Mr. J. L. Tupper on an Optical Illusion. 427 



to the mind. To procure this result we first fix the eye upon 

 the radiant point E till we see the pin head upward, as in the 

 illusion; then steadily increasing the distance, as stated, we 

 presently get sight of the actual pin as it is, head downward, with 

 now a little light circle in its head, and a little upright pin 

 within that circle. Remove the transparent paper, and the bright 

 cones obliterate the smaller image; or cut off the cones by 

 opaque paper, still retaining the radiant point, and the larger 

 image vanishes. It is only by reducing the intensity of the 

 cone-depicted image that we make the other image appreciable. 



The deductions from these induced facts are, first, concerning 

 erect vision with an inverted image. 



However conclusive the reasoning against there being any 

 direct deliverance to the retina, or other neurine, as to the position 

 of the object, however consistent the theory that experience is the 

 sole interpreter of sensory excitement, and that there never is 

 " an inverted retinal image," inasmuch as there is no image till 

 experience has made it (made it in accordance with muscular 

 evidence, and so erect), however conclusive all this to reason, by 

 various analogies, direct experimental evidence has been wanting. 

 It is here supplied. For we have here one object before us and 

 two opposite perceptions of its position at the same moment, a 

 present voucher that there is no direct deliverance from the ob- 

 ject as to its position. If there were (as still contended by a 

 high authority) " a law " by which intuitively we interpret an 

 effect on the retina in behoof of the object's position, we should 

 here see both images in one and the true position. But we do 

 not. And what is it we do ? We fall back upon hitherto expe- 

 rience, and interpret the retinal effect accordingly. It is of no 

 avail that the pin falls on the retina in its true position; we 

 must see it as we have hitherto found it. 



A second deduction regards "the law of visible direction," by 

 which, it is said, an object is seen in the direction of a perpendi- 

 cular to the retina at the object's point of incidence; and it is 

 cited by its promulgator to prove the sense of vision, primary 

 and unacquired; therefore its operation should be absolutely 

 efficient and unindebted to experience, unconditionally indicating 

 the direction of the object. But (fig. 6) the law proves falla- 

 cious, the object not being in the direction of the perpendicular 

 AB : it is seen where it is not. That it is seen under novel and 

 unexperienced conditions cannot be urged without admitting ex- 

 perience as a prerequisite to sense of direction. 



Thirdly, there are some points of interest in regard to focus, 

 which I omit for want of space, but would observe that the pin 

 neither is, nor is presented as in focus, its outline being every- 

 where jagged with rays of light. I have shown the attempt to 



