OF NEWTON'S OPTICS. 

 "X 



likewise affected the organ. The effect 

 is thus compounded of the influences 

 of the several colours upon the eye, as 

 much as if they all affected it at the 

 same moment. 



(44.) In this explication of the pheno- 

 menon just described, we assume the fact, 

 that when a visible object affects the eye, 

 it continues to be perceived after it has 

 ceased to be present. Thus, if a light 

 be suddenly extinguished, the light itself 

 and all the objects which it rendered 

 visible continue to be seen for a certain 

 short space of time after the extinction. 

 This curious fact admits of very simple 

 proof. If a burning coal or lighted stick 



be moved rapidly in a circle, it will be 

 seen in every part of the circle at once, 

 so as to have the appearance of a ring 

 of fire ; which proves that the impres- 

 sion which the light in one part of the 

 circle makes upon the eye, continues 

 until it returns again to the same part of 

 the circle, to make another impression. 



The colours of the spectrum may be 

 recomposed, so as to form white light, by 

 a second prism, instead of the lens men- 

 tioned in the last experiment. Let R V, 

 fig. 37, be the spectrum formed by the 

 prism ABC, and let this be viewed 

 through another prism a b c, placed in 

 such a manner that the rays which con- 



verge from RVwill be received as if 

 they emerged from a circular image of 

 the sun at s. In this case the rays enter 

 the eye exactly as they would if it were 

 placed before the aperture F, and pre- 

 sented towards it. The colours pro- 

 ceeding from R V are thus mixed on 

 entering the eye, and appear white. 



If any of the colours of the spectrum 

 RV be removed by intercepting a part 

 of the light between R V and the prism 



ABC, the colour which will be per- 

 ceived through the prism a be, will be 

 that which would be formed by the mix- 

 ture of the remaining colours. But if 

 the comb mentioned in the last experi- 

 ment be quickly moved between RV 

 and ABC, so as to throw the several 

 colours on the screen in rapid suc- 

 cession, the eye will again perceive 

 white, for the reason already explained. 

 The same result was obtained by va- 

 D2 



