_Stereoscopic Angles._--As I presume that MR. T. L. MERRITT is, like
myself, only desirous of arriving at truth, I beg to offer the following
reply to his last communication (Vol. viii., pp. 275-6.), in which he
misinterprets some observations of mine upon the subject in question.
With regard to the distance quoted by me of 2-1/4 inches, I look upon it as
the same thing as intended by MR. MERRITT--that is, the _average_ distance
between the centres of the eyes; and it amounts simply to a difference of
_opinion_ between us; but, so far as that point is concerned, I am quite
ready to adopt 2-1/2 inches as a standard, although I believe that the former
is nearer the truth: however, I require more than a mere _assertion_ that
"the _only_ correct space for the cameras to be apart is 2-1/2 inches, and
this under every circumstance, and that _any_ departure from this _must_
produce error." I quote verbatim, having merely Italicised three words to
point my meaning more clearly. An object being 5 feet distant, and another
at 10 feet from the observer, a line between the eyes will subtend a very
_much larger_ angle in the former than in the latter instance: hence the
inclination of the axes of the eyes is the chief criterion by which people
with the usual complement of those useful organs judge of proximity: but if
half a dozen houses are made to appear as if 10 or 12 feet distant (by
means of the increase of the angle between the points of formation of the
pictures), while the angle which each picture subtends is relatively small;
it is clear that both eyes will see in relief at a short distance half a
dozen houses in a space not large enough for a single brick of one of them,
and, _consequently_, _the view will appear as if taken from a model_. MR.
MERRITT will object that an erroneous effect is produced; if he will refer
to my statement (Vol. viii., p. 228.), he will find that it is precisely
what I admitted; and he appears to have overlooked the _proviso_ attached
to my next observation (judging by his comment thereon), so I shall make no
farther remark upon that point, beyond inquiring why the defect he is
content to put up with is called a _trifling exaggeration_, while that
which is less offensive to me is designated as _absolute deformity_ and
error? Persons with one eye are _not good judges_ of distance, and this may
be easily tested thus:--Close one eye, and endeavour to dip a pen in an
inkstand at some little distance not previously ascertained by experiment,
with both eyes open; it will be found far less easy than would be imagined.
One-eyed people, from habit, contrive to judge of distance mainly by
_relative position_, and by moving the head _laterally_ cause a change
therein: to them, all pictures are, to an extent, stereoscopic.
I am really amazed that my advocacy of the radial, instead of the parallel,
position of the cameras should have been so misunderstood. Surely, it
cannot be seriously asserted that the former will produce _two_ vanishing
points, and the latter only one? And as to the supposition connected with
the boy, the ass, and the drum, a camera that would produce the effect of
showing both sides of the ass, both legs of the boy, and both heads of the
drum, _with a movement of only 2-1/2 inches_, whether radially or parallel,
would indeed be a curiosity. But if the motion of the camera extended over
a space sufficiently large to exhibit the phenomena alluded to, then it
would confirm what I have before advanced, viz. present the idea of a
_small model_ of the objects, which could be so placed as to show naturally
these very effects.
That the axes of the eyes are inclined when viewing objects, is readily
proved thus:--Let a person look across the road at any object--say a
shop-window; but stand so that a _lamp-post near him_ shall intervene, and
be in a _direct line_ between the observer's nose and the object viewed. If
he be requested to observe the post instead of the distant object, the
pupils of his eyes will be seen to approach one another; and on again
looking to the distant object, will instantly recede. The _range_ of vision
is another point that appears to be misunderstood, as we are differing
about words instead of facts. The column is an illustration that will
_exactly_ suit my views; for I call the _range_ of vision the same if taken
from side to side of the column, although it is perfectly true that the
tangents to the two eyes differ by the angle they subtend: but certainly
MR. WILKINSON'S case (Vol. viii., p. 181.) of seven houses and five
bathing-machines in one picture, and five houses and eight machines in the
other, illustrates an instance where the range of vision is not the same;
but I contend that the stereoscopic effect is then _confined_ to five {349}
houses and five machines, otherwise MR. WILKINSON'S supposititious case
(_ibid._), of all machines in one, and all houses in the other, might be
considered as stereoscopic.
In concluding this very lengthened and, I fear, tedious reply, I beg to
assert that I am most willing to recant any proposition I may have put
forth, if _proved_ to be erroneous; but I must have proof, not mere
assertion. And farther, my willing thanks are always tendered to any one
kind enough to correct an error.