AN OPTICAL PHENOMENON. 



' By FRANCIS E. NIPHER. 

 {Read April 31,1911.) 



In 1871 in a letter to Tyndall, Joseph LeConte gave an interest- 

 ing discussion of an ocular illusion which had been previously 

 described. Tyndall communicated it to the Philosophical Magazine 

 (XLL, p. 266). The phenomenon was observed in the manner here 

 described : 



Pierce a card with a pin. Hold it before the eye at a distance 

 of four to six inches, looking through the hole at a bright back- 

 ground. Place the pin in front of the eye with the head central 

 in the pupil and in close proximity. The pin head will be " seen 

 in the hole," and in an inverted position. 



As was pointed out by LeConte, this is not an optical image but 

 a shadow. As proof of this he cites the fact that if a series ot 

 holes are made in the card, a similar appearance of the pin head is 

 seen in each hole. He adopted the idea that objects are seen erect, 

 because the nerve fibers at the lowest point on the image see the top 

 of the object in the direction along which those rays have come. 

 He also argued that the inverted appearance of this shadow, which 

 was erect on the retina, was in harmony with this explanation. 



The well-known fact that this point in the image is the vertex 

 of a cone of rays, whose base is the pupil of the eye, and that this 

 diverging bundle of rays, when traced outward, does not define the 

 position of any external point, is sufficient explanation of the fact 

 that this line of reasoning has not been generally adopted. Evi- 

 dently the fact that there are no rays has also been taken into con- 

 sideration. It does not seem quite evident that nerve fibers at the 

 lower point of the image on which ether waves collapse and deliver 

 their impulses could " see " that these waves had their origin at a 

 definite point, at the top of an object, at a definite distance from the 

 refracting media, in w'hich the radii of curvature of these waves 

 were reversed in direction. And these waves from this point on the 

 object are involved in a summation of waves from other and adjoin- 

 ing points. 



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