THE PERCEPTION OF SIGHT. 285 



picture g in the mirror 6, while the left saw the picture 

 k in the mirror a. 



A more convenient instrument, though it does not 



FIG. 35. 



give sucli sharply defined effects, is the ordinary stereo- 

 scope of Brewster, 1 shown in Fig. 36. Here the two 



FIG. 36. 



pictures are placed on the same slide and laid in the 



1 Sir David Brewster, Professor of Mathematics at Edinburgh, boru 

 1781, died 1868. TR. 



