322 Dr. Darwin’s Experiments on the 
M. Beguelin,iii the Berlin Memoires, V. II. 1771, obferves, 
that, when he held a book fo that the fun {hone upon his 
half clofed eyelids, the black letters, which he had long in- 
ipefted-, became red, which muft have been thus occafioned. 
Thofe parts of the retina which had received for fome time 
the black letters, were fo much more fenfible than thofe parts 
which had been oppofed to the white paper, that to the former 
the red light, which paffied through the eyelids, was percepti- 
ble. There is a fimilar ftory told, I think, in M. de Voltaire’s. 
Hiftoricai Works, of a Duke of Tufcany, who was playing 
at dice with the general of a foreign army, and, believing he 
faw bloody fpots upon the dice, portended dreadful events, and 
retired in confufion. The obferver, after looking for a minute 
on the black fpots of a die, and careleffly doling his eyes, oil 
a bright day, would fee the image of a die with red fpots upon 
it, as above explained. 
5. On emerging from a dark cavern, where we have long 
continued, the light of a bright day becomes intolerable to the 
eye for a conliderable time, owing to the excefs of fenfibility 
exifting in the eye, after having been tong expofed to little or 
no ftimulUs. This cccafions us immediately to contra£i the 
iris to its fmalleft aperture, which becomes again gradually 
dilated, as the retina becomes accuftomed to the greater ftknulus 
of the daylight. 
The twinkling of a bright ftar, or of a diftant candle in the- 
night, is perhaps owing to the fame caufe. While we con- 
tinue to look upon thefe luminous obje&s, their central parts, 
gradually appear paler, owing to the decrealing fenfibility of 
the part of the retina expofed to their light ; whilft, at the 
fame time, by the unfteadinefs of the eye, the edges of them 
are perpetually falling on parts of the retina that were juft 
before 
