It has the same disadvantages as other modes on paper from inequality in the strength of the image. The photographic pons asinorum appears however to be got over by the process, viz. taking the picture at once in the camera, and it is very possible that it can be made perfect. A small quantity of chromate of potash, about one grain to three ounces of solution of iodide of iron, gives a little more force to the picture.
I find the nitrate of lead a very useful salt in iodizing paper. Six grains of the salt to the ounce of water, and tincture of iodine added till a pale yellow, will give additional sensitiveness to iodized paper, if the sheets are floated upon the solution. This will shorten the time in the camera nearly five minutes; but it requires care, as it is apt to solarize.
A weak solution of iodide of iron has also the same effect, and, if blotted off at once, it will not blacken by the use of gallic acid.
Weld Taylor.
Bayswater.
Stereoscopic Angles.—When I last addressed you, I fancied I should set the stereoscopic-angle question at rest. It appears, however, that Mr. G. Shadbolt is unconvinced, and as I alone (to the best of my knowledge) have defined and solved the problem in relation to this subject, you will perhaps allow me to offer a few words in rejoinder to Mr. S.'s arguments which, had that gentleman thought more closely, would not have been advanced. This is also requisite, because, from their speciousness, they are likely to mislead such as take what they read for granted. Mr. S. says that when the stereographs are placed at the same distance from the eyes as the focal length of the lens, that 2¼ inches is the best space for the cameras to be apart; and that were this space increased, the result would be as though the pictures were taken from models. To this I reply, that the only correct space for the cameras to be apart is 2½ inches (i. e. the space usually found to be from pupil to pupil of our eyes), and this under every circumstance; and that any departure from this must produce error. As to the model-like appearance, I cannot see the reason of
it. Next Mr. Shadbolt says, and rightly, that when the pictures are seen from a less distance than the focal length of the lens, they appear to be increased in bulk. But the "obvious remedy" I pronounce to be wrong, as it must produce error. The remedy is nevertheless obvious, and consists in placing the stereographs at the same distance from the eyes as the focal length of the lens. But, if this cannot be done, it were surely better to submit to some trifling exaggeration than to absolute deformity and error. Mr. S. says also, that as we mainly judge of distance, &c. by the convergence of the optic axis of our eyes (Query, How do persons with only one eye judge?), so, in short or medium distances, it were better to let the camera radiate from its centre to the principal object to be delineated. The result of this must be error, as the following illustration will show. Let the sitter (for it is especially recommended in portraits) hold before him, horizontally, and in parallelism with the picture, a ruler two feet long; and let planes parallel to the ruler pass through the sitter's ears, eyes, nose, &c. The consequence would be that the ruler, and all the other planes parallel to it, would have two vanishing points, and all the features be erroneously rendered. This, to any one conversant with perspective, should suffice. But, as all are not acquainted with perspective, perhaps the following illustration may prove more convincing. Suppose an ass to stand facing the observer; a boy astride him, with a big drum placed before him. Now, under the treatment recommended by Mr. G. Shadbolt, both sides of the ass would be visible; both the boy's legs; and the drum would have two heads. This would be untrue, absurd, ridiculous, and quite as wonderful as Mr. Fenton's twelve-feet span view from across the Thames.
Once more, and I shall have done with the present arguments of Mr. G. Shadbolt. He says that the two pictures should have exactly the same range of vision. This I deny: for, were it so, there would be no stereoscopic effect. Let the object be a column: it is evident that a tangent to the left side of the column from the right eye, could not extend so far to the left as a tangent to the left side of the column from the left eye, and vice versâ. And it is only by this difference in the two pictures (or, in other words, the range of vision) that our conceptions of solidity are created. This is not exactly the test to suit the views of Mr. Shadbolt, as I am quite aware; but I chose it for its simplicity, and because it will bear demonstration; and my desire has been to elicit truth, and not to perpetuate error.
In conclusion, I beg to refer Mr. G. Shadbolt to my definition and solution of the stereoscopic problem—which I then said I believed—but which I now unhesitatingly assert to be correct.
T. L. Marriott.