Vision, tracitig them to Functional Actions of the Brain. 257 



were shaded from, the bright light, and thus reconciled the 

 result of the second experiment to that of the others. But when 

 the exciting cause of this affection (See Note*, p. 255, &c.) is 

 understood, when it is known that it occurs only when one 

 of the objects is seen indistinctly from any cause (such as the 

 image falling on the retina before or behind the focal point of 

 the rays composing it)f, all difficulty in regard to the brighter 



always occurs at the same time an increase of the sensibility to the stronger 

 light next to a weaker ; for when the white tube is strongly, and the white 

 surface weakly, lighted, the appearances of the latter are always such as I 

 have stated above, to the two eyes. If the lights are now placed so as to 

 make the illumination of the tube and surface more equal, the diiference 

 to the two eyes becomes less and less, till when both tube and surface are 

 equally illuminated, the area seen through the tube appears as white as 

 the rest of it does to the naked eye — a satisfactory proof, as appears to 

 me, that the sensibility is only diminished to the weaker light in the vicinity 

 of a stronger. But if the lights are so placed as to illuminate the white 

 surface more than the tube, the former begins to appear brighter through 

 the tube than to the naked eye ; and when the difference of illumination 

 is very great, as when we view a white surface through a black tube, the 

 surface, which appears of its proper brightness to the naked eye, will ap- 

 pear over the circular part seen through the tube, as if the sun shone upon 

 it and not upon the rest, — a decided proof that the sensibility is increased to 

 the stronger light in the vicinity of a weaker. That the increase and decrease 

 of sensibility here demonstrated occur at the same time and in the same 

 degree, may be shown by carefully performing the following experiment. 

 Take two pieces of paper, about four inches long and two inches broad 

 each, the one bright white, the other painted dark, but not entirely black. 

 Join the tvvo pieces together so as to make one square piece, and at right 

 angles to the line of their junction draw three straight lines with black ink 

 through the white division, one of the lines being in the middle of it, and 

 the other two at the distance of about three quarters of an inch from it on 

 each side. Direct the two eyes to any distant point, and without altering 

 their direction, introduce the square piece of paper between the eyes and 

 the point they are directed to, holding the line of junction of the white 

 and dark parts horizontally, and so that the two outer black lines shall 

 appear united through their whole length with the middle black line. If 

 strict attention is now paid to the appearance of the white and dark divi- 

 sions, it will be found, if the experiment is properly performed, that the 

 white portion will appear to terminate in the dark one by a very bright 

 belt ; while the dark portion will appear at the same time to terminate in 

 the white one by a darker or black belt of the same breadth, and both of 

 these belts will be seen to have evanescent edges at the sides where they 

 disappear in their respective surfaces. These appearances are manifestly 

 results of the same affection of sight that occurs in looking through co- 

 loured tubes, and show clearly that the increased sensibility to the greater 

 light, and the diminished sensibility to the lesser, in the vicinity of each 

 other, take place at the same time and in the same degree, the one being 

 dependent upon the other. 



f If a white wafer or round piece of white paper is steadily viewed on 

 the middle of an extensive red ground, about a foot from the eyes, the 

 retina will evidently be in the same physical condition in regard to the 

 light falling on it, as when a white surface is viewed through a red tube 

 duly illuminated, except that in the latter case the line of junction between 



Third Series. Vol. 1. No. 4-. Oct. 1832. 2 L 



