oil the Mechanism of the Eye. 61 
spectrum distinct enough without inconvenience ; and no light 
is sufficiently strong to cause a permanent impression on any 
part of the retina remote from the visual axis. I therefore had 
recourse to another experiment, I placed two candles so as 
exactly to answer to the extent of the termination of the optic 
nerve, and, marking accurately the point to which my eye was 
directed, I made the utmost change in its focal length ; expect- 
ing that, if there were any elongation of the axis, the external 
candle would appear to recede outwards upon the visible space. 
(Plate III. Fig. 15.) But this did not happen ; the apparent place 
of the obscure part was precisely the same as before. I will 
not undertake to say, that I could have observed a very minute 
difference either way: but I am persuaded, that I should have 
discovered an alteration of less than a tenth part of the whole. 
It may be inquired if no change in the magnitude of the 
image is to be expected on any other supposition ; and it will 
appear to be possible, that the changes of curvature may be so 
adapted, that the magnitude of the confused image may remain 
perfectly constant. Indeed, to calculate from the dimensions 
which we have hitherto used, it would be expected that the 
image should be diminished about one-sixtieth, by the utmost 
increase of the convexity of the lens. But the whole depends 
on the situation of the refracting surfaces, and the respective 
increase of their curvature, which, on account of the variable 
density of the lens, can scarcely be estimated with sufficient 
accuracy. Had the pupil been placed before the cornea, the 
magnitude of the image must, on any supposition, have been 
very variable : at present, this inconvenience is avoided by the 
situation of the pupil ; so that we have here an additional 
instance of the perfection of this admirable organ. 
