STEREOSCOPIC PICTURES. 



not the total impracticability, of accomplishing this, with the 

 extreme precision which is indispensable, by any process of hand- 

 drawing, will be apparent ; and if the stereoscope were dependent 

 on such a process, the most remarkable effects manifested by it 

 would never have been witnessed. Fortunately, however, contem- 

 poraneously with this beautiful optical invention, another, still 

 more remarkable, was in progress of improvement. Photography 

 lent its powerful aid to the stereoscope, and supplied an easy and 

 perfectly accurate and efficient means of producing the right and 

 left monocular pictures. If two lines be imagined to be drawn 

 from the object inclined to each other at the angle which measures 

 theproposedbinocularparallax, two photographic instruments placed 

 one on each of these lines, at the proper distance from the object, 

 will produce the two desired pictures ; or the same instrument 

 would do so, placed successively in the directions of the two lines. 



The stereoscopic pictures are accordingly produced by this 

 method either upon daguerreotype plates, photographic paper, or 

 glass. On daguerreotype plates they are necessarily opaque ; on 

 glass they are transparent ; and on paper may be either opaque or 

 transparent, according to the thickness and quality of the paper. 



10. Since the greater number of stereoscopic pictures represent 

 views of objects which must be so distant from the observer as to 

 have no sensible binocular parallax, it may be asked how it is 

 that stereoscopic effects, so remarkable as those which are mani- 

 fested by such pictures, can be produced. If the stereoscopic 

 effects be the consequences of binocular parallax, and of that 

 alone, how can such effects be produced by pictures of objects, 

 which have no such parallax ? 



This brings us back to a statement made in the commence- 

 ment of this notice, that the appearance of perspective and relief 

 produced by the stereoscope is, in most cases, exaggerated, as 

 compared with that produced by an immediate view of the objects 

 themselves, and that it is consequently such as can never be 

 perceived when the objects themselves are looked at ; and that 

 hence arises the sensation of surprise that such stereoscopic effects 

 never fail to excite. 



If we desire to obtain a pair of stereoscopic pictures of any 

 object of considerable magnitude, a palace or a cathedral, for 

 example, we take a position at such a distance from it as will 

 enable us to obtain, in the camera obscura of the photographic 

 apparatus, a picture of it on a sufficiently small scale. Supposing, 

 then, two lines to be drawn from the centre of the object to the 

 place selected for the camera, making with each other an angle 

 equal to the amount of binocular parallax, which is necessary to 

 produce the stereoscopic effect of perspective and relief; let two 



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