250 RECENT PROGRESS OF THE THEORY OF VISION. 



saw the picture g in the mirror b, while the left saw the picture 

 k in the mirror a. 



A. more convenient instrument, though it does not give such 

 FIG. 35. 



sharply denned effects, is the ordinary stereoscope of Bre \vster, 1 



shown in Fig. 36. Here the two pictures are placed on the 



same slide and laid in the lower part of the stereoscope, which 



FIG.. 



is divided by a partition s. Two slightly prismatic glasses with 



1 Sir David Brewstcr, Professor of Mathematics at Edinburgh, born 1781, 

 died 1868. TK. 



