Page:Mind (New Series) Volume 12.djvu/579

 PHILOSOPHICAL PERIODICALS. 565 ness is shown chiefly in warming-up and getting into swing. Attention is of great importance : the feelings (which vary as for quantity) are merely concomitant phenomena. (3) Reaction exju'rinn-nlx. For every observer there is an optimal rate for a rhythmical series of reactions. As the rate increases the time of reaction and the length and height of the curve of lift are reduced ; at a very quick rate, the curves have practically the same form for all observers. Rhythm and mastery of the work (au- tomatism) increase the regularity of the reactions. Every weight has its special rate. The amount of weight employed, however, affects simply the form of the curves of lift, not their height or length or the time of re- action. The individual reaction time is affected by rhythm only within narrow limits. (4) Writiny. Preliminary report of results (to be pub- lished in full later) as regards the influence of rate, the pressure exerted in writing, the innervation of writing movements, types of writing (masculine, feminine, childish) and their characteristics, and the general conditions of writing.] W. Wirth. ' Der Fechner-Helmholtzsche Satz liber negative Nachbilder und seine Analogien.' [(1) Observational. The perception of a uniformly coloured surface, after long fixation of a bright- ness or colour difference, shows subjective differences, which can be compensated by the withdrawal of the same fraction of the fatiguing- stimuli from those portions of the visual field which have been fatigued by them. This fraction may be considered as the value of a determinate after-image under the different conditions of reaction. The value of all negative after-images is for all qualities directly proportional to the inten- sity of the reacting stimulus (Fechner-Helmholtz law). The ratio of the intensities, in which the different colour tones react with equal absolute values to a pure brightness after-image, varies from the equality of their apparent brightness as follows : the equivalent reacting intensity is brighter inyellow than in blue, while pure red and green have an intermediate value. The law holds, for all fatigue and for all reaction colours, that the values of coloured after-images correspond approximately to their value of equivalence for brightness after-images. The fatigue colour reacts rela- tively most strongly, the complementary colour least strongly. All these statements apply, in practically the same degree, to bright and dark adaptation. The after-image may be considered as a modification of visual sensation, decreasing continuously with the cessation of fatigue influences, and persisting throughout the whole course of the process. It disappears the more quickly, on the various reacting stimuli, the greater its absolute value on the occurrence of the new stimulus. (2) Theoretical. The brightness difference of the equivalent reacting intensi- ties is explicable from the effect of the colour tone on the total mental impression of brightness, apart from the independent colourless process : this is not to be confused with the ' specific brightness ' of Hering and Hillebrand. All after-images may be explained in two ways : either as the result of simple changes of excitability in the normal substrates, or as due to the coexcitation of an independent remainder substrate, propor- tionately to the reacting intensity. The former hypothesis requires the further assumption of a diffusion of every stimulus-effect over the entire colour substrate as understood by a four-component theory (best taken as simplest case of Wundt's periodicity theory) : it affords the simplest ex- planation of brightness after-images. The latter hypothesis requires the assumption of an excitation of a secondary substrate, in its specific after- image quality, by all stimuli : as applied to brightness after-images, it requires still other auxiliary hypotheses. It can be harmonised more readily than its rival with a general theory of colour vision.] W. Wirth. ' Das Spiegeltachistoskop ' ; ' Ein neuer Apparat fur Gediichtnissversuche '