Page:Mind (New Series) Volume 12.djvu/299

 PHILOSOPHICAL PERIODICALS. 'JH5 shortening of the lines of the figures ; (6) a tendency to errors of the same kind ; (c) preference for some forms, to the neglect of others. We cannot here enter into the author's explanations of these laws.] E. Duerr. * Ueber das Ansteigen der Netzhauterregungen.' [The results of Exner and Kunkel are not completely concordant ; and there are obvious objections to the methods of both observers. Especially must one take account of adaptation. (1) Experiments with Dark Adaptation. Series of experiments with moving stimuli (slits in a revolving black cylinder) gave no result. The arrangement finally employed was, in brief, as follows : the normal stimulus, exposed for more than the time necessary to reach its intensive maximum, was varied by means of an episcotister ; the stimulus of comparison was varied in duration, by change of length of slit in a revolving drum. Since the brightness relations of the stimuli were known, it was only necessary to give the stimulus of comparison such a duration as should make it just equal in intensity, subjectively, to the standard : this duration gave the time required for the intensive rise of the sensation. With colourless stimuli, of the intensity employed, this time of rise was 0'266 sec. Coloured stimuli were obtained by gelatine plates and (yellow) by a 'filter'. The times of rise (although the stimuli were so dark as to be practically colourless for the observer) were : red, 0-541 sec. ; blue, O543 sec. ; yellow, 0-573 and 0'541 sec. ; green, 0-541 and 0-691 sec. (the latter value is suspicious). Control exneriments with colourless light gave 0*266 and 0-272 sec., the same value as before. (2) Light Adaptation. The standard stimulus was again regulated by an episcotister ; the stimulus of comparison (intrinsically weaker) had its duration varied by varying sections in the periphery of a rotating disc. Results : white light, 0-269, 0-253, 0-271 sec. ; weaker stimuli, 0'288 sec. ; red, 0-519, 0'535, 0'546 sec. ; green, 0-529, 0'519, 0-533 sec. ; blue, 0-523, 0'496, 0-521 sec. ; yellow. 0-534, 0'497 sec. (For green, blue and yellow the method was slightly modified.) We find, then, as before, a remarkable constancy for the different kinds of homogeneous light, and a marked difference be- tween the times of intensive rise with coloured and colourless stimuli. The coincidence of the times in dark and light adaptation must be explained on the assumption that the time of rise does not change with change in the intensity of stimulus. (3) This assumption is further justified by the results of new experiments with dark adaptation, made with the stimuli of the last series, which were much stronger than those of the first dark series. Finally, the maximal effect that a stimulus can produce, under different conditions, may be calculated from the fact that the stimulus of comparison that can appear equal to a given standard in dark adapation is 2'75 times as small as that which is judged equal to the same standard in light adaptation, if the stimulation be cut short at the time required for the arousal of the sensation maximum. The author ends with an explanation of the discrepancies between his own results and those of his two predecessors.] F. S. Wrinch. ' Ueb^r das Verhalt- niss der ebenmerklichen zu den ubermerklichen Unterschieden im Gebiet des Zeitsinns.' [Record of experiments (chiefly with times ' filled ' by a tuning-fork tone) and theoretical discussion. The principal results are as follows. (1) Work with mean gradations offers no confirmation of Weber's Law ; the relative deviation from the geometrical mean in- creases, for all observers, with increase of the ratio of the two given .time-stimuli. On the other hand, minimal change confirms Weber's Law for times between the limits 0'25 and 1'20 sees. This relation between the two methods agrees with the results of Merkel and Ament in the sphere of intensity. (2) The difference limen, for the times indi-