57 
on the Mechanism of the Eye . 
eye occupies on the surface of a glass half its real dimensions, 
at whatever distance it is viewed, its true size is always double 
the measure thus obtained. I illuminated the glass strongly, 
and made a perforation in a narrow slip of black card, which I 
held between the images ; and was thus enabled to compare 
them with the scale, although their apparent distance was dou- 
ble that of the scale. I viewed them in all states of the eye ; 
but I could perceive no variation in the interval between them. 
The sufficiency of these methods may be thus demonstrated. 
Make a pressure along the edge of the upper eyelid with any small 
cylinder, for instance a pencil, and the optometer will show that 
the focus of horizontal rays is a little elongated, while that of 
vertical rays is shortened ; an effect which can only be owing to a 
change of curvature in the cornea. Not only the apparatus here 
described, but even the eye unassisted, will be capable of discover- 
ing a considerable change in the images reflected from the cor- 
nea, although the change be much smaller than that which is re- 
quisite for the accommodation of the eye to different distances. 
On the whole, I cannot hesitate to conclude, that if the radius 
of the cornea were diminished but one-twentieth, the change 
would be very readily perceptible by some of the experiments 
related ; and the whole alteration of the eye requires one-fifth. 
But a much more accurate and decisive experiment remains. I 
take out of a small botanical microscope, a double convex lens, of 
eight-tenths radius and focal distance, fixed in a socket one-fifth 
of an inch in depth ; securing its edges with wax, I drop into it a 
little water, nearly cold, till it is three-fourths full, and then apply 
it to my eye, so that the cornea enters half way into the socket, 
and is every where in contact with the water. (Plate III. Fig. 
13.) My eye immediately becomes presbyopic, and the refractive 
MDCCCI. I 
