BINOCULAR VISION. 



351 



inches from the face and the eyes are focused on the point of inter- 

 section of any two of the lines, a third line will be seen perpen- 



\ 



Fig. 154. Zollner's lines. 



dicular to the plane of the other two, and projecting vertically 

 from the surface of the page. A row of these vertical lines will 

 be seen, if the distance is properly chosen. As one bends the 



Fig. 155. Optical illusion in projection. (Franklin.") 



head from side to side the lines sway in the same direction. It 

 forms a very striking instance of the fact that we may see most 

 distinctly a thing that has no real existence, a case, therefore, 



