Mr Smith on the Jluctuating Se7isibiUt^ of the Retma. 9? 
That the ^nsibility of the retina, however, is less impaired 
in certain circumstances by the action of light, than it is in- 
creased in darkness, appears evident from the following facts. 
If we look on an uniformly red surface, occupying the whole 
sphere of vision, for about half a minute, it will not appear 
darker at the end of that time than it did at first, provided the 
eye was before accustomed to that degree of light. But if the 
eye is then turned from the red to a white surface, the lattef 
will appear greenish ; which shews that the sensibility of the 
retina, although not perceptibly diminishing to red light while 
viewing the red surface, is yet increasing to all the other prima- 
ry colours to which, at the time, the eye is in a comparative 
state of darkness. 
Again ; after looking long find steadily on the centre of a red 
circle on a white or black ground, the red colour becomes 
fainter and fainter, till at last the circle appears almost pc alto- 
gether white. Dr Darwin was of opinion that the perception, 
in this case, becomes every instant weaker and weaker. But 
the circumstance of the red circle appearing white^ which seems 
to have escaped his notice, indicates a greater increase of sensi- 
bility to the other simple rays of light proceeding from the co- 
loured circle, than decrease to the red light. 
But neither the decrease of the sensibility by the action of 
light, nor its increase in darkness, can alone account for all the 
phenomena of accideutal colours. Thus the accidental colou^' 
of a small white or black circle upon a red, yellow, or other 
primary coloured ground, is not the reverse of the circ}e vieiy- 
ed, but the same as the ground it was viewed upon, 
The cause of this may be traced tP a remarkable fluctuation 
in the state of the sensibility of the retina, which occurs during 
and after the eye is exposed to a stronger light in the vicinity 
of a weaker. As no notice, I believe, has been taken of this 
fluctuating sensibility of the retina by writers on vision, a brief 
account of it may perhaps be not unacceptable to some of the. 
readers of this Journal. 
It may appear a singular position, that at the time of view-? 
ing a stronger and weaker light together, the sensibility of the 
retina is increased to the stronger light, and diminished to the 
weaker. Yet if the facts I am about to mention shall, upou 
