PICTURES ON THE RETINA. 131 



impressions of luminous objects after the objects them- 

 selves are withdrawn. 



In the healthy state of the mind and body, the relative 

 intensity of these two classes of impressions on the retina 

 are nicely adjusted. The mental pictures are transient 

 and comparatively feeble, and in ordinary temperaments 

 are never capable of disturbing or effacing the direct 

 images of visible objects. The affairs of life could not be 

 carried on if the memory were to intrude bright repre- 

 sentations of the past into the domestic scene, or scatter 

 them over the external landscape. The two opposite 

 impressions, indeed, could not coexist : the same nervous 

 fibre which is carrying from the brain to the retina the 

 figures of memory, could not at the same instant be carry- 

 ing back the impressions of external objects from the 

 retina to the brain. The mind cannot perform two 

 different functions at the same instant, and the direction 

 of its attention to one of the two classes of impressions 

 necessarily produce the extinction of the other : but so 

 rapid is the exercise of mental pawer, that the alternate 

 appearance and disappearance of the two contending 

 impressions is no more recognized than the successive 

 observations of external objects during the twinkling of 

 the eyelids. If we look, for example, at the facade of 

 St. Paul's, and, without changing our position, call to 

 mind the celebrated view of Mont Blanc from Lyons, the 

 picture of the cathedral, though actually impressed upon 

 the retina, is momentarily lost sight of by the mind, 

 exactly like an object seen by indirect vision ; and during 

 the instant the recollected image of the mountain, tower- 

 ing over the subjacent range, is distinctly seen, but in a 

 tone of subdued colouring, and indistinct outline. When 

 the purpose of its recall is answered, it quickly disappears, 

 and the picture of the cathedral again resumes the ascen- 

 dency. 



