o9 
on the Mechanism of the Eye. 
were elongated one-seventh, its transverse diameter must be 
diminished one-fourteenth, and the semi-diameter would be 
shortened a thirtieth of an inch. 
I therefore placed two candles so that when the eye was 
turned inwards, and directed towards its own image in a glass, 
the light reflected from one of the candles by the sclerotica 
appeared upon its external margin, so as to define it distinctly 
by a bright line ; and the image of the other candle was seen in 
the centre of the cornea. I then applied the double eye-glass, 
and the scale of the looking-glass, in the manner already de- 
scribed ; but neither of them indicated any diminution of the 
distance, when the focal length of the eye was changed. 
Another test, and a much more 'delicate one, was the appli- 
cation of the ring of a key at the external angle, when the eye 
was turned as much inwards as possible, and confined at the 
same time by a strong oval iron ring, pressed against it at the 
internal angle. The key was forced in as far as the sensibility 
of the integuments would admit, and was wedged, by a mode- 
rate pressure, between the eye and the bone. In this situation, 
the phantom caused by the pressure extended within the field 
of perfect vision, and was very accurately defined ; nor did it, 
as I formerly imagined, by any means prevent a distinct percep- 
tion of the objects actually seen in that direction ; and a straight 
line coming within the field of this oval phantom, appeared 
somewhat inflected towards its centre; (Plate III. Fig. 14.) 
a distortion easily understood by considering the effect of the 
pressure on the form of the retina. Supposing now, the dis- 
tance between the key and the iron ring to have been, as it 
really was, invariable, the elongation of the eye must have been 
either totally or very nearly prevented; and, instead of an 
I 2 
