TRANSACTIONS OF THE SECTIONS. 553 
which no traces of these vessels appear. Mr. W. remarked, 
that the absence of these minute obstructions to light will pro¬ 
bably account for the greater distinctness with which small ob¬ 
jects are there seen, and also for the difference of colour ob¬ 
served by anatomists in that spot of the retina. 
On the effect of Compression and Dilatation upon the Retina. 
By Sir David Brewster, K.H. LL.D. F.R.S. V.P.R.S.E. 
In repeating many times the well known experiment, particu¬ 
larly described by Newton in the sixteenth query at the end of 
his Optics, of the production of light by gentle pressure upon 
the eye-ball, or a stroke upon the eye, Sir David Brewster 
saw reason to correct the statement of Newton, that “the co¬ 
lours vanish in a second when the eye and the finger remain 
quiet,” having found them to continue as long as the pressure 
is kept up. With respect also to the character of the light 
thus produced in the eye, the author’s experience has only 
shown him black and white circles, with a general red tinge, 
arising from the light passing through the closed eye-lids, 
whereas Newton speaks of the colours as like those in the fea¬ 
ther of a peacock’s tail. 
The author states,—When a gentle pressure is first applied, 
so as to compress slightly the fine pulpy substance of the retina, 
a circular spot of colourless light is produced, though the eye 
be in total darkness, and have not been exposed to light for 
many hours. If light be now admitted to the eye, the com¬ 
pressed part of the retina is found to be more sensible to the 
light than any other part, and consequently appears more lumi¬ 
nous. Hence it follows, that a slight compression of the retina 
increases its sensibility to the light which falls upon it , and 
creates a sensation of light when the eye is in absolute darkness. 
If we now increase the pressure, the circular spot of light gra¬ 
dually becomes darker, and at last black, and is surrounded with 
a bright ring of light. By augmenting the pressure still more, alu¬ 
minous spot appears in the middle of the central dark one, and an¬ 
other luminous spot diametrically opposite, and beneath thepoint 
of pressure. Considering the eye as an elastic sphere, filled with 
incompressible fluids, it is obvious that a ring of fluids will rise 
round the point depressed by the finger, and that its pressure 
from within outwards will dilate the part of the retina under 
the finger which was formerly compressed, and will compress 
all that part of the retina in contact with the elevated ring. An 
increase of pressure will be resisted by the opposite part of the 
retina, and will thus produce a compression at both extremities 
