Physiological Perspective. 311 



of visual triangulation, he was right in laying stress upon the 

 successive changes of relation between the visual lines in 

 attaining the illusion of binocular relief. In many cases this 

 is indispensable to perfect success ; but it is not a fully 

 sufficient explanation. When the stereoscopic displacement 

 on the conjugate pictures is small, the perception of relief is 

 instantaneous, as has been abundantly established by the 

 experiments of Dove, Helmholtz, Le Conte, and others, who 

 illuminated the stereograph with the electric spark. But 

 when the stereoscopic displacement is large, if a pair of 

 properly constructed diagrams be employed, the superposed 

 external images may be made apparently to spring into full 

 relief only by play of the eyes, and to flatten out into a 

 confused network of intersecting and partially coalescent 

 lines by making the gaze rigidly fixed. The perception of 

 relief is then confined to those parts where the displacement 

 is small. The fact that this residual relief is always more 

 or less perceptible, indicates that the generally accepted 

 theory of corresponding retinal points cannot be interpreted 

 mathematically. The so-called minimum vinbile is not a point, 

 but corresponds to a retinal area whose diameter is variously 

 estimated from "OOo mm. to *005 mm. It is impossible to 

 perceive separately an object whose retinal picture is smaller 

 than this. But it may be quite possible for the quality of a 

 retinal sensation to be modified while the additional im- 

 pression, apart from that which it modifies, would be imper- 

 ceptible. This indeed is quite comparable to our perception 

 of musical quality by the ear. Through the auditory nerve 

 it has always been possible to distinguish between notes of 

 the same fundamental pitch from different sources. It was 

 reserved for Hemholtz to analyze by instrumental aid what 

 had previously eluded analysis by the unaided ear, and show 

 that minute modifications upon the sensations that had been 

 regarded as identical were due to additional impressions that 

 were separately imperceptible without resonators. The sensible 

 coalescence between a fundamental and the overtones which 

 give it character, or between the sensations produced by two 

 well-trained unisonant voices, each with its attendant over- 

 tones which make them produce slightly dissimilar sound- 

 images, is in no way more remarkable than the sensible 

 coalescence of slightly dissimilar light-images, with the pro- 

 duction of new and recognizable quality. Whether the com- 

 plete analysis of such complex images will ever be made is 

 for the future to develop. 



Wheatstone therefore was correct in his theory of mental 

 fusion of retinal images, so far as this goes ; but it does not 

 2B2 



