sect, in.] DISSERTATION SECOND. C9 



refrangibility of the rays of light, and are now too well known 

 to stand in need of a particular description. 



Having admitted a beam of light into a dark chamber, 

 through a hole in the window-shutter, and made it fall on a 

 glass prism, so placed as to cast it on the opposite wall, he 

 was delighted to observe the brilliant colouring of the sun's 

 image, and not less surprised to observe its figure, which, in- 

 stead of being circular, as ho expected, was oblong in the 

 direction perpendicular to the edges of the prism, so as to 

 have the shape of a parallelogram, rounded at the two ends, 

 and nearly five times as long as it was broad. 



When he reflected on these appearances, he saw nothing 

 that could expain the elongation of the image but the suppo- 

 sition that some of the rays of light, in passing through the 

 prism, were more refracted than others, so that rays which 

 were parallel when they fell on the prism, diverged from one 

 another after retraction, the rays fliat differed in refrangibi- 

 lity differing also in colour. The spectrum, or solar image, 

 would thus consist of a series of circular images partly cover- 

 ing one another, and partly projecting one beyond another, 

 from the red or least refrangible rays, in succession, to the 

 orange, yellow, green, blue, indigo, and violet, the most re- 

 frangible of all. 



It was not, however, till he tried every other hypothesis 

 which suggested itself to his mind by<4.he test of experiment, 

 and proved its fallacy, that he adopted this as a true inter- 

 pretation of the phenomena. Even after these rejections, 

 his explanation had still to abide the sentence of an experi- 

 fnentum crucis. 



Having admitted the light and applied a prism as before, 

 he received the coloured spectrum on a board at the dis- 

 tance of about twelve feet from the first, and also pierced 

 with a small hole. The coloured light which passed through 

 this second hole was made to fall on a prism, and afterwards 



