16 Mil. J. SMITH ON THE OKIGIN OF COLOUR 



26. But in attempting to introduce a new term in 

 explanation of the cause of colour, in attempting to speak 

 of shadows and penumbrse, there are great difficulties 

 attending the subject, for optical science is not familiarized 

 to the use these terms are put to, and besides the negative 

 element is as undefined as the positive; for between the 

 faintest shadow and absolute darkness there are an infinite 

 variety of shades, just as there are between the faintest 

 light and the brightest, if indeed there is a limit to either. 

 But if white light be admitted to be composed of an infi- 

 nite number of rays crossing each other at all angles — or 

 of one great wave — then the interception of any amount 

 of these rays or of any portion of the wave on any given 

 spot is a shadow. There must be a body to intercept the 

 light, and that body may be either opaque or transparent, 

 for the most transparent body does not transmit all the 

 light which falls upon it. This shadow is generally some 

 degree of grey, unless it be illuminated by a vivid ray at 

 an angle to the ordinary ray, in which case it is coloured. 

 But it is so customary to argue on this subject from the 

 phenomena of the prism, that I shall be told that in all 

 coloured shadows this light had been previously refracted, 

 and the shadow did nothing more than prepare a place on 

 which to reflect it, and prevent its being extinguished by 

 a greater light. Whereas, on the contrary, there is more 

 reason for saying that the number of vibrations being 

 reduced by the interposition of a substance causing a 

 shadow, the light reflected from this shadow alternating 

 with the shadow itself produces a blue, purple, or red 

 effect. In our ignorance of the value of the ray imping- 

 ing on the shadow, as well as of the value of the shadow 

 (for it is well known that shadows have diflerent values), 

 we have no authority to call them refracted but from a 

 belief in the received theory. 



27, Were a ray of light like an infinitely fine wire 



