_Stereoscopic Angles._--Like many of your correspondents, I have been an
inquirer on the subject of stereoscopic angles, which seems to be still a
problem for solution. What is this problem? for until that be known, we
cannot hope for a solution. I would ask, is it this?--_Stereoscopic
pictures should create in the mind precisely such a conception as the two
eyes would if viewing the object represented by the stereograph._ If this
be the problem (and I cannot conceive otherwise), its solution is simple
enough, as it consists in placing the cameras _invariably_ 2-1/2 inches
apart, on a line parallel to the building, or a plane passing through such
a figure as a statue, &c. In this mode of treatment we should have two
pictures possessing like stereosity with those on the retinas, and
consequently with like result and as our eyes enable us to conceive
perfectly of any solid figure, so would the stereograph. I believe,
therefore, that this is, under every circumstance, the correct treatment;
simply because every other mode may be proved to be false to nature.
Professor Wheatstone recommends 1 in 25 when objects are more than 50 feet
distant, and this rule seems to be pretty generally followed. Its
incorrectness admits of easy demonstration. Suppose a wall 300 feet in
extent, with abutments, each two feet in front, and projecting two feet
from the wall, at intervals of five feet. The proper distance from the
observer ought to be 450 feet, which, agreeably with this rule, would
require a space of 18 feet between the cameras. Under this treatment the
result would be, that both of the _sides, as well as the fronts_, of the
three central abutments would be seen; whilst of all the rest, only the
front and one side would be visible. This would be outraging nature, and
false, and therefore should, I believe, be rejected. The eyes of an
observer situated midway between the cameras, could not possibly perceive
either of the sides of the buttress opposite to him, and only the side next
to him of the rest. This seems to me conclusive.
Again, your correspondent [Phi]. (Vol. vii., p. 16.) says, that for
portraits he finds 1 in 10 a good rule. Let the sitter hold, straight from
the front, _i.e._ in the centre, a box 2-1/2 inches in width. The result
would be, that in the stereographs the box would have both its sides
represented, and the front, instead of being horizontal, consisting of two
inclined lines, _i.e._ unless the cameras were {110} placed on _one line_,
when it would be horizontal. In such treatment the departure from both is
as great as in the first example, and the outrage greater, inasmuch as,
under these circumstances (I mean a boy with a box), to any person of
common sense, the caricature would be at a glance obvious. This rule, then,
although it produces stereosity enough, being false, should also be
rejected.
I believe that 2-1/2 inches will be found to be right under any
circumstance; but should sufficient reasons be offered for a better rule, I
trust I am open to conviction, and shall hail with great pleasure a
demonstration of its correctness.
Should it, however, turn out that I have given a right definition, and a
correct solution of this most interesting problem, I shall rejoice to know
that I have rendered an essential service to a great number of anxious
students in photography.