LXXIII.] 



PERIMETRY, IRRADIATION, ETC. 



349 



the eye, when by indirect vision the broad white and black areas 

 of the peripheral parts, bounded by hyperbolic curves, will appear 

 as small and the lines bounding them as straight as the smaller 

 areas in the middle zone. 



9. Convergence of the Visual Axes Influences one's Concep- 

 tions of Size and Distance. 



(a.) Place a blackened paper tube before each eye, look at a fixed 

 object, and then gradually converge the tubes ; the object appears 

 larger and nearer. 



FIG. 270. 



(6.) Look at an object through two pieces of glass (2^x2^x| in.), held 

 at first in the same plane, one in front of each eye. Let the adjoining edges of 

 the two plates of glass be moved each on a vertical axis, so that they form either 

 a more or less obtuse angle with each other. In order to see the object dis- 

 tinctly the axes of tbe eyeballs must converge to a greater or less extent, as 

 the case may be, with the result that the object appears larger or smaller, or 

 appears to approach or recede as the plates are rotated. Special forms of 

 apparatus contrived by Rollett, and another by Landois, are used for this 

 purpose. 



