94 PSYCHOLOGY. 



was two or three feet, and lie seemed against the wall of 

 the room.* Of these vacillations we shall have to speak 

 again in the ensuing chapter.f 



Mrs. C. L. Franklin has recently described and explained 

 with rare acuteness an illusion of which the most curious 

 thing is that it was never noticed before. Take a single 

 pair of crossed lines (Fig. 49), hold them in a horizontal plane 

 1,2, before the eyes, and look along them, at such a 

 distance that with the right eye shut, 1, and with 

 the left eye shut, 2, looks like the projection of a 

 vertical line. Look steadily now at the point of 

 intersection of the lines with both eyes open, and 

 vou will see a third line sticking up like a pin 

 through the paper at right angles to the plane of the 

 Fig. 49. ^wo first lines. The explanation of this illusion is 

 very simple, but so circumstantial that I must refer for it to 

 Mrs. Franklin's own account.:}; Suffice it that images of the 

 two lines fall on ' corresponding ' rows of retinal points, 

 and that the illusory vertical line is the only object capable 

 'of throwing such images. A variation of the experiment 

 is this : 



" In Fig. 50 the lines are all drawn so as to pass through a common 

 point. With a little trouble one eye can be put into the position of this 

 point — it is only necessary that the paper be held so that, with one eye 

 shut, the other eye sees all the lines leaning neither to the right nor to 

 the left. After a moment one can fancy the lines to be vertical staffs 

 standing out of the plane of the paper. . . . This illusion [says Mrs. 

 Franklin] I take to be of purely mental origin. When a line lies any- 

 where in a plane passing through the apparent vertical meridian of one 

 eye, and is looked at with that eye only. ... we have no very good 

 means of knowing how it is directed m that plane. . . . Now of the 

 lines in nature which lie anywhere within such a plane, by far the 



* Physiol. Optik, p. 602. 



f It seems likely that the strains iu the recti muscles have something to 

 do with the vacillating judgment iu these atropin cases. The internal recti 

 contract whenever we accommodate. They squint and produce double 

 vision when the innervation for accommodation is excessive. To see 

 singly, when straining the atropinized accommodation, the contraction of 

 our internal recti must be neutralized by a correspondingly excessive con- 

 traction of the external recti. But this is a sign of the object's recession, eta 



X American Journal of Psychology, i. 101 ff. 



