4 8 



THE POPULAR SCIENCE MONTHLY. 



little smaller than that constructed by him. In the background of 

 each picture is the moon, the stereographic interval between them be- 

 ing 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 pict- 

 ure, 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 



Fig. 8. Modified Brewster Stereoscope. 



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 Bap- 



