332. Dr. Darwins Experiments on the 
my eyes, this appeared of a dull yellow % and at other times 
mixed with the colours of other objeds on which it was 
thrown. It may be imagined, that this part of the retina was 
become infenfible ta white light, and thence a bluifh fpe&rum 
became visible on all luminous objedls ; but as a yellowifh fpec- 
trum was alfo feen in the clofed and covered eye, there can 
remain no doubt of this being the fpedtrum of the fun. A 
limilar appearance was obferved by M. ^Epinus, which he 
acknowledges he could give no account of. (Nov. Com. Petrop* 
V. io, p. 2. and 6.) 
The locked jaw, and fome cataleptic fpafms, are refembledi 
by this phenomenon ; and from hence we may learn the dan- 
ger to the eye by infpefling very luminous obje&s too^ long & 
time, 
IX. A quantity of jiimulus greater than the preceding induces a 
temporary paralyfs of the organ of vifon . 
1. Place a circular piece of bright red (ilk, about half an 
inch in diameter, on the middle of a fheet of white paper ; 
lay them on the floor in a bright funfbine, and fixing your 
eyes fteadily on the center of the red circle* for three or four 
minutes, at the diftance of four or fix feet from the objedt, the 
red filk will gradually become paler* and finally ceafe to appear 
red at all. 
2, Similar to thefe are many other animal fa£ts ; as purges* 
opiates, and even poifons, and contagious matter* ceafe to 
ftimulate our fyftem, after we have been habituated to their 
life. So fome people deep undifturbed by a clock, or even by 
a forge hammer in their neighbourhood: and not only conti- 
nued irritations, but violent exertions of any kind, are fucceeded 
by 
