70 DISSERTATION SECOND. [pabth. 



received on the opposite wall. It was then fonnd that the 

 rays which had been most refracted, or most bent from their 

 course by the first prism, were most refracted also by the 

 second, though no new colours were produced. " So," says 

 he, " the true cause of the length of the image was detected 

 to be no other than that light consists of rays differently re- 

 frangible, which, without any respect to a difference in their 

 incidence, were, according to their degrees of refrangibility, 

 transmitted towards divers parts of the wall." 1 



It was also observed, that when the rays which fell on the 

 second prism were all of the same colour, the image formed 

 by refraction was truly circular, and of the same colour with 

 the incident light. This is one of the most conclusive and 

 satisfactory of all the experiments. 



When the sun's light is thus admitted first through one 

 aperture, and then through another at some distance from the 

 first, and is afterwards made to fall on a prism, as the rays 

 come only from a part of the sun's disk, the spectrum has 

 nearly the same length as before, but the breadth is greatly- 

 diminished ; in consequence of which, the light at each point 

 is purer, it is free from penumbra, and the confines of the 

 different colours can be more accurately traced. It was in 

 this way that Newton measured the extent of each colour, 

 and taking the mean of a great number of measures, he as- 

 signed the following proportions, dividing the whole length of 

 the spectrum, exclusive of its rounded terminations, into 360 

 equal parts ; of these the 



1 Phil, Trans. Vol. VI. (1672), No. 80. p. 3075. 



