256 Mr. T. Smith's Investigation of certain Pha:nomena of 



the second experiment soon convinced me that the action of 

 the stronger lights F and F', and of the weaker hght from 

 S, and all surrounding objects, placed the retina in precisely 

 the same condition in which the affection now mentioned 

 had been developed in the experiments referred to. This 

 principle, therefore, accounted for the two images of S appear- 

 ing darker when the eyes were exposed to, than when they 



to that of the tube ? M. Meusnier observed that, when the sun shone 

 through a hole a quarter of an inch in diameter in a red curtain, the image 

 of the luminous spot was green. M. Meusnier's retina in this case was 

 evidently in a state closely analogous to that of the eye looking through a 

 tube in any of the above instances. It has been proposed to explain the false 

 perception in M. Meusnier's case on the principle, " that when the whole, 

 or a great part, of the retina has the sensation of any primitive colour, a 

 portion of the retina protected from the impression of the colour is actually 

 thrown into that state which gives the accidental or harmonic colour." 

 — {Vide Lardner's Cyclopredia, " Optics," p. 310.) If this principle exists, 

 the appearance of the white surface seen through any of the coloured tubes 

 would be owing to an increase of sensibility to the complementary colour in 

 that part of the retina where the white light acts ; but till direct proof of 

 that principle is obtained, it cannot be denied that the phenomena would be 

 equally well accounted for, by supposing the sensihility diminished to those 

 rays of the white surface, which are of the same colour as the tube ; for a 

 white surface will appear as distinctly green by the abstraction of red as 

 by the addition of green. To determine, therefore, to which of these causes 

 the phsenomena were due, I adopted the following plan. 



As wJiiteness is the result of the united action of all the primary coloured 

 rays, so a white tube may be regarded as an assemblage of all the primary 

 coloured tubes into one. Hence, in viewing a white surface through a white 

 tube, we ought to have a combination of all the effects ihat are obtained 

 on viewing the same white surface through the primary coloured tubes 

 separately. Consequently, if the green appearance of a white surface seen 

 through a red tube is owing to an increase of the sensibility to green, and 

 so on of the others, then it is obvious that a union of all these increased 

 sensibilities will constitute an increased sensihility to white ; and, therefore, 

 that a white surface, viewed through a white tube, ought to appear brighter 

 than it does to the naked eye. But if the appearance of the white surface 

 through each coloured tube is owing to diminished sensibility to those rays 

 from the white surface that give the same colour as the tube, then a union 

 of all the diminished sensibilities will form a diminished sensibility to ichite, 

 consequently make the white surface seen through the white tube appear 

 darker than it does to the naked eye. Experiment declares unequivocally 

 in favour of the latter state of the sensibility ; for the circular area of the 

 white surface, which was seen through a white tube highly illuminated, 

 appeared exceedingly dark, and if the illumination of the tube was very 

 brilliant, almost black, compared with the other parts of it, which were 

 seen of theii ordinary white colour by the naked eye. In this manner, 

 therefore, we are enabled to pronounce with certainty, that the false vision 

 of the white surface seen through a coloured tube, is owing to the sensi- 

 bility being diminished to the action of that kind of rays in the white 

 image, which predominates in the tube employed. 



By a proper management of the illumination of the white tube and 

 white surface seen through it, we soon find that this diminished sensibility 

 is only to a weaker light in the vicinity of a stronger ; and further, that there 



