_Stereoscopic Angles_.--The discussion in "N. & Q." relative to the best
angle for stereoscopic pictures has gone far towards a satisfactory
conclusion: there are, however, still a few points which may be
beneficially considered.
In the first place, the kind of stereoscope to be used must tend to modify
the mental impression; and secondly, the _amount_ of reduction from the
size of the original has a considerable influence on the final result.
If in viewing a stereoscopic pair of photographs, they are placed _at the
same distance_ from the eyes as the _length of the focus of the lens used
in producing them_, then without doubt the distance between the eyes, viz.
about two and a quarter {228} inches, is the best difference between the
two points of view to produce a perfectly natural result; and if the points
of operation be more distant from one another, as I have before intimated,
an effect is produced similar to what would be the case if the pictures
were taken from a _model_ of the object instead of the object itself.
When it is intended that the pictures taken are to be viewed by an
instrument that requires their distance from the eyes to be _less_ than the
focal length of the lens used in their formation, what is the result? Why,
that they subtend an angle larger than in nature, and are consequently
apparently _increased_ in bulk; and the obvious remedy is to _increase_ the
angle between the points of generation in the exact ratio as that by which
the visual distance is to be lessened. There is one other consideration to
which I would advert, viz. that as we judge of _distance_, &c. mainly by
the degree of _convergence_ of the optic axes of our two eyes, it cannot be
so good to arrange the camera with its two positions quite parallel,
especially for objects at a short or medium distance, as to let its centre
radiate from the principal object to be delineated; and to accomplish this
desideratum in the readiest way (for portraits especially), the ingenious
contrivance of Mr. Latimer Clark, described in the _Journal_ of the
Photographic Society, appears to me the best adapted. It consists of a
modification of the old parallel ruler arrangement on which the camera is
placed; but one of the sides has an adjustment, so that within certain
limits any degree of convergence is attainable. Now in the case of the
pictures alluded to by MR. H. WILKINSON in Vol. viii., p. 181., it is
probable they were taken by a camera placed in two positions parallel to
one another, and it is quite clear that only a _portion_ of the two
pictures could have been really stereoscopic. It is perfectly true that two
indifferent negatives will often combine and form one good stereoscopic
positive, but this is in consequence of one possessing that in which the
other is deficient; and at any rate two _good_ pictures will have a
_better_ effect; consequently, it is better that the two views should
contain exactly the same _range_ of vision.