398 Intelligence and Miscellaneous Articles. 



it ; but this extent expressed in time is always constant ; the band 

 takes always the same time to pass in front of a point of the 

 retina ; it commences about -fa or -^ after the passage of the white 

 and lasts appreciably the same time. It is more visible the brighter 

 the illumination of the white; but when it has been seen in these 

 conditions it is found again easily under feebler illumination, in 

 which it is only less striking. But this enfeeblement of the sensa- 

 tion, more or less marked according to the intensity of the excita- 

 tion, is always observed. 



It may be said that in this experiment there is spread out in 

 space what occurs in time. The black band is in fact only a kind 

 of reaction of the retina against the excitation by light, a reaction 

 which may be made evident in a totally different way. 



I have in fact observed that if in complete darkness we produce 

 an instantaneous luminous excitation, or rather one of a duration 

 which can be neglected in comparison with the first, the sensation 

 appears doubled ; that is to say, that when once it is formed it 

 disappears, and again shows itself anew. This takes place, for 

 instance, if we pass either through a Crookes or Geissler tube, or 

 simply, but with less effect, through air, a single discharge from a 

 Euhmkorff coil. This doubling is more or less precise according 

 to various circumstances which I have mentioned, but it is readily 

 found again when it has once been observed : it is particularly 

 marked with indirect vision. I have shown that it cannot be 

 attributed to a reaction of the pupil, as I at first believed when I 

 made my observations in a room not perfectly dark ; it is in fact 

 a retinal phenomenon. 



There is then in this experiment, as in the first, a negative 

 reaction under the influence of the excitation ; the difference is 

 that in the first case the excitation lasts when this reaction is seen, 

 while here the excitation has come to an end, and only an obscu- 

 ration of its persistent or consecutive image is seen. 



Is this reaction, this negative excitation, unique ? I do not think 

 so ; for in certain cases I have seen the dark band followed by other 

 analogous bands uniformly spaced but much less distinct. The 

 observation is moreover difficult, for a pretty great velocity of the 

 disk is required, and then the bands, as they become larger, con- 

 trast less strongly with the ground, and become less perceptible. 

 What is certain is that the first dark band is much less marked, 

 and it dominates the phenomenon. 



It would be difficult, and in any case premature, to indicate the 

 causes of tin's appearance, but it may be permitted to characterize 

 it as the result of a retinal oscillation formed under the influence 

 of the start of the luminous excitation. What confirms this inter- 

 pretation is that this oscillation travels along the retina with a 

 uniform velocity from the point where it starts; and that by 

 placing ourselves under certain experimental conditions we may, 

 thanks to it, produce true phenomena of interference in the 

 sensation. 



The most convenient way of realizing these interferences is to 

 turn a large black disk of about 0-4 metre with a velocity of about 



