﻿Prof. J. LeConte on an Optical Illusion. 267 



A very simple and easy way of making the experiments, I 

 have found, is as follows. Make a pinhole (a) in one end of a 

 card-paper box (fig. 2). Through the Fig. 2. 



bottom at the other end thrust the pin 

 (p) until the head stands a little above 

 the margin of the box. Now with the 

 eye at E work through the pinhole a 

 at the sky or any strongly illuminated 

 surface. The head of the pin will be seen in the pinhole inverted 



thus (7). 



Mr. Tupper's explanation of this illusion is substantially as 

 follows. In ordinary vision, as is well known, the retinal im- 

 pression is made by conical bundles of rays from radiant points 

 of the object converged to corresponding focal points in the image; 

 while in this experiment " the rays of light pass from the radiant 

 point E (fig. 1) 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 corresponding to a section of the pin." 

 Now the impress in this case differs from that of natural vision 

 in two respects : 1st, it is produced by single rays passing by the 

 margin of the pin instead of bundles of rays converged to focal 

 points; 2ndly, the impress in this case is erect, while in natural 

 vision it is inverted. " Now it is manifest that two diametrically 

 opposed positions of the pin on the sensitive retina will produce 

 perceptions of diametrically opposite positions of the object." 

 Pictures formed on the retina in the manner of this experiment, 

 to distinguish them from ordinary retinal pictures, Mr. Tupper 

 calls "single-ray delineations." 



1. Now there is no doubt that this explanation is substantially 

 correct ; but it would have been much clearer if Mr. Tupper had 

 distinctly expressed the fact that the retinal impression in his 

 experiment is not an image, as in ordinary vision, but a shadow* 

 Mr. Tupper seems to have perceived the distinction, but has not 

 kept it clearly in his mind ; and hence some confusion in his de- 

 ductions. The extreme nearness of the pin is unfavourable for 

 making an image and therefore for vision, but very favourable 

 for making a strong shadow. The truth is we can hardly be 

 said to see the pin at all. The shadow on the retina, by a well- 

 known law, is projected outward into the field of vision and seen 

 there inverted. The phenomenon is precisely similar to musci 

 volitantes, which are also retinal shadows projected into the field 

 of view and seen inverted. What proves incontestably that the 

 retinal impress in Mr. Tupper' s experiment is a shadow and not 

 an image is, that, like shadows, it may be multiplied (as Mr. 

 Tupper himself recognizes) to any extent by multiplying, not the 

 pins, but the radiant points. In the experiment with the card- 



