Stereoscopic Angles.—As I presume that Mr. T. L. Merritt is, like myself, only desirous of arriving at truth, I beg to offer the following reply to his last communication (Vol. viii., pp. 275-6.), in which he misinterprets some observations of mine upon the subject in question.
With regard to the distance quoted by me of 2¼ inches, I look upon it as the same thing as intended by Mr. Merritt—that is, the average distance between the centres of the eyes; and it amounts simply to a difference of opinion between us; but, so far as that point is concerned, I am quite ready to adopt 2½ inches as a standard, although I believe that the former is nearer the truth: however, I require more than a mere assertion that "the only correct space for the cameras to be apart is 2½ inches, and this under every circumstance, and that any departure from this must produce error." I quote verbatim, having merely Italicised three words to point my meaning more clearly. An object being 5 feet distant, and another at 10 feet from the observer, a line between the eyes will subtend a very much larger angle in the former than in the latter instance: hence the inclination of the axes of the eyes is the chief criterion by which people with the usual complement of those useful organs judge of proximity: but if half a dozen houses are made to appear as if 10 or 12 feet distant (by means of the increase of the angle between the points of formation of the pictures), while the angle which each picture subtends is relatively small; it is clear that both eyes will see in relief at a short distance half a dozen houses in a space not large enough for a single brick of one of them, and, consequently, the view will appear as if taken from a model. Mr. Merritt will object that an erroneous effect is produced; if he will refer to my statement (Vol. viii., p. 228.), he will find that it is precisely what I admitted; and he appears to have overlooked the proviso attached to my next observation (judging by his comment thereon), so I shall make no farther remark upon that point, beyond inquiring why the defect he is content to put up with is called a trifling exaggeration, while that which is less offensive to me is designated as absolute deformity and error? Persons with one eye are not good judges of distance, and this may be easily tested thus:—Close one eye, and endeavour to dip a pen in an inkstand at some little distance not previously ascertained by experiment, with both eyes open; it will be found far less easy than would be imagined. One-eyed people, from habit, contrive to judge of distance mainly by relative position, and by moving the head laterally cause a change therein: to them, all pictures are, to an extent, stereoscopic.
I am really amazed that my advocacy of the radial, instead of the parallel, position of the cameras should have been so misunderstood. Surely, it cannot be seriously asserted that the former will produce two vanishing points, and the latter only one? And as to the supposition connected with the boy, the ass, and the drum, a camera that would produce the effect of showing both sides of the ass, both legs of the boy, and both heads of the drum, with a movement of only 2½ inches, whether radially or parallel, would indeed be a curiosity. But if the motion of the camera extended over a space sufficiently large to exhibit the phenomena alluded to, then it would confirm what I have before advanced, viz. present the idea of a small model of the objects, which could be so placed as to show naturally these very effects.
That the axes of the eyes are inclined when viewing objects, is readily proved thus:—Let a person look across the road at any object—say a shop-window; but stand so that a lamp-post near him shall intervene, and be in a direct line between the observer's nose and the object viewed. If he be requested to observe the post instead of the distant object, the pupils of his eyes will be seen to approach one another; and on again looking to the distant object, will instantly recede. The range of vision is another point that appears to be misunderstood, as we are differing about words instead of facts. The column is an illustration that will exactly suit my views; for I call the range of vision the same if taken from side to side of the column, although it is perfectly true that the tangents to the two eyes differ by the angle they subtend: but certainly Mr. Wilkinson's case (Vol. viii., p. 181.) of seven houses and five bathing-machines in one picture, and five houses and eight machines in the other, illustrates an instance where the range of vision is not the same; but I contend that the stereoscopic effect is then confined to five
houses and five machines, otherwise Mr. Wilkinson's supposititious case (ibid.), of all machines in one, and all houses in the other, might be considered as stereoscopic.
In concluding this very lengthened and, I fear, tedious reply, I beg to assert that I am most willing to recant any proposition I may have put forth, if proved to be erroneous; but I must have proof, not mere assertion. And farther, my willing thanks are always tendered to any one kind enough to correct an error.
Geo. Shadbolt.
Mr. Pumphrey's Process for securing black Tints in Positives.—The importance that appears to be attached by some of thy correspondents to the stereoscopic appearance of photographs, induces me to call the attention of those who may not have noticed it to the fact that, as all camera pictures are monocular, they are best seen by closing one eye, and then they truly represent nature; and the effect of distance (which so often appears wanting in photographs) is given with marvellous effect, so well indeed as to render the use of a stereoscope unnecessary. Like other photographers, I have been long seeking for a method, easy, cheap, and certain, for obtaining the black tints that are so highly prized by many in the French positives; and having at last attained the object of my search, I lose no time in laying it before my fellow-operators.
I obtain these results with a twenty-grain solution of nitrate of silver, a fact that will, I think, commend the plan to most operators. Thou wilt be able to judge of the result from the inclosed specimen.[[7]] I use Canson's paper, either albumenized or plain (but the former is far preferable). If albumen is used, I dilute it with an equal measure of water, and add half a grain of common salt (chloride of sodium) to each ounce of the mixture. This is applied to the paper with a soft flat brush, and all bubbles removed, by allowing a slender stream of the mixture to flow over its surface: it is then hung up to dry, and afterwards the albumen is coagulated with a hot iron. If the paper is used plain, a solution of common salt (half a grain to one ounce of water) is placed in a shallow tray, and the paper floated on its surface for a minute, and then hung up to dry. Excite, in either case, with an ammonio-nitrate of silver solution (twenty grains to one ounce of water), by floating the paper, prepared side downwards, for one minute, and hang up to dry.
Print tolerably strongly, and the proof will be of a reddish-brown. Fix in tolerably strong solution of hypo. sodæ (I never weigh my hypo., so cannot give the proportion), that either has been in use some time, or else, if new, has been nearly saturated with darkened chloride of silver. When fixed, remove the proofs into another vessel of the same solution of hypo., to which has been added chloride of gold and acetic acid. The way I do this is to dissolve one drachm of chloride of gold in two and a half ounces (1200 minims) of water. Of this I take twenty minims (which will contain one grain Au Cl3) and forty minims of acetic acid (Beaufoy's) for every dozen proofs (of the size of 7 × 9 in.), that I mean to operate on, and having mixed the gold and acetic acid with the solution of hypo., place the proofs in it till they attain the desired colour: they are then to be washed and dried in the usual way.