being a Defence agains! the Sun’s Rays. 33 
Exp. 6.—September 8, 1820, at eleven o’clock, the heat in 
the sun 90°; the concentrated rays applied to my naked arm 
produced a vesicle. This experiment was repeated when the 
heat was 84°, and in seven minutes a blister formed on the arm. 
Exp. 7.—September 9th, eleven o’clock, the thermometer in 
the sun at 90°. The concentrated rays applied to a piece of black 
kerseymere cloth, made tight round my arm for 15 minutes, gave 
no real pain, and left no impression whatever on the skin, al- 
though the nap of the cloth had been destroyed. 
This experiment was repeated with white kerseymere, the heat 
at 86°; in 15 minutes a blister was formed. 
Repeated with Irish linen, the thermometer 86°. In 15 mi- 
nutes a blister was formed, and coagulable lymph thrown out, 
which had become vascular. 
The same experiment was made with a white handkerchief 
loose upon the hand, the heat 83°, In 15 minutes an inflam- 
matory blush was produced over a surface of several inches ex- 
tent, which almost immediately disappeared on withdrawing the 
hand from the sun’s rays. 
Exp. 8.—September 12th. The sun’s heat at noon 85°. The 
concentrated rays applied to the back of the hand of a Negro 
from Grenada, for 15 minutes, produced no visible effect; at the 
first moment he felt a stab going inwards, but that went off, and 
afterwards he had no pain. 
From these experiments, it is evident that the power of the 
sun’s rays to scorch the skin of animals is destroyed when ap- 
plied to a black surface, although the absolute heat, in conse- 
quence of the absorption of the rays, is greater. 
The same wise providence which has given so extraordinary a 
provision to the Negro for the defence of his skin, while living 
within the tropics, has extended it to the bottom of the eye, which 
otherwise would suffer in.a greater or less degree when exposed 
to strong light; the retina, from its transparency, allowing it to 
pass through without injury. 
That the nigrum pigmentum ‘is not necessary for vision, but 
only provided as a defence against strong light, is proved by its 
being darker in the Negro than the European, and being of a 
lighter colour in fair people than in dark, and therefore lightest 
in those countries furthest removed from the effects of the sun. 
In the monkey it is dark, and in all animals that look up- 
wards, 
In all birds exposed to the sun’s rays the ni igrum pigmentum 
is black. In fishes, the basking shark, which lies upon the sur- 
face of the ocean, has a nigrum pigmentum, The turbot and 
skate, which lie upon banks of sand in shallow water, have ni- 
grum pigmentum. 
Vol, 58. No, 279, July 1821. E In 
