On the black Rete mucosum of the Negro. 31 
it is equally at variance with observation and the theory of ba- 
rometrical measurements. 
It is extremely probable that, in regard to the astronomical re- 
fractions, Professor Leslie’s formula will be found to be equivalent 
to the law of a uniform gradation of temperature in the atmo- 
sphere, the rate at which the heat decreases depending upon the 
constant coefficient. If this conjecture shall turn out to be just, 
it must be allowed that the introducing of the formula can have 
no other effect than to lead off the attention from the true prin- 
ciples and the real difficulties of the problem. 
July 5, 1821. J. Ivory. 
N. B. The readers of the Philosophical Magazine are desired 
to read sin. A for sin. in the formula at p. 406 of the last Num- 
ber. 
V. On the black Rete mucosum of the Negro being a Defence 
against the scorching Effect of the Sun’s Rays. By Sir 
EverarD Home, Bart. F.R.S.* 
To ascertain the use of the black colour of the rete mucosum in 
the Negro, has occupied the attention of many physiologists ; 
and I confess that this subject formed the first investigation in 
which I ever engaged. Fruitless, indeed, were my attempts; and 
when I learnt that black surfaces absorbed heat, and raised the 
temperature several degrees beyond any others, I] gave the matter 
up in despair. Two years ago my attention was again called to 
this inquiry, upon being told by our late excellent President, that 
a silver fish, in a pond at Spring Grove, during a very hot sum- 
mer, immediately after some trees by which the pond was shaded 
were cut down, was so much exposed to the sun’s rays as to have 
its back scorched, the surface putting on the same appearance as 
after a burn, and rising above the scales of the surrounding skin. 
I saw the fish several times, and directions were given to send it 
to me when it died ; but I was not so fortunate as'to receive it. 
This extraordinary circumstance brought to my recollection 
one not less so. In crossing the Tropic in April 1781], at twelve 
o'clock at noon, in a voyage to the West Indies, I had fallen 
asleep upon deck, lying upon my back, having a thin linen pair 
of trowsers on, and I had not slept half an hour, when [ was 
awakened by the bustle attending the demand of forfeits on cross- 
ing the Line, and found the inside of the upper part of both thighs 
scorched, the effects of which have never gone off: but till now 
1 could not imagine how it happened, always suspecting it to be 
* From the Transactions of the Royal Society for 192), Part I. 
the 
