258 Mr. T. Smith on certain Phanome7ia of Vision. 



appearance of the two images of S when they coalesce par- 

 tially, as at h c, than when they coalesce totally at S, will 

 vanish ; for the image on the retina is distinct in the latter 

 case, and indistinct in the former. 



Allowance being made, therefore, for the eifects on the sen- 

 sibility produced b}' the action of a greater light in the vicinity 

 of a lesser, the results in all the preceding expei'iments will be 

 found to be in strict accordance with the following conclusion : 

 viz. that the green appearance of a "white object through all parts 

 of the retina around the bright image in the exposed eye is owing 

 to diminished sensibilitij to red light; and the red appearance of 

 a lahite object through all the corresponding parts of the shaded 

 eye is owing to an equal increase of the sensibility to the same 

 kind of light. , , ^ , 



[To be continued.] ' ■"' " ^» " 



he red and white images is rendered indistinct by the scattered red rays ; 

 whereas in the former, both the surfaces and their line of junction are per- 

 fectly distinct : but when the eyes are fixed steadily on the centre of the 

 wafer, it never fails to be seen of its proper white colour as long as it is 

 seen without any dazzling or fatigue of the eyes. I fell into an important 

 error in my first experiments on this subject, and thus was led, in my first 

 publication adverted to in the text, to a misstatement of this fact, having 

 too hastily averred that these changes in the sensibility took place in every 

 case in which a greater and a lesser light were viewed together. I had no 

 suspicion that indistinctness of vision could have anything to do in pro- 

 ducing the changes ; and therefore when, on fixing my eyes on the centre 

 of a white circle on an extensive red ground till they became dazzled and 

 fatigued, I saw the white circle appear green, I concluded, erroneously 

 indeed, but perhaps not unnaturally, that the change in the sensibility 

 from W'hich the green appearance arose, had actually commenced the mo- 

 ment I fixed my eyes on the white circle. So convinced, indeed, was I, for 

 a long time, of the correctness of this conclusion, that I was not a little 

 surprised and disappointed, on submitting my observations to several 

 friends, to find that though they unanimously agreed with me respecting 

 the appearances of a white surface viewed through coloured tubes, yet not 

 one of them could be got, without prompting, to declare that a white circle 

 on a wide coloured ground presented the same appearances. At length, 

 on finding that I myself never experienced the changes in the sensibility 

 but when the object became indistinct, I was led to attend to the import- 

 ance of that condition, which enabled me to reconcile my own experience 

 with that of intelligent persons who formerly differed from me. The power 

 of indistinctness to excite this affection is well illustrated by the effects of 

 twilight, -which is well known to throw a shade of indistinctness over ob- 

 jects. I lately witnessed a very striking instance of this. Some white and 

 pink-coloured paper were lying together on my table one evening, when the 

 light of day was just enough to show the colour of the red paper. One 

 of the companj' present said, pointing to the white paper. What a beautiful 

 green colour that is ! and every person present agreed that it was so, 

 though I assured them it was actually white : and not a little surprise was 

 excited when it was distinctly perceived to be so, after I had removed the 

 pink paper. A white image, reflected by coloured glass, appearing of its 

 complementai-y colour, is also a fine instance of this effect of indistinctness. 



