30 On Vision. 
with the dignity of candid philosophical discussion.” Then, 
After giving an imperfect account of my theory, he com-' 
mences his FoBservatlon’ thus : 
<< Tt will be seen, that, although Mr. Walker's hypo- 
thesis is supported by the authority ‘of mathematical demon-. 
stration, it may, notwithstanding, be inadmissible, since 
there is a principle ‘assumed in the demonstration which 
is not completely established. Is Mr. Walker certain that 
the figure of the crystalline, its position in the eye, the di- 
stance from the pupil to the retina at the back of the eye, 
or, in other words, the general dimensions and conforma- 
tion of the eye, continue unchanged, while the magnitude 
of the pupil varies? If he be, he has attained conviction on 
@ point respecting which Harris, Wood, Porterfield, and 
others of our best writers on vision, have still many doubts. 
Yet, unless this be a decided point, we conceive the whole 
of Mr. Walker’s demonstration nugatory.” 
Ft does not appear, from the above observations, that the 
writer knows whether my theory is true or false; all that he 
has been able to discover is, (and that, it seems, not without 
some difficulty,) that if my data be pie ** he conceives the 
whole of my demonstration nugatory.’ 
. Now, sir, my deibuirsteation: depends no more upon the | 
string of questions mentioned by this writer, nor on the 
doubts which, he says, our best writers on vision have re- 
specting them, than it does upon the shining of a Troston 
glow-worm*, 
For all the data that T have used in my demonstration 
are. well-known truths. . As the crystalline lens converges 
the rays of light to a focus, mot only when it is in the eye, 
but after it is taken out, it is evident, that when heteroge~ 
neal rays are thus collected by the crystalline,a circle of aber- 
ration is formed in its focus, which varies in magnitude: 
with the pupil; for the same law of aberration obtains i 
all convex lenses composed of dense refracting mediums. 
Hence the image of an object in the focus of the crystalline 
Jens increases with the pupil; and this is a property of the 
* See Monthly Magazine for August 1806, p. 103. 
3" eye 
