SECOND REPORT- 
of the axis of pressure, occasioning the diametrically opposite 
spot of light, and also the luminous spot in the middle of the 
circular black space. Hence the author concludes, that when 
the retina is dilated under exposure to light , it becomes abso¬ 
lutely blind or insensible to all luminous impressions. 
These properties of the retina often exhibit themselves invo¬ 
luntarily, with different variations, according to the state of 
sensibility of the retina, in consequence of the movement of the 
eye-ball by its own muscles during the act of sneezing, and on 
other occasions. 
The phenomena above described are those produced in the 
parts of the retina which are most affected by any given pres¬ 
sure : but it is obvious that this pressure is propagated over the 
whole retina ; and even when it is too weak to produce a lumi¬ 
nous impression, it may yet modify other impressions previously 
produced on the retina. If, from looking at the sun, the eye 
sees a pinkish brown spectrum, a pressure upon another part of 
the retina will change it to a green spectrum, which, wdien the 
pressure is removed, will again become brown. If the pressure 
is such as to diminish the sensibility of the retina, it will either 
diminish or entirely remove a weak spectral impression. 
When the eye is pressed in front, by putting the finger on 
the eyelid above the cornea, no luminous spectrum is seen, and 
the author did not venture to increase this pressure so as to 
produce an impression on the back of the eye. He however 
mentions a case where this effect was produced accidentally. 
A person, in a state of intense grief, had been sitting for some 
time withhis hand pressed against his eye;—the moment his hand 
was removed, and the eye opened, a black spot, the size of a 
sixpence, was seen in the axis of vision. The pressure of the 
blood-vessels upon the retina, in particular states of indisposi¬ 
tion, occasion floating masses of light, visible in the dark, at 
first faint blue , then green , then yellow , and sometimes even red, 
all these colours being occasionally seen at the edge of the lumi¬ 
nous mass. 
The preceding observations on the influence of dilatation in 
making the retina insensible to light, render it extremely pro¬ 
bable that the disease in that membrane, called amaurosis, may 
sometimes arise from a general distension of the eye-ball, ari¬ 
sing from a superabundance of the fluids which it incloses. If 
this be the case, the removal of the pressure might be effected 
by puncturing the eye-ball, (wdien this can be done with safety,) 
and letting out a portion of the aqueous humour. How far such 
an operation would be effectual when the disease is of long 
standing, can be determined only by experiment. 
