254) Mr. T. Smith's Investigation of certain Phcenomena of 



the fifth experiment ; and found that, compared with it, the 

 image S" seen by the shaded eye was incomparably brighter : 

 whence it is quite certain that the sensibility is increased to 

 red, and not impaired to green in the unexposed eye. 



As the sensibility being increased to red light is the cause 

 of the appearance to the shaded eye, it follows, from what has 

 been remarked above, that the appearance to the exposed eye 

 arises from equally diminished sensibility to the same colour. 

 As this induction, however, may not appear so complete as it 

 ought to be, on account of the phaenomena mentioned under 

 the third experiment, the state of the sensibility to red light 

 in the exposed eye may be determined by direct experiment, 

 thus : — 



Ej:p. 7. Having found, by examination with the prism, that 

 red morocco leather reflected few or none of the green rays 

 when exposed to moderate white light, I put a slip of it in the 

 position S, expecting that the image seen by the exposed 

 eye would appear very dark or nearly black if the sensibility 

 was remarkably diminished to red, but that it would appear 

 bright white if the sensibility was increased to green : the re- 

 sult was, that it appeared a very dark red, and even (when 

 the li£fht F was very brilliant) almost black. 



Airhough these experiments appear to establish the truth 

 of the conclusion, that the gree7i and 7-ed appeai'ances of the 

 white surface S, in the first and second experiments, were 

 owing to changes in the sensibility to, or perception of, red 

 light alone; yet it is proper that we should pause here, to con- 

 sider the fact disclosed by the third experiment, viz. that the 

 two images of S appeared whiter when the two eyes were 

 shaded from, than when they were both exposed to, the 

 same quantity of bright light F and F'. The result in the 

 third experiment may be thus expressed — diminished sensi- 

 bility to white light by the equal action of bright light on both 

 eyes. Now the results obtained by the first and second expe- 

 riments have been proved to arise from the sensibility to red 

 liffht being diminished in the exposed eye, and increased 

 equally, as appeared, in the shaded eye : the effect, there- 

 fore, to be expected from exposing both eyes, is deci'eased, and 

 equally increased sensibility to red light in each eye, or, in 

 other words, an unchanged state of the perception of the white 

 of both images of S, — a result manifestly at variance with that 

 of the third experiment. But this was not the only perplexing 

 circumstance encountered in this stage of the inquiry ; for in 

 repeating the experiments with white, red, and green slips of 

 paper by turns, it could not long escape observation, that a 

 jmrtial coalescence of the two images, as at b c, gave a brighter 

 colour than a total coalescence at S. Happily, however, an 



