P.S.--I may add that I have only lately tried the addition of the iodide of
potassium to my setting liquid, and so must qualify my recommendation of it
by saying so.
Florian, Torquay.
_Stereoscopic Angles._--I am obliged to MESSRS. SHADBOLT and WILKINSON for
the information given in reply to my Queries (Vol. vii., p. 505.) My mode
of operation is precisely that of MR. WILKINSON: "I obtain all the
information I can from every source; then try, and judge for myself." Hence
the present letter.
I regret to be obliged to differ from MR. SHADBOLT, but there is a point in
his communication which appears to me to arise from a misconception of the
stereoscopic problem. He says (p. 557.), "for _distant_ views there is in
nature scarcely any _stereoscopic_ effect." Now, surely visual distance is
merely visual stereosity; for, to see an object solid is merely to see its
parts in relief, some of them appearing to project or recede from the
others. It is the difficulty of producing this effect in landscapes, by the
ordinary camera process, that renders views taken by such means so
deficient in air, or, as the artists term it, aerial perspective, most
distant objects seeming almost as near as those in the foreground. This
indeed is the main defect of all photographs: they are true representations
of nature to one eye--cyclopean pictures, as it were--appearing perfectly
stereoscopic with one eye closed, but seeming absolutely flattened when
viewed by the two eyes. I remember being shown a huge photograph of the
city of Berlin, taken from an eminence; and a more violent caricature of
nature I never set eyes upon. It was almost Chinese in its perspective: the
house-tops appeared to have been mangled. It was a wonderful work of art,
photographically considered; but artistically it was positively hideous.
But the same defect exists in _all_ monophotographic representations,
though in a less degree, and consequently less apparent than in views to
which a sense of distance is essential. In portraits, the features appear
slightly flattened; and until photographers are able to overcome this, the
chief of all obstacles to perfection, it is idle to talk of the art giving
a correct rendering of nature. This is what is wanted, more than colour,
diactinic lenses, multiplication of impressions, or anything else. And when
it is remembered that the law of an ordinary convex lens is, the farther
the object from the lens the nearer the focus, and, _vice versa_, the
nearer the object the farther the focus, it becomes evident that by such an
instrument distant objects must be made to appear near, and near objects
distant, and nature consequently mangled.
The stereoscope gives us the only demonstrably correct representation of
nature; and when that instrument is rendered more simple, and the peep-show
character of the apparatus disconnected from it, the art of photography
will transcend the productions of the painter--but not till then.
I am anxious to obtain all the information I can from such of your
photographic readers as are practically acquainted with the stereoscopic
portion of the art relative to the angles under which they find it best to
take their pictures for given distances.
Mr. Fenton, the secretary of the Photographic Society, takes his
stereoscopic pictures, when the objects are 50 feet and upwards from the
camera, at 1 in 25. This is, as MR. SHADBOLT states, Professor Wheatstone's
rule for distances. {17}
MR. WILKINSON, on the other hand, asserts that 3 feet in 300 yards is
sufficient separation for the cameras: this is only 1 in 300,--a vast
difference truly.
"For views across the Thames," says the editor of the _Photographic
Journal_, "the cameras should be placed 12 feet apart, and with this
separation the effect is declared to be astonishing."
MR. WILKINSON, however, asserts that from 4 to 6 feet in a mile will do
_well enough_!
Farther, Mr. Latimer Clark (the inventor of an ingenious stereoscopic
camera) states that with regard to the distance between the two positions
of the cameras, he knows no good reason why the natural distance of the
eyes, viz. 2-1/2 inches, should be much exceeded. "A little extra relief is
obtained," he adds, "without visible distortion, by increasing the
separation to about 4 or 5 inches; but if this distance be greatly
exceeded, especially for near objects (I give the gentleman's own words),
they become apparently diminished in size, and have the appearance of
models and dolls rather than natural objects."
The reason for making the separation between the cameras greater than that
between the two eyes, is exceedingly simple. The stereograph is to be
looked at much _nearer_ than the object itself, and consequently is to be
seen under a much larger angle than it is viewed by the two eyes in nature.
Hence the two pictures should be taken at the angle under which they are to
be observed in the stereoscope. Suppose the object to be 50 feet distant,
then of course it is seen by the two eyes under an angle of 2-1/2 inches in
50 feet, or 1 in 240. But it is intended that the stereograph should be
seen by the two eyes when but a few inches removed from them, or generally
under an angle of 2-1/2 in 12 inches, or nearly 1 in 5. Hence it is
self-evident that the stereoscopic angle should be considerably larger than
that formed by the optic axes of the two eyes when directed to the object
itself.
But there is great diversity of opinion as to the extent of the angles
requisite for producing the precise stereoscopic or distantial effect of
nature. For myself I prefer Professor Wheatstone's rule, 1 in 25 for
objects beyond 50 feet distant. For portraits I find the best angle 1 in 10
when the sitter is 10 feet off, and for busts about 1 in 5 when placed
about 5 or 6 feet from the cameras. But I should be happy to receive
information from any of your readers concerning this important branch of
the photographic art. For months past I have been engaged in a series of
experiments in connexion with the subject, and wish for larger experience
than it is possible for any single operator to acquire for himself.
Mr. Fenton, I may observe, does not keep the cameras parallel in taking
landscapes, but inclines them so that the same object may occupy as nearly
as possible the centre of the ground glass plate.
Nor is it essential that perfect horizontality or parallelism of the
cameras should be maintained in copying trees. For buildings, however, it
is absolutely necessary that the cameras be kept straight.
I am sorry thus to trespass on your space, but being anxious, as MR.
WILKINSON says, to collect information from every source, and your
periodical being a happy medium for conveying and receiving instruction, I
am glad to avail myself of such a channel.
[Phi]. (2)
P.S.--Mr. Claudet has, I perceive, been awarded the prize given by the
Society of Arts for the best essay on the stereoscope. Can you, or any of
your readers, inform me whether this is likely to be published, and when
and at what price?
_Query respecting Mr. Pollock's Process._--In MR. POLLOCK's directions for
obtaining positives which appeared in "N. & Q." (Vol. vii, p. 581.), iodide
of silver is to be dissolved in a saturated solution of hypo. Can you give
me the quantity of iodide of silver to be dissolved, and the quantity of
the saturated solution of hypo. in which it is to be dissolved?