96 Mr S 1 ^^^ith on the Jhictuating SensibUity of the Retina. 
merits lead me to conclude : but that it should at the same time 
become so much more sensible to all the other kinds of light, as 
to perceive, not dimly, but distinctly, the very few, if any there 
were, of these rays which, in the way the board was placed, 
could be reflected from the black surface, is highly improbable ; 
and the more so, that I have never found this halo produced 
but when the object viewed was brilliant. I am therefore in- 
clined to believe that the part of the retina answering to the 
halo in this experiment, was, without the direct agency of purple 
light, thrown into a state similar to that which is produced by 
such light, and that the external yellowish halo was prpduced 
ip a similar manner. 
These are the principal results of my observations on the 
state of the sensation produced, and sensibility of the retina, at 
the 'time of viewing unequally illuminated surfaces. 1 shall 
now proceed to shew the singular reflux which occurs after- 
wards. 
Experiment 5. — After looking steadily for about half a mi- 
nute on the centre of a small white circle, upon an extensive 
blackish ground, as in the 1st experiment, if the eyes are turned 
to another part of the black ground, a very black circle will ap- 
pear surrounded by a bright halo ; from which it is evident 
that the sensibility undergoes a remarkable change of vie\^'ing 
the white circle, being diminished where the sensation was in- 
creased., and increased where the sensation was diminished. 
Experiment 6. — When the eyes are steadily fixed on the 
centre of a red circle an inch in diameter, on a white ground a 
foot or more square, the red circle appears surrounded by a 
greenish halo. If the distance between the red circle and the 
eyes is then increased, still looking steadily at the centre of the 
red circle, part of the spectrum or accidental colour of the red 
circle will be allowed to form, and the red circle will appear 
surrounded by a green ring and red halo. 
Experiment T.-r- After looking steadily at the centre of the 
oiled paper mentioned in the 5th experiment, till the halo ap- 
peared brilliant, I turned my eyes to another part of the black 
paper, and saw a circular purple spectrum, surrounded by a 
greenish- yellow halo. When my eyes were turned to a white 
wall, tlie spectrum and halo were less vivid ; but what is still 
1 
