A Manual Of Photography

CHAPTER XI. THE STEREOSCOPE. 1ROM the interest which this very interesting optical instru- ment has excited, and the very intimate relation which exists between it and photography, since it is only practical to pro- duce images suited for the instrument by the agency of the camera obscura, it is thought advisable to devote a short chapter to some notice of it. It is not intended that any examination of the phenomena of vision, or of the application of the stereoscope to the explanation of single vision with a pair of eyes, shall be attempted; these questions would be somewhat out of place in the present manual, and would occupy too large a space if pro- perly dealt with. J b 1 P’ The stereoscope is before the world: a simple description, therefore, of the forms under which it may be constructed, and a sufficient explanation of its principles, is all that can here with propriety find a place. The name is compounded from two Greek words, signifying solid, and I see, and adopted from the fact that two pictures on a plane surface, will, when adjusted in the instrument, resolve themselves into one image, and that image will acquire an apparently distinct solidity, being repre- sented as an object having three dimensions,—length, breadth, and thickness. “The theory ”—of single vision with a pair of eyes-says Mr. Wheatstone, in his valuable Memoir ‘ On some remarkable and hitherto unobserved Phenomena of Binocular Vision’_____ “ The theory which has obtained the greatest currency is that which assumes that an object is seen single because its pictures bill on corresponding points of the two retime ; that is, on points which are similarly situated with respect to the ’two centres, both in distance and position. This theory supposes that the pictures projected on the retinae are exactly similar to 6ach other, corresponding points of the two pictures falling on corresponding points of the two retinae.” Leonardo da Vinci ill his Treatise on Painting, has some remarks on the peculiarity of vision, which bear in a singular manner on the phenomena of the stereoscope, to the effect, that a painting, though con-