Note 574 — Chapter XVI

The primary after-image itself cannot be utilized if the stimulus is too brief. Mr. Cattell found (Psychologische Studien, iii, p. 93 ff.) that the color of a light must fall upon the eye for a period varying from 0.00275 to 0.006 of a second, in order to be recognized for what it is. Letters of the alphabet and familiar words require from 0.00075 to 0.00175 sec.—truly an interval extremely short. Some letters, E for example, are harder than others. In 1871 Helmholtz and Baxt had ascertained that when an impression was immediately followed by another, the latter quenched the former and prevented it from being known to later consciousness. The first stimulus was letters of the alphabet, the second a bright white disk. "With an interval of 0.0048 sec. between the two excitations [I copy here the abstract in Ladd's Physiological Psychology, p. 480], the disk appeared as scarcely a trace of a weak shimmer; with an interval of 0.0096 sec., letters appeared in the shimmer—one or two which could be partially recognized when the interval increased to 0.0144 sec. When the interval was made 0.0192 sec. the objects were a little more clearly discerned; at 0.00336 sec. four letters could be well recognized; at 0.0432 sec., five letters; and at 0.0528 sec. all the letters could be read." (Pflüger's Archiv, iv, 325 ff.)