82 MR. J. SMITH ON THE ORIGIN OF COLOUR 



the contrary happens in the posture of the paper S e, the 

 more refrangible rays being then predominant which 

 always tinge light with blues and violets." ' 



Newton appears to have been too easily satisfied with 

 his experiments on penumbrse, for this experiment seems 

 to me to prove the very reverse of what he supposes. I 

 consider that the doctrine of more and less refrangible 

 rays completely fails him here^ and I find from a note 

 which I pencilled on this part of his Optics many years 

 ago that this was my first impression. I, however, never 

 ascribed this to anything but my inability to comprehend 

 the profundity of Newton^s doctrine of light. But nature 

 herself speaks to me now in a diiferent language to that 

 of Newton. ¥/hen we incline the paper to the rays, as 

 directed by Newtou in this experiment, we do two things; 

 we remove a portion of the paper farther from the prism, 

 and thus increase the penumbra; and at the same time we 

 cut off nearly one half of the rays which fell directly on 

 the paper, when it was kept perpendicular, and thereby 

 diminish the light. My experiments lead to the con- 

 clusion that it is necessary not only to have penumbrse in 

 order to obtain colour, but that it is also necessary to 

 diminish the momentum of the incident light, similar to 

 what is done in this experiment of Newton^s. But further, 

 if v/e consider the paper as an eye looking into the prism, a 

 little study will soon convince us that at every change of 

 the angle the object seen from the the paper is totally 

 different, that the lights and shadows are completely 

 altered. We have merely to trace the lines of the image 

 backwards through the prism to convince ourselves of this. 



124. Though additional proof may be required by others, 

 I am satisfied that my experiment proves that the spectrum 

 is caused by coloured penumbrse in the way I have shown, 

 and consequently solves the difficulty of its length compared 

 to its breadth. It even solves the difficulty which Newton 



