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. 2½ 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 2½ 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 2½ 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.
Φ. (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?
N. T. B.