ZOLLNER'S DESIGN. 



disappear, if we fix on a point of the object in order to develop an accidental 

 image, and when we have obtained one very distinctly, which is quite pos- 

 sible with Zollner's design (fig. 121), this image will present not the slightest 

 trace of illusion. In fig. 1 1 8 the displacement of the gaze will exercise no 

 very decided influence on the strengthening of the illusion ; on the contrary, 

 it disappears when we turn our eyes on the narrow line, ad. On the other 

 hand, the fixing of the eyes causes the illusion to disappear with relative 

 facility in fig. 120, and with more difficulty in fig. 121; it will, however, 

 disappear equally in the latter design, if we fix it immovably, and instead of 

 considering it as composed of black lines on a white background, we compel 

 ourselves to picture it as white lines on a black foundation ; then the illusion 

 vanishes. But if we let our eyes wander over the illustration, the illusion 



Fig. 121. The vertical strips are parallel ; they appear convergent or divergent under the influence 



of the oblique lines. 



will return in full force. We can indeed succeed in completely destroying 

 the illusion produced by these designs by covering them with a sheet of 

 opaque paper, on which we rest the point of a pin. Looking fixedly at the 

 point, we suddenly draw away the paper, and can then judge if the gaze has 

 been fixed and steady according to the clearness of the accidental image 

 which is formed as a result of the experiment 



The light of an electric spark furnishes the surest and simplest means 

 of counteracting the influence of movements of the eyes, as during the 

 momentary duration of the spark the eye cannot execute any sensible move- 

 ment. For this experiment the present writer has made use of a wooden 

 box, A B c D (fig. 122), blackened on the inside. Two holes are made for 

 the eyes on each side of the box, / and g. The observer looks through the 

 openings, f y and in front of openings, g, the objects are placed ; these are 

 pierced through with a pin, which can be fixed by the eyes in the absence 



