PHOTOGRAPHIC CORRESPONDENCE.
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
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.
Geo. Shadbolt.
Protonitrate of Iron.—"Being in the habit of using protonitrate of iron for developing collodion pictures, the following method of preparing that solution suggested itself to me, which appears to possess great advantages:—
Water 1 oz.
Protosulphate of iron 14 grs.
Nitrate of potash 10 grs.
Acetic acid ½ drm.
Nitric acid 2 drops.
In this mixture nitrate of potash is employed to convert the sulphate of iron into nitrate in place of nitrate of baryta in Dr. Diamond's formula, or nitrate of lead as recommended by Mr. Sisson; the advantage being that no filtering is required, as the sulphate of potash (produced by the double decomposition) is soluble in water, and does not interfere with the developing qualities of the solution.
"The above gives the bright deposit of silver so much admired in Dr. Diamond's pictures, and will be found to answer equally well either for positives or negatives. If the nitric acid be omitted, we obtain the effects of protonitrate of iron prepared in the usual way.—John Spiller."
(From the Photographic Journal.)
Photographs in natural Colours.—As "N. & Q." numbers among its correspondents many residents in the United States, I hope you will permit me to inquire through its columns whether there is really any foundation for the very startling announcement, in Professor Hunt's Photography, of Mr. Hill of New York having "obtained more than fifty pictures from nature in all the beauty of native coloration," or whether the statement is, as I conclude Professor Hunt is inclined to believe, one of those hoaxes in which many of our transatlantic friends take so much delight.
Matter-of-Fact.
Photographs by artificial Lights.—May I ask for references to any manuals of photography, or papers in scientific journals, in which are recorded any experiments that have been made with the view of obtaining photographs by means of artificial lights? This is, I have no doubt, a subject of interest to many who, like myself, are busily occupied during the day, and have only their evenings for scientific pursuits: while it is obvious, that if such a process can be successfully practised, there are many objects—such as prints, coins, seals, objects of natural history and antiquity—which might well be copied by it, even though artificial light should prove far slower in its action than solar light.
A Clerk.