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seen, and the same picture exhibited as a panorama or a diorama, in which no light reaches the eyes but that which radiates from the painting itself, the reflexion from the varnish being removed by oblique or lateral illumination.
The great value of transparent binocular slides, when the picture is to be upon glass, is obvious from the preceding considerations. The illumination is uniform and excellent, but care must be taken to have the ground glass in front of the picture, or the paper, when it is used, of a very fine grain, so that it may throw no black specks upon the sky or the lights of the picture. Another advantage of the transparent slides is, that the pictures are better protected from injury than those upon paper.
It is obvious from these considerations that the size of the pictures is determined, as well as the distance at which they are to be viewed. Much ignorance prevails upon this subject, both among practical photographers and optical writers. Large binocular pictures have been spoken of as desirable productions, and it has been asserted, and claimed too, as a valuable property of the reflecting stereoscope, that it allows us to use larger pictures than other instruments. There never was a greater mistake. If we take a large picture for the stereoscope we must place it at a great distance from the eye, and consequently use a large stereoscope. A small picture, seen distinctly near the eye, is the very same thing as a large picture seen at a greater distance. The size of a picture, speaking optically and correctly, is measured by the angle which it subtends at the eye, that is its apparent magnitude. A portrait three inches high, for example, and placed in the lenticular stereoscope five inches from the eye, has the same apparent size as a Kit
Cat portrait in oil the size of life, three feet high, seen at the distance of five feet, the distance at which it is commonly examined; and if we increase the magnifying power so as to see the three-inch picture at the distance of two inches, it will have the same apparent size as the three feet oil portrait seen at the distance of two feet. If the pictures used in the stereoscope were imperfect pictures that would not bear being magnified, it would be improper to use them; but the Daguerreotypes, and the transparent pictures, which are taken by the first artists, for the lenticular stereoscope, will bear a magnifying power ten times greater than that which is applied to them.
If we take a large picture for the stereoscope, we are compelled by pictorial truth to place it at a distance from the eye equal to the equivalent focal distance of the camera. Every picture in every camera has the same apparent magnitude as the object which it represents; whether it be a human figure, or the most distant landscape; and if we desire to see it in its true relief in the stereoscope, we must place it at a distance from the eye equal to the focal length of the lens, whether it be an inch or a foot high. There is, therefore, nothing gained by using large pictures. is, on the contrary, much inconvenience in their use. are in themselves less portable, and require a larger stereoscope; and we believe, no person whatever, who is acquainted with the perfection and beauty of the binocular slides in universal use, would either incur the expense, or take the trouble of using pictures of a larger size.
In the beautiful combination of lenticular stereoscopes, which was exhibited by Mr. Claudet, Mr. Williams, and others, in the Paris Exhibition, and into which six or eight
persons were looking at the same time, binocular pictures of a larger size could not have been conveniently used.
But, independently of these reasons, the question of large pictures has been practically settled. No such pictures are taken by the Daguerreotypists or Talbotypists, who are now enriching art with the choicest views of the antiquities, and modern buildings, and picturesque scenery of every part of the world; and even if they could be obtained, there are no instruments fitted for their exhibition. magnificent collection of stereoscopic pictures, amounting to above a thousand, advertised by the London Stereoscopic Company, there are no fewer than sixty taken in Rome, and representing, better than a traveller could see them there, the ancient and modern buildings of that renowned city. Were these sixty views placed on the sides of a revolving polygon, with a stereoscope before each of its faces, a score of persons might, in the course of an hour, see more of Rome, and see it better, than if they had visited it in person. At all events, those who are neither able nor willing to bear the expense, and undergo the toil of personal travel, would, in such a panorama,- -an analytical view of Rome, -acquire as perfect a knowledge of its localities, ancient and modern, as the ordinary traveller. In the same manner, we might study the other metropolitan cities of the world, and travel from them to its river and mountain scenery,―admiring its noble castles in our descent of the Rhine, its grand and wild scenery on the banks of the Mississippi, or the Orinoco,-the mountain gorges, the glaciers, and the peaks of the Alps and the Ural,—and the more sublime grandeur which reigns among the solitudes of the Himalaya and the Andes.
The following general rule for taking and combining binocular pictures is the demonstrable result of the principles explained in this chapter :
Supposing that the camera obscura employed to take binocular portraits, landscapes, &c., gives perfect representations of them, the relief picture in the stereoscope, produced by their superposition and binocular union, will not be correct and truthful, unless the dissimilar pictures are placed in the stereoscope at a distance from the eyes, equal to the focal distance, real or equivalent, of the object-glass or object-glasses of the camera, and, whatever be the size of the pictures, they will appear, when they are so placed, of the same apparent magnitude, and in the same relief, as when they were seen from the object-glass of the camera by the photographer himself.
APPLICATION OF THE STEREOSCOPE TO PAINTING.
HAVING explained the only true method of taking binocular portraits which will appear in correct relief when placed in the stereoscope, we shall proceed in this chapter to point out the application of the stereoscope to the art of painting in all its branches. In doing this we must not forget how much the stereoscope owes to photography, and how much the arts of design might reasonably expect from the solar pencil, when rightly guided, even if the stereoscope had never been invented.
When the processes of the Daguerreotype and Talbotype, the sister arts of Photography, were first given to the world, it was the expectation of some, and the dread of others, that the excellence and correctness of their delineations would cast into the shade the less truthful representations of the portrait and the landscape painter. An invention which supersedes animal power, or even the professional labour of man, might have been justly hailed as a social blessing, but an art which should supersede the efforts of genius, and interfere with the exercise of those creative powers which represent to us what is beautiful and sublime in nature, would, if such a thing were possible, be a social evil.
The arts of painting, sculpture, and architecture have in