436 EXPERIMENTAL INVESTIGATIONS. 



tice, it is astonishing to what an extent and for how long a 

 time images may be thus reproduced. 



2nd. Reverse the experiment, looking from the bright 

 object at white paper, and a dark image of the object will be 

 seen ; when this has faded away, move between the eye and 

 the paper a dark substance, held so as to reflect as little light 

 as possible to the eye, and the image is reproduced on the 

 white paper ; or may be, in the first instance, produced, as with 

 the converse experiment. 



The explanation which occurs to me is, that the effect is 

 one of contrast between the portions of the retina which have 

 not been strongly affected and those which have. 



The white paper dulls or deadens the sensitive portion of 

 the retina for an instant, more than the part which has been 

 previously rendered non-sensitive to other impressions than 

 that which it has received by the bright light, and the black 

 supervenes as a contrast to the parts affected by the white, 

 but not to those unaffected. In the converse experiment the 

 black relieves or renders more sensitive the comparatively un- 

 affected portions of the retina, but has little or no operation 

 on the non-sensitive parts ; thus, at the moment of removing 

 the black body, the unimpressed portions of the eye are 

 affected by the white substance, but the impressed portion is 

 comparatively dead to it. 



PHOTOGRAPHIE NATURELLE. 



Cosmos ; Janvier 12, 1858. 



Aux faits si inte"ressants et si curieux enumeres par M. Millot- 

 Brule, vient se joindre une observation curieuse de M. Grove. 

 II s'agit cette fois de photographic naturelle des truites. M. 

 Grove avait pris une grande truite, et n'ayant pas de baquet 

 ou il put la deposer, il 1'avait jetee au pied d'un arbre. Quand 

 il revint une heure apres, il s'apergut qu'elle etait couverte de 

 larges taches blanches. En examinant avec grand soin, il 

 remarqua que ces taches reproduisaient 1'image de feuilles et 

 d'herbes qui avaient etc en contact avec la truite, ou placees 



