202 Experimental Philosophy. [Lecture 14. 



glass. In the image PT the- colours succeeded 

 in this order from the bottom at T, to the top 

 at P, namely red, orange, yellow, green, blue, 

 indigo, violet*. 



Unable as yet to account for the phenomenon, 

 he was induced to try the effect of two prisms, 

 and he found that the light, which by the first 

 prism was diffused into an oblong, was by the 

 second reduced to a circular form, as regularly 

 as if it had passed through neither of them. 

 After various conjectures and experiments, he 

 had recourse, at length, to what he calls the 

 experimentum crucis. At the distance of about 

 twelve feet from the prism, which was close to 

 tiie aperture F, he placed a board which might 

 receive the image in the same manner as the 

 sheet of paper MN. In this board there was 

 also a small hole, through which some of the 

 light might pass ; behind this hole, then, he 

 placed a second prism, and, by moving the first 

 prism, he made the several parts of the image 

 cast by it on the board to pass successively 

 through the hole, so as to be refracted again 

 upon the wall by the second prism. He found 

 then, that the different colours of the spectrum, 

 when permitted to pass through the hole in the 

 board, were incapable of further decomposition : 



* These, taken in an inverse order, are readily called 

 to mind, by means of the word vilgyor, formed of the 

 successive initials of violet, indigo, Mue green, yellow, 

 orange, red. 





