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



conditions of the experiment, they pass in a cone from the ra- 

 diant point E, where they surround the pin, and proceed to the 

 lens, whence, after refraction, they slightly converge and end on 

 the retina in a circle of light containing a dark space correspond- 

 ing to a section of the pin. But observe here, the impress is 

 not erect, as in fig. 5 ; neither is every point in its outline de- 

 picted by the convergence of a cone of rays (as in fig. 5), but, 

 on the contrary, it is obvious that no more than one ray can go 

 to the depicting of any one point of this impress, which is con- 

 sequently the faintest that can be produced. 



Now, whilst it is manifest that two diametrically opposed posi- 

 tions of the pin on the sensitive retina will produce perceptions 

 of diametrically opposite position, the erect pin on retina fig. 5 

 giving perception of an inverted pin, and the inverted pin on 

 retina fig. 6 giving perception of an erect pin (and so explain- 

 ing the illusion), it is equally clear that in fig. 5 there will be 

 the same faint impact of rays on the retina that we find in fig. 6, 

 seeing that there is nothing to hinder a cone of rays from any 

 radiant point impinging on retina fig. 5, inasmuch as fig. 6 dif- 

 fers only from fig. 5 in point of deficiency; i. e. retina fig. 5 has 

 all the light of retina fig. 6, and a great deal more. Now the 

 experiment is here interesting because, by a special arrangement, 

 it makes this faint impact of light appreciable ; and this it does 

 by excluding the more vigorous impact of light ; i. e. it negatives 

 delineation by cones and admits of it by single rays only ; and it 

 shows once more, supposing perception to be appreciated nervous 

 differentiation, how such differentiation may exist unappreciated. 



For it may, as I shall now show, be proved that both impres- 

 sions, the stronger and the fainter, do indeed coexist upon the 

 retina, and that the exclusive appreciation of the former is owing 

 solely to its greater intensity. For by introducing the means of 

 delineation by cones of faint light, both images (the one erect, 

 the other inverted) may be made visible at once. At E, fig. 6, 

 have a hole (J inch diameter) in the card and cover it with trans- 

 parent paper. The faint light from this enlarged source will 

 depict all points of the pin by cones of rays, which cross a little 

 behind the crystalline, and consequently give a tracing of the 

 pin head upward on the retina, head downward to perception. 

 Now let a small hole be pierced in the transparent paper to do 

 the office of radiant E, i. e. to delineate by single rays ; and a 

 little nice adjustment, somewhat increasing the distance of Gr 

 from the eye, will bring this single-ray tracing of the pin head 

 downward on the retina, and head upward to perception. So 

 that we have on the retina at the same time two impressions in 

 contrary positions, and consequently perceive two pins in con- 

 trary positions, each retinal impression being respectively reversed 



