A sheet of the best linen paper is dipped in salt water and suffered to dry. Then a solution of nitrate of silver is poured over it and it is dried in a dark place. This paper is now sensitive; it has a conscience, and is afraid of daylight. Press it against the glass negative and lay them in the sun, the glass uppermost, leaving them so for from three to ten minutes. The paper, having the picture formed on it, is then washed with the solution of hyposulphite of soda, rinsed in pure water, soaked again in a solution of hyposulphite of soda, to which, however, the chloride of gold has been added, and again rinsed. It is then sized or varnished.
Out of the perverse and totally depraved negative,—where it might almost seem as if some magic and diabolic power had wrenched all things from their proprieties, where the light of the eye was darkness, and the deepest blackness was gilded with the brightest glare,—is to come the true end of all this series of operations, a copy of Nature in all her sweet gradations and harmonies and contrasts.
We owe the suggestion to a great wit, who overflowed our small intellectual home-lot with a rushing freshet of fertilizing talk the other day,—one of our friends, who quarries thought on his own premises, but does not care to build his blocks into books and essays,—that perhaps this world is only the negative of that better one in which lights will be turned to shadows and shadows into light, but all harmonized, so that we shall see why these ugly patches, these misplaced gleams and blots, were wrought into the temporary arrangements of our planetary life.
For, lo! when the sensitive paper is laid in the sun under the negative glass, every dark spot on the glass arrests a sunbeam, and so the spot of the paper lying beneath remains unchanged; but every light space of the negative lets the sunlight through, and the sensitive paper beneath confesses its weakness, and betrays it by growing dark just in proportion to the glare that strikes upon it. So, too, we have only to turn the glass before laying it on the paper, and we bring all the natural relations of the object delineated back again,—its right to the right of the picture, its left to the picture's left.
On examining the glass negative by transmitted light with a power of a hundred diameters, we observe minute granules, whether crystalline or not we cannot say, very similar to those described in the account of the daguerreotype. But now their effect is reversed. Being opaque, they darken the glass wherever they are accumulated, just as the snow darkens our skylights. Where these particles are drifted, therefore, we have our shadows, and where they are thinly scattered, our lights. On examining the paper photographs, we have found no distinct granules, but diffused stains of deeper or lighter shades.
Such is the sun-picture, in the form in which we now most commonly meet it,—for the daguerreotype, perfect and cheap as it is, and admirably adapted for miniatures, has almost disappeared from the field of landscape, still life, architecture, and genre painting, to make room for the photograph. Mr. Whipple tells us that even now he takes a much greater number of miniature portraits on metal than on paper; and yet, except occasionally a statue, it is rare to see anything besides a portrait shown in a daguerreotype. But the greatest number of sun-pictures we see are the photographs which are intended to be looked at with the aid of the instrument we are next to describe, and to the stimulus of which the recent vast extension of photographic copies of Nature and Art is mainly owing.
3. THE STEREOSCOPE.—This instrument was invented by Professor Wheatstone, and first described by him in 1838. It was only a year after this that M. Daguerre made known his discovery in Paris; and almost at the same time Mr. Fox Talbot sent his communication to the Royal Society, giving an account of his method of obtaining pictures on paper by the action of light. Iodine was discovered in 1811, bromine in 1826, chloroform in 1831, gun-cotton, from which collodion is made, in 1846, the electro-plating process about the same time with photography; "all things, great and small, working together to produce what seemed at first as delightful, but as fabulous, as Aladdin's ring, which is now as little suggestive of surprise as our daily bread."
A stereoscope is an instrument which makes surfaces look solid. All pictures in which perspective and light and shade are properly managed, have more or less of the effect of solidity; but by this instrument that effect is so heightened as to produce an appearance of reality which cheats the senses with its seeming truth.
There is good reason to believe that the appreciation of solidity by the eye is purely a matter of education. The famous case of a young man who underwent the operation of couching for cataract, related by Cheselden, and a similar one reported in the Appendix to Müller's Physiology, go to prove that everything is seen only as a superficial extension, until the other senses have taught the eye to recognize depth, or the third dimension, which gives solidity, by converging outlines, distribution of light and shade, change of size, and of the texture of surfaces. Cheselden's patient thought "all objects whatever touched his eyes, as what he felt did his skin." The patient whose case is reported by Müller could not tell the form of a cube held obliquely before his eye from that of a flat piece of pasteboard presenting the same outline. Each of these patients saw only with one eye,—the other being destroyed, in one case, and not restored to sight until long after the first, in the other case. In two months' time Cheselden's patient had learned to know solids; in fact, he argued so logically from light and shade and perspective that he felt of pictures, expecting to find reliefs and depressions, and was surprised to discover that they were flat surfaces. If these patients had suddenly recovered the sight of both eyes, they would probably have learned to recognize solids more easily and speedily.
We can commonly tell whether an object is solid, readily enough with one eye, but still better with two eyes, and sometimes only by using both. If we look at a square piece of ivory with one eye alone, we cannot tell whether it is a scale of veneer, or the side of a cube, or the base of a pyramid, or the end of a prism. But if we now open the other eye, we shall see one or more of its sides, if it have any, and then know it to be a solid, and what kind of a solid.