94< Mr on the Jluctuatmg Sensibility of the Retma. 
trialj be found to agree with the experience of others, it will not 
be denied that the sensation produced by the greater light is 
greater, and that produced by the weaker light is less, than 
what is produced by these different degrees of light when they 
occupy singly the whole sphere of vision. 
Experiment 1. — When I fix my eyes steadily on the centre 
of a white circle, about an inch in diameter, upon an extensive 
blackish ground, the white circle appears surrounded by a halo 
darker than the rest of the ground, shewing a decrease of the 
sensibility of the retina in the parts adjacent to that which was 
acted upon by the greater light. But when I view a black 
circle upon a white ground, the black circle appears surrounded 
by a halo whiter than the rest of the ground, shewing an in- 
crease of sensibility to the greater light in the vicinity of the 
lesser. 
There is a degree of steadiness of the eyes required at the 
time of performing this experiment, which those who are not 
accustomed to the like, may find difficult to practise at first ; 
for the eyes are apt to turn to whatever part the attention is di- 
rected, which prevents the results I have mentioned from being 
obtained. 
The halo mentioned above, and in other parts of this paper, 
becomes fainter as it receded from the object it surrounds, and 
with moderate light is found to occupy an area equal, as far as 
I could ascertain by measurement, to that of the object viewed. 
But with very brilliant objects, such as the sun, the area of the 
halo is considerably greater. 
Experiment 2. — I provided myself with two tubes, each 
about an inch in diameter, and four inches long, one of them 
opaque, and blackened within ; the other thin, white, and trans- 
mitting freely through its sides the rays of light. Then ascer- 
taining that both eyes were equally sensible to light, I looked 
at a uniform white wall moderately illuminated, with one eye 
through the white tube, and with the other eye naked, and ob- 
served that when the inside of the tube was more illuminated 
than the wall^ that part of the latter which was seen through 
the tube appeared darker than it did to the naked eye. Again, 
the eyes being equally sensible, on looking at the same wall 
with one eye through the black tube, and with the other eye 
