THE STEREOSCOPE : ITS HISTORY. 49 



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 instru- 

 ment on whose co-existence 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 im- 

 posed, few artists can compete with the sunbeam where perfect accu- 

 racy in every detail is required. Without the stereoscope, on the 

 other hand, there would be little or no raison cVttre 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 suc- 

 cessful application for stereoscopic purposes. This epoch in photog- 

 raphy, indeed, came after Brewster's double camera had been devised. 

 The latter was itself the timely and natural outcome of the develop- 

 ment of this art of sun-drawing, in conjunction with Brewster's in- 

 vention of a far more convenient form of stereoscope than that em- 

 ployed 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, in- 

 deed, 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 



Fig. 9. Arrangement of Semi-Lenses. 



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, con- 

 sists in giving the proper direction to the eyes, which, in spite of the 

 VOL. xxi. 4 



