204 



ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



object through two blackened tubes of paper. Cause the tubes to 

 converge slowly and observe the apparent increase in the size of the 

 object. 



X 



\ 



FIG. 116. ZOLLNER'S LINES. 



22. Optical Illusions. Examine a series of figures, such as the fol- 

 .lowing, and give reasons for the illusion : 



D . B 



FIG. 117. To ILLUSTRATE THE ILLUSION OF SUBDIVIDED SPACE. 



(a) Make two lines parallel and draw short oblique lines through 

 them. Note that the lines now slope in accordance with the direction 

 of the oblique lines (Fig. 116). 



(6) Make three black dots (A, B, and C) at equal distances from 



