Page:Popular Science Monthly Volume 21.djvu/56

48 little smaller than that constructed by him. In the background of each picture is the moon, the stereographic interval between them being two and a half inches, which is about the average distance between the pupils of a pair of eyes. Next comes a cross, and in the fore-ground is the withered branch of a tree. In the picture on the right it is seen that the branch is nearly aligned with the cross, which is projected against the sky on one side of the moon; in that on the left one limb of the cross is projected against the moon, while the branch is wholly on the right of both. If the reader will place one edge of a card on the line between the two pictures, while the other edge touches his nose and forehead, he will perceive but a single picture, in which the branch, cross, and moon are successively farther away, the two former standing out in clear relief. By a little attention, moreover, he will see two phantom-cards, one on each side of the combined picture, and between the two his Cyclopean eye is regarding the landscape before him.

At the exhibition of Wheatstone's reflecting stereoscope, and the reading of his paper before the British Association at Newcastle, in August, 1838, one of the most interested auditors present was Sir David Brewster, who remarked on its important bearing upon the



theory of single vision to which he himself had given much attention. In his subsequent investigations he devised two important instruments which, with others of less value, were described in papers published in 1849. These were the binocular camera and the lenticular stereo-scope. During the following year they were exhibited in Paris; and here it was that stereoscopy first became the delight of the people after having been confined for a dozen years to the laboratory of the physicist.

The binocular camera needs but little description. Every one is familiar with the instrument, first devised in its simplest form by