ORGANS OF PERCEPTION". 187 



specticm^ 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 DUNGLISON, countenances the view 

 taken above : " A portion of the newly prepared extract of Belladona, for 

 the sake of experiment, wag 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 sudder mission of light ; and although the pupil approached by al- 

 most 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 



