OF OPTICS. 



273 



Having already shewn, that the rays of light are LECi 

 refracted when they pass obliquely through different ^^\^ 

 mediums, we come now to prove that some rays are 

 more refrangible than others ; and that, as they are 

 differently refracted, they excite in our minds the ideas 

 of different colours. This will account for the colours 

 seen about the edges of the images of those objects 

 which are viewed through some telescopes. 



Let the sun shine into a dark room through a small hole 

 as at e e, in a window-shutter ; and place a triangular 



prism B C in the beam of rays A, in such a manner, 

 that the beam may fall obliquely on one of the sides 

 a b C of the prism. The rays will suffer different re- The prism. 

 fractions by passing through the prism, so that instead 

 of going all out of it on the side d c C, in one direc- 

 tion, they will go on from it in the different directions 

 represented by the lines f, g, h, i, k, I, m, n ; and falling 

 on the opposite side of the room, or on white paper 

 placed as at p q, to receive them, they will paint upon 

 it a series of most beautiful and lively colours (not to be The co- 

 equalled by art) in this order, viz. those rays which {""/,(." 

 are least refracted by the prism, and will therefore go 

 on between the lines n and m, will be of a very bright 

 intense red at n, degenerating from thence gradually 



mirrors, jointed together with hinges, and opening like a book. So 

 that the polished pistes, being set upon a geometrical drawing, and 

 the observer placed in the front : the lines of the drawing are seen 

 multiplied by repeated reflections. Brief notices of the principle of 

 this invention may also be found in Wood's Optics ; and, in a similar 

 Work, by Harris ; bat it is to Dr. Brewster, that we are indebted 

 for the present improved form of this beautiful instrument. 

 18. T 



