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Fechner's conception of a 'psycho-physical threshold' as connected with his 'wave-scheme' is little known to English readers. I accordingly subjoin it, in his own words, abridged:—

"The psychically one is connected with a physically many: the physically many contract psychically into a one, a simple, or at least a more simple. Otherwise expressed: the psychically unified and simple are resultants of physical multiplicity; the physically manifold gives unified or simple results. . ..

"The facts which are grouped together under these expressions, and which give them their meaning, are as follows: . . . With our two hemispheres we think singly; with the identical parts of our two retinæ we see singly. . . . The simplest sensation of light or sound in us is connected with processes which, since they are started and kept up by outer oscillations, must themselves be somehow of an oscillatory nature, although we are wholly unaware of the separate phases and oscillations. . ..

"It is certain, then, that some unified or simple psychic resultants depend on physical multiplicity. But, on the other hand, it is equally certain that the multiplicities of the physical world do not always combine into a simple psychical resultant,