tista Porta, as ordinarily employed by the modern photographer. It consists now of a dark chamber, into which light from the object to be pictured is converged with a combination of carefully corrected achromatic lenses upon a prepared plate whose distance can be readily adjusted. If provided with two such combinations a few inches apart (Fig. 6), so that two pictures of the same object can be simultaneously taken thus from slightly different standpoints, it becomes the instrument on whose coexistence depends the value of the stereoscope. Without it the preparation of the stereograph would be practically impossible in many cases, for a living object, and even many inanimate objects, such as clouds, may move during the interval consumed in changing the position of the single camera and taking the two pictures successively. In the absence of photography dissimilar pictures must be made with the brush or pencil; and, aside from the labor thus imposed, few artists can compete with the sunbeam where perfect accuracy in every detail is required. Without the stereoscope, on the other hand, there would be little or no raison d'être for the binocular camera. Photography can scarcely be said to have had an existence before the publication, in 1839, of the labors of Talbot and Daguerre; and until Archer discovered, in 1851, that collodion could be employed as a vehicle for silver salts, the art was incapable of very wide or successful application for stereoscopic purposes. This epoch in photography, indeed, came after Brewster's double camera had been devised. The latter was itself the timely and natural outcome of the development of this art of sun-drawing, in conjunction with Brewster's invention of a far more convenient form of stereoscope than that employed by his distinguished contemporary. Wheatstone could hardly have entertained any idea of utilizing the evanescent images in silver nitrate obtained prior to 1802 by Wedgwood and Davy, or even those secured in 1814 by the elder Niepce on bituminized plates, which, indeed, were more permanent, but still far from satisfactory. Scarcely a year elapsed after Wheatstone's invention before the first photograph ever obtained from the human face was successfully taken by the leader in photography on our own side of the Atlantic, Dr. John W. Draper; but the art was not yet enough developed, even in such
hands, to suggest the application for stereoscopic purposes which was afterward so happily made by Brewster. To this physicist, therefore, we must credit the invention of the means by which stereoscopy was made to become co-extensive with photography.
The only difficulty in viewing a stereograph, as we have seen, consists in giving the proper direction to the eyes, which, in spite of the