Mr. Young's Observations on Vision. 179 
fourth query. When a broadish pressure, as that of the 
finger, is made on the opaque part of the eye in the dark, an 
orbicular spectrum appears on the part opposite to that which 
is pressed : the light of the disc is faint, that of the circum- 
ference much stronger ; but when a narrow surface is ap- 
plied, as that of a pin's head, or of the nail, the image is nar- 
row and bright. This is evidently occasioned by the irritation 
of the retina at the part touched, referred by the mind to the 
place from whence light coming through the pupil would fall 
on this spot ; the irritation is greatest where the flexure is 
greatest, viz. at the circumference, and sometimes at the centre, 
of the depressed part. But in the presence of light, whether 
the eye be open or closed, the circumference only will be lu- 
minous, and the disc dark ; and if the eye be viewing any 
object at the part where the image appears, that object will be 
totally invisible. Hence it follows, that the tension and com- 
pression of the retina destroys all the irritation, except that 
which is produced by its flexure ; and this is so slight on the 
disc, that the apparent light there is fainter than that of the 
rays arriving at all other parts through the eyelids. This ex- 
periment demonstrates a truth, which may be inferred from 
many other arguments, and is indeed almost an axiom, viz. that 
the supposed rectification of the inverted image on the retina 
does not depend on the direction of the incident rays. Newton, 
in his sixteenth query, has described this phantom as of pavo- 
nian colours, but I can distinguish no other than white ; and it 
seems most natural that this, being the compound or average 
of all existing sensations of light, should be produced when 
nothing determines to any particular colour. This average 
seems to resemble the middle form, which Sir Joshua Rey- 
