﻿Prof. J. LeConte on an Optical Illusion. 269 



which will happen not only when the radiant point is behind 

 the object, showing it as a shadow, but also when an object, itself 

 reflecting rays, is seen through a pinhole. The effect in both cases 

 is faint vision and an uninverted image on the retina, which would 

 result in both cases in inverted vision, save that in the latter 

 there has been a previous inversion which sets things right." 

 Now in this sentence there is an association, under the term* 

 single-ray delineation, of things which are entirely distinct. 

 Nothing can be more certain than that vision through pinholes, 

 in all essential respects, is precisely similar to ordinary vision. 

 It is vision by retinal image, and not vision by retinal shadow. 

 The image on the retina in this case, as in all cases of true vision, 

 is formed by cones of rays from radiant points of the object con- 

 centrated to corresponding focal points in the image — the only 

 difference being that the radiant cones in pinhole vision are very 

 small, and the light therefore feeble. The axial rays of the ra- 

 diant cones cross each other as in ordinary vision (though per- 

 haps not exactly at the same point as in ordinary vision), and 

 produce therefore an inverted image on the retina. In other 

 words, a pinhole acts much as a very small pupil. The retinal 

 impression formed by pinhole vision is therefore entirely different 

 from that in Mr. Tupper's experiment. The one is a true image, 

 the other a shadow ; the one is inverted, the other erect. 



What Mr. Tupper means by the last part of the sentence 

 quoted above I cannot tell. 



I fear I weary your patience with this long letter. Only one 

 word more. Whether the seeing of objects in their true position 

 be, in man, a primary or an acquired faculty may perhaps always 

 be a vexed question. My own belief is that it is partly the one 

 and partly the other — that a capacity is innate by virtue of which 

 the very complex knowledge involved in vision is rapidly, almost 

 immediately acquired. The actions of all animals are determined 

 partly by experience and partly by instinct. But instinct itself 

 is probably but an inherited capacity, improved by the experience 

 of thousands of generations — a sort of inherited experience. In 

 insects the wealth of inherited experience is great, and of indi- 

 vidual experience is small ; while in man the reverse is true. I 

 think no one can doubt that a newborn chick or a newborn ru- 

 minant sees objects in their true position. Is it improbable, 

 then, that in man also erect vision is partly the result of inhe- 

 rited experience, though confirmed and strengthened by indivi- 

 dual experience — the former inherited through all human gene- 

 rations, or. even perhaps (Darwinians would say) through all 

 vertebrate generations ? 



Very truly and respectfully yours, 



Joseph LeConte. 



