We see quite plainly that all the images of visible objects that lie
before us, whether large or small, reach our sense by the minute
aperture of the eye; and if, through so small a passage the image
can pass of the vast extent of sky and earth, the face of a
man--being by comparison with such large images almost nothing by
reason of the distance which diminishes it,--fills up so little of
the eye that it is indistinguishable. Having, also, to be
transmitted from the surface to the sense through a dark medium,
that is to say the crystalline lens which looks dark, this image,
not being strong in colour becomes affected by this darkness on its
passage, and on reaching the sense it appears dark; no other reason
can in any way be assigned. If the point in the eye is black, it is
because it is full of a transparent humour as clear as air and acts
like a perforation in a board; on looking into it it appears dark
and the objects seen through the bright air and a dark one become
confused in this darkness.