ORGANS OF PERCEPTION. 187 
spection, within which the pupil enlarges and contracts, ac- 
cording-to circumstances. Within this range vision is dis- 
tinct ; beyond it, at either extremity, it is obscure. When, 
however, we are able to assist the iris by any contrivance, 
the range of distinct vision is enlarged. Thus, in reading 
a book, the page is kept at least six inches from the eye ; 
and when brought much nearer, the rays entering the eye to 
form the image of the letters possess two great a degree of 
divergence, and vision is obscure. When, however, we 
look at the letters through a pin-hole in paper, held close to 
the eye, we prevent the rays of greatest divergence from 
entering the pupil, admitting only those which are nearly 
parallel, and the resulting image is so distinct, that we can 
read print brought within an inch and a half of the eye. 
The pin-hole in paper is therefore the simplest kind of micro- 
scope, but the eye is fatigued by using it. When an object, 
beyond the limit of minute vision, is viewed through a smal- 
ler aperture than the pupil, a more obscure image is formed 
than when the eye is uncovered, and the same object ceases 
to be visible at a shorter distance *. 

* The following experiment by Mr Dunexison, countenances the view 
taken above: ‘* A portion of the newly prepared extract of Belladona, for 
the sake of experiment, was inserted between, and applied to the eye-lids ; in 
consequence, in the space of about twenty minutes, the pupil was so much 
dilated, that the iris was almost totally invisible. From the time the pupil 
attained to three times its natural dimensions, objects presented to this eye, 
with the other closed, were seen as through a cloud ; and as it proceeded to 
the point of extreme dilation, this effect gradually increased, so that minute 
and near objects, as letter-press, &c. could not be at all distinguished. By 
means of a double convex lens, the focus of this eye was found to be at 
twice the distance of that of the sound eye; the iris, however, dilated upon 
the sudden admission of light; and although the pupil approached by ai- 
mest imperceptible degrees for six days to its natural size, yet, at the end 
of that time, it was dilated to twice the size of the other; and, in propor- 
tion as the contraction took place, the sight became more distinct, and the 
