PHOTOGRAPHIC CORRESPONDENCE.
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 2½ 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 Φ. (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 2½ 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
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 2½ 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.
T. L. Merritt.
Maidstone.
Yellow Bottles for Photographic Chemicals.—The proposal of your correspondent Ceridwen to employ yellow glass bottles for preventing the decomposition of photographic solutions has been anticipated. It was suggested by me, in some lectures on Photography in November 1847, and in January of the present year, that yellow bottles might be so used, as well as for preventing the decomposition, by light, of the vegetable substances used in pharmacy, such as digitalis, ipecacuanha, cinchona, &c. For solutions of silver, however, the most effectual remedy against precipitation is the use of very pure water, procured by slow redistillation in glass vessels at a temperature much below the boiling point.
Hugh Owen.