PROFESSOR AT HEIDELBERG 245 



increased in fever. It is excellent for you, and I rejoice that it 

 explains your first statement of the 6o-meter velocity.' 



The continuation of these experiments, presented to the 

 Academy on June 8, 1871, under the title 'On the Time neces- 

 sary to bring a Visual Impression to Consciousness. Results 

 of work done by Herr N. Baxt in the Heidelberg Laboratory', 

 gave a further series of results that were very interesting, and of 

 the greatest importance in optics. Since positive after-images 

 last as long as 12 seconds under favourable conditions, and 

 during this time the forms of the larger objects are still 

 recognizable in them, there will always, even with the shortest 

 duration of light-stimulus, be a certain time during which the 

 observer is able by means of the after-image to perceive a series 

 of details in the object viewed, for the observation of which the 

 direct light-stimulus could have given no time. In order to 

 ascertain the time that is necessary for recognizing a more or 

 less composite visual image, the positive after-image must be so 

 submerged in a new and powerful light impression, that it loses 

 its value for perception. 



Helmholtz had previously constructed the Tachistoscope, in 

 which the observer looks at the object through the slit of a 

 rotating disk for a very brief period, while the slit is immediately 

 replaced first by a black and then by a brightly illuminated 

 white sector, the illumination of which is designed to extinguish 

 the after-image. With the help of this apparatus he found, as 

 expressed in a definite numerical ratio, that large spatial differ- 

 ences in the field of vision, as well as large differences in 

 brightness, were perceived more quickly than small ones ; the 

 influence of different figures used as objects was also strikingly 

 evident, according as they were more or less well known, 

 simpler or more complex. In conclusion Helmholtz appended 

 another observation, which he had made much earlier. If he 

 employed a persistently bright spot in the dark field before him 

 as the fixation point he was able, without leaving this point of 

 fixation, to direct his attention upon this or that portion of the 

 dark field, even before its illumination by a spark, and then to 

 see what appeared there. 



' This fact seems to me of great importance, since it shows 

 that what we term the voluntary direction of attention is a 

 change in our nervous system, independent of the motions of 



