AND THE THEORY OF LIGHT. 41 



other much. These two phenomena, the setting sun and 

 the sun seen through a fog, may be adduced as leading to 

 the conckision, that red is produced by motion in compa- 

 ratively straight lines, without any intersection of rays. I 

 think that this conclusion is supported by Halley's diving- 

 bell experiment. 



But the question^ What indication is there of a negative 

 element ? still remains unanswered. 



64. Were I to adduce arguments founded on observa- 

 tions on the eye, I should be met by the objection, that 

 the eye is not a perfectly achromatic instrument — an 

 assertion, however, which no one can believe unless he 

 also believes in the heterogeneous nature of light; since 

 certain phenomena, connected with the eye, cannot be 

 explained on any other supposition by those who adopt 

 that theory. My eye, I know, is not now perfectly achro- 

 matic, but I remember the time when it was so; and I 

 believe every good eye to be achromatic. But letting 

 this pass, every one who has experimented on light knows 

 that he can convert his eye into a prism, so as to produce 

 all the coloured dispersion which is seen in the prism. 

 If we look at a candle over the edge of anything — the 

 finger will do as well as any thing else — and cover some- 

 what more than one half of the pupil of the eye, yet so 

 as to see the candle distinctly, thus intercepting a mul- 

 titude of the rays, but allowing those which do enter the 

 eye to enter in one direction only, the lower part of the 

 candle will appear red, the middle yellow, and beyond there 

 will be a line of blue. Of course this order depends on what 

 part of the pupil is covered. It is evident that the whole of 

 this red or yellow is not produced by refraction, allowing 

 that the refractive power of the eye is the sole cause 

 of the colour. It is impossible to omit observing that the 

 dispersion of light on the penumbra caused by the object 

 has more to do with the colour than refraction. A minute 



SEE. III. VOL. I. ' G 



