AND THE THEORY OF LIGHT. 81 



coming into a dark chamber through a hole in the win- 

 dow-shutter be refracted by a large prism ABC, Vt^hose 

 refracting itngle C is more than sixty degrees^ and so soon 

 as it comes out of the prism let it fall upon the white 

 paper D E glued upon a stiff plane ; and this light^ when 

 the paper is perpendicular to it as it is represented in D E, 

 will appear perfectly white upon the paper : but when the 

 paper is very much inclined to it in such a manner as to 

 keep always parallel to the axis of the prism, the white- 

 ness of the whole light upon the paper will, according to 

 the inclination of the paper this way or that way, change 

 either into yellow and red as in the posture d e, or into 

 blue and violet as in the posture 8 e. And if the light 

 before it fall upon the paper be twice refracted the same 

 way by two parallel prisms, "these colours will become the 

 more conspicuous. Here all the middle parts of the broad 

 beam of white light which fell upon the paper did, without 

 any confine of shadow to modify it, become coloured all 

 over with one uniform colour, the colour being always the 

 same in the middle of the paper as at the edges, and this 

 colour changed according to the various obliquity of the 

 reflecting paper, without any change in the refractions or 

 shadows, or in the light which fell upon the paper. And 

 therefore these colours are to be derived from some other 

 cause than the new modifications of light by refractions 

 and shadows. 



" If it be asked what then is their cause ? I answer that 

 the paper in the posture d e, being more oblique to the 

 more refrangible rays than to the less refrangible ones, is 

 more strongly illuminated by the latter than by the 

 former, and therefore the less refrangible rays are predo- 

 minant in the reflected light. And wherever they are 

 predominant in any light they tinge it with red or yellow, 

 as may in some measure appear by the first proposition of 

 the first book, and will more fully appear hei-eafter. And 



SER. III. VOL. I. M 



