AND THE THEORY OV LIGHT. 47 



the existence of rays of light in the sun for every colour in 

 the solar spectrum or even in nature. He concludes, prop. 2, 

 bk. i. pt. ii. of his Optics, in these words : " From all which 

 it is manifest that if the sun's light consisted of but one 

 sort of rays, there would be but one colour in the whole 

 world, nor would it be possible to produce any new colour 

 by reflections and refractions, and by consequence that the 

 variety of colour depends upon the composition of light." 

 To which he appends the following definition : 



" The homogeneal light and rays which appear red, or 

 rather make objects appear so, I call rubrific or red- 

 making; those which make objects appear yellow, green, 

 blue and violet, I call green-making, yellow-making, blue- 

 making, violet-making, and so of the rest. And if at any 

 time I speak of light and rays as coloured or indued with 

 colours, I would be understood to speak not philosophically 

 and properly, but grossly, and according to such concep- 

 tions as vulgar people, in seeing all these experiments, 

 would be apt to frame. For the rays, to speak properly, 

 are not coloured. In them there is nothing else than a 

 certain power and disposition to stir up a sensation of 

 this or that colour. For as sound in a bell, or musical 

 string, or other sounding body, is nothing but a trembling 

 motion, and in the air nothing but that motion propagated 

 from the object, and in the sensorium it is a sense of 

 that motion under the form of sound ; so colours in the 

 object are nothing but a disposition to reflect this or that 

 sort of rays more copiously than the rest. In the rays 

 they are nothing but their dispositions to propagate this 

 or that motion into the sensorium, and in the sensorium 

 they are the sensations of those motions under the form of 

 colours." 



In his letter to the Royal Society he says : " Nor are 

 there only rays proper and particular to the more eminent 

 colours, but even to all their intermediate gradations." In 



