ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



675 



P P, which give the single images of a stereogram and represent the 

 relative positions in space of the points X, Y. 



Fig. 160 shows the final arrangement of the single pictures i. and ii. 

 for the stereogram. The combination-points (x and x{) corresponding 

 to X lie closer together in the stereogram than those for Y (jj and y') ; 

 X is therefore presented to the observer nearer than Y. Two conditions 



Fig. 159. 

 P. 



n 



y * 



are necessary for success : first, that the points X and Y must lie within 

 the penetrating power of the objective ; secondly, the object must not 

 project so far from the object-plane that the side views exceed those 

 same limits of penetration. The " angle of inclination " is the inclina- 

 tion of the optic axis of the camera to the vertical, and it is found that 

 an inclination angle of 8° gives the best results. 



