Now, when any one with no covering on his head
(as must have been aboriginally the case with mankind), strives
to the utmost to distinguish in broad daylight, and especially
if the sky is bright, a distant object, he almost invariably
contracts his brows to prevent the entrance of too much light;
the lower eyelids, cheeks, and upper lip being at the same time raised,
so as to lessen the orifice of the eyes. I have purposely asked
several persons, young and old, to look, under the above circumstances,
at distant objects, making them believe that I only wished to test the power
of their vision; and they all behaved in the manner just described.
Some of them, also, put their open, flat hands over their eyes to keep
out the excess of light. Gratiolet, after making some remarks to nearly
the same effect,[5] says, "Ce sont la des attitudes de vision difficile."
He concludes that the muscles round the eyes contract partly for
the sake of excluding too much light (which appears to me the more
important end), and partly to prevent all rays striking the retina,
except those which come direct from the object that is scrutinized.
Mr. Bowman, whom I consulted on this point, thinks that the contraction
of the surrounding muscles may, in addition, "partly sustain the consensual
movements of the two eyes, by giving a firmer support while the globes
are brought to binocular vision by their own proper muscles.
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