LXXIII.] PERIMETRY, IRRADIATION, ETC. 347 



they appear to run into each other and to be joined together by a 

 white bridge. 



(c.) Look at fig. 266, placed at such a distance that the accommodation ia 

 imperfect. The white stripe, which is of equal breadth throughout, appears 

 wedge-shaped, being wider below between the broad black patches, and 

 narrower above. To me also the narrow black patches appear to be broader 

 above and narrower below. 



{d.) Gum on to a sheet of white paper two strips of black paper 5 mm. wide, 

 and parallel to each other, leaving a white interspace of 8 mm. between them. 

 Look at the object, and, especially if it be not sharply focussed, the smaller 

 black strips will appear broader tlian the white one. 



5. Imperfect Visual Judgments. 



(a.) ]\Iake three round black dots, A, B, C, of the same size, in 

 the same line, and let A and C be equidistant from B. Between 

 A and B make several more dots of the same size. A and B will 

 then appear to be farther apart than B and C. 



(h.) Make on a white card two squares of equal size, omitting 

 the outlines. Across the one draw horizontal lines at equal dis- 



SSSSSSSS 88888888 



Fia. 267. 



tances, and in the other make similar vertical lines. Hold them at 

 some distance. The one with horizontal lines appears higher than 

 it really is, while the one with vertical lines appears broader, i.e., 

 both appear oblong. 



(c.) Look at the row of letters (S) and figures (8). To some 

 the upper halves of the letters and figures may appear to be the 

 same size as the lower halves, to others the lower halves may 

 appear larger. Hold the figure upside 

 down, and observe that there is a con- 

 siderable difference between the two, 

 the lower half being considerably larger 

 (fig. 267). 



{(J.) Zollner's Lines.— ISfake two lines 

 parallel to each other. Note that one 

 can judge very accurately as to their 

 parallelism. Draw short oblique lines 

 through them. The lines now no longer 

 appear to be parallel, but seem to slope 

 inwards or outwards, according to the fiq. zes.-Zoiiner's Lines, 



direction of the oblique lines. 



(e.) Look at fig. 268 ; the long lines do not appear to be parallel, 

 although they are so. 



