ll venture just one remark more on these very interesting
drawings. The subject is so suggestive of thought at the present
stage, that it would be no easy matter to exhaust it; and it will, we
have no doubt, be still more suggestive of thought by and by; but we
are encroaching on our limits, and must restrain ourselves, therefore,
to the indication of just one of the trains of thought which it has
served to originate. Many of our readers must be acquainted with Dr.
Thomas Brown's theory of attention,--'a state of mind,' says the
philosopher, 'which has been understood to imply the exercise of a
peculiar intellectual power, but which, in the case of attention to
objects of sense, appears to be nothing more than the co-existence of
desire with the perception of the object to which we are said to
attend.' He proceeds to instance how, in a landscape in which the
incurious gaze may _see_ many objects without _looking_ at or knowing
them, a mere desire to know brings out into distinctness every object
in succession on which the desire fixes. 'Instantly, or almost
instantly,' continues the metaphysician, 'without our consciousness of
any new or peculiar state of mind intervening in the process, the
landscape becomes to our vision altogether different. Certain parts
only--those parts which we wished to know particularly--are seen by
us; the remaining parts seem almost to have vanished. It is as if
everything before had been but the doubtful colouring of enchantment,
which had disappeared, and left us the few prominent realities on
which we gaze; or rather as if some instant enchantment, obedient to
our wishes, had dissolved every reality beside, and brought closer to
our sight the few objects which we desired to see.' Now, in the
transcript of the larger _tableau vivant_ before us--that which
represents Dr. Chalmers seated among his friends on the Moderator's
chair--we find an exemplification sufficiently striking of the laws on
which this seemingly mysterious power depends. They are purely
structural laws, and relate not to the mind, but to the eye,--not to
the province of the metaphysician, but to that of the professor of
optics. The lens of the camera obscura transmits the figures to the
prepared paper, on quite the same principle on which in vision the
crystalline lens conveys them to the retina. In the centre of the
field in both cases there is much distinctness, while all around its
circumference the images are indistinct