_Stereoscopic Articles._--I cannot agree to my opponent's assumed amendment
(?) (Vol. viii., p. 419.) _space_, for the simple reason that it would be
virtually abandoning the whole of the points in dispute between us; when
farther discussion and more mature consideration, only tend to convince me
more firmly of the correctness of the propositions I have advocated, viz.:
1st. That circumstances _may_ and _do_ arise in which a better result is
obtained in producing stereographs, when the chord of the angle of
generation is more or less than 2-1/2 inches.
2nd. That the positions of the camera should _not_ be parallel but radial.
I certainly thought that I had, as I intended, expressed the fact that I
treat the cameras _precisely as two eyes_, and moreover I still contend
that they should be so treated; my object being to present to each eye
_exactly such a picture and in such a direction as would be presented under
certain circumstances_. The plane of delineation being a flat, instead of a
curved surface, has nothing whatever to do with this point, because the
curves of the retinas are not portions of one curve having a common centre,
but each having its own centre in the axis of the pupil. That a plane
surface for receiving the image is not so good as a spherical one would be,
is not disputed; but this observation applies to photographs _universally_,
and is only put up with as the lesser of two evils. A plane surface
necessarily contracts the field of view to such a space as could be cut out
of the periphery of a hollow sphere, the versed sine of which bears but a
small ratio to its chord.
There is another misunderstanding into which my opponent has fallen, viz.
the part of the object to be delineated, which should form the centre of
radiation, is not the most contiguous visible point, but the most remote
principal point of observation. I perceive that this is the case from two
illustrations he was kind enough to forward me, being stereographs of a
[T-square] square, placed with the points of junction towards the observer,
and the tail receding from him; and in one case the angle of the square is
made the centre of radiation, and while its distance from the camera is
only six feet, the points of delineation are no less than three feet apart.
To push an argument to the extreme to test its value, is quite right; but
this goes far beyond the extreme, if I may be allowed such a very Hibernian
expression.
No object, however minute, can be clearly seen if brought nearer to the
eyes than a certain point, because it will be what is technically called
out of focus. It is true that this point differs in different individuals,
but the _average distance_ of healthy vision is 10 inches. Now, adopting
MR. MERRITT'S own standard of 2-1/2 inches between the eyes, it is clear that
supposing the central point had been rightly selected, the distance between
the cameras was _only double_ what might have been taken an extreme
distance. It is scarcely necessary to suggest what a person devoid of taste
(in which category I am no doubt included) might do in producing
monstrosities by adopting the radial method, as such an one is not very
likely to produce good results at all.
I now address myself to another accusation. It is quite true that I am
unacquainted with the _scholastic dogmas_ of perspective, but equally true
that I am familiar with _the facts_ thereof, as any one must be who has
studied optical and geometrical science generally; and while I concur in
the propositions as enunciated for a one-eyed picture, I by no means agree
to the assumption that the "vanishing points," in the two stereographs
taken radially with the necessary precautions, "would be so far apart, that
they could not in the stereoscope flow into one;" on the contrary, direct
experiment shows me, what reason also suggests, that they do flow into one
as _completely as in nature when viewed by both eyes_.
I put the proposition thus, because I do not hesitate to avow that in
nature, as interpreted by binocular vision, these points do not
_absolutely_, but only approximately, flow _into one_; otherwise one eye
would be as effective as two.
I have not the smallest objection to my views being considered "false to
art," as, alas! her fidelity to nature is by no means beyond suspicion.
{477}
Lastly, as to the model-like appearance of stereographs taken at a large
angle, for the fact I need only refer the objector to most of the beautiful
foreign views now so abundant in our opticians' shops: for the reason, is
it not palpable that increasing the width of the eyes is analogous to
decreasing the size of the object? and if naturally we cannot "perceive at
one view three sides of a cake, two heads of a drum, nor any other like
absurdity," it is only because we do not use objects sufficiently _small_
to permit us to do so. Even while I am writing this, I have before me a
small rectangular inkholder about 1-1/4 inches square, and distant from my
eyes about one foot, in which the very absurd phenomenon complained of does
exist, the front, top, and _both_ sides being perfectly visible at once:
and being one of those obstinate fellows who will persist in judging
personally from experience if possible, I fear I shall be found
incorrigible on the points on which your correspondent has so kindly
endeavoured to enlighten me.