 

 a beam falls on a triangular prism of glass, its constituent ravs are not all equally re- 



