Intelligence and Miscellaneous Articles. 231 



section, usually closed by a plate lined with black, which plate, 

 heavy and metallic, was retained in its position by the attraction of 

 an electromagnet, but without coming into immediate contact with 

 the latter, so that as soon as a person placed behind the box inter- 

 rupted the current animating the electromagnet the stopping-plate 

 instantly fell and disclosed the window placed before the eye which 

 was under experiment. A current supplied by a laboratory Gramme 

 machine, after passing through the electromagnet, put in action a 

 small Deprez signal, the pen of which left its trace on a regis- 

 tering cylinder with a Toucault regulator. The signal immediately 

 announced the interruption of the current, and consequently the 

 precise moment of appearance of the light. Then the subject under 

 experiment, directly after perceiving the light, restored the current 

 in the signal through a derived path, by pressing on a spring the 

 index finger of his right hand ; precisely at this moment a new sign 

 is traced upon the registering cylinder. 



The interval which had elapsed between the interruption and 

 reestablishment of the current, measured by comparison with the 

 vibrations of a Marey electric chronograph, indicated directly the 

 time which had been required for the subject to perceive and signal 

 the light. Eor shortness, I shall call that time simply the dura- 

 tion of the luminous perception. 



Here are the principal results which I have obtained in this in- 

 vestigation : — 



(1) For one and the same person, under the same conditions, 

 the duration of the perception varies from single to double without 

 any apparent regularity. But if in one and the same experiment the 

 mean of a sufficiently large number of successive determinations be 

 taken (ten for example), a duration constant during the whole time 

 of the experiment is found. I have found for myself, in direct 

 vision, a mean duration of 0-13 second with daylight. 



(2) The duration of the direct perception varies according to the 

 individuals. I have seen it vary, according to the persons, from 

 0-09 to 0-15 second. 



(3) The duration of the perception is sensibly the same for the 

 right and for the left eye when they are sound. 



(4) The duration of the luminous perception is notably increased 

 by another cerebral occupation imposed on the subject during the"* 

 experiment. Thus, when he speaks, when he listens attentively to 

 a reading or a discourse, while at the same time applying himself 

 to the experiment, he must have, for the reaction, 0-04 or 0*06 

 second more than before. 



(5) The duration of the luminous perception is always more con- 

 siderable in indirect than in direct vision ; it is more considerable 

 in proportion as the point of the retina struck by the light is more 

 distant from the centre. This cannot be due to a difference of sen- 

 sitivity, since, as I together with M. Landolt have shown, the retina 

 is everywhere nearly equally sensitive to light. 



(6) The difference between the duration of indirect and that of 

 direct vision showed itself especially considerable at the beginning 

 of our experiments. There was then between the duration of per-. 



