50 LETTEES ON NATTJBAL MAGIC. 



could not be carried on if the memory were to 

 intrude bright representations of the past into 

 the domestic scene, or scatter them over the 

 external landscape. The two opposite impres- 

 sions, indeed, could not co-exist: the same 

 nervous fibre which is carrying from the brain to 

 the retina the figures of memory, could not at 

 the same instant be carrying back the impres- 

 sions 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 produces the extinction 

 of the other: but so rapid is the exercise of 

 mental power, that the alternate appearance and 

 disappearance of the two contending impressions 

 are 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, towering over the subjacent range, 

 is distinctly seen, but in a tone of subdued colour- 

 ing and indistinct outline. When the purpose 

 of its recall is answered, it quickly disappears, 

 and the picture of the cathedral again resumes 

 the ascendancy. 



In darkness and solitude, when external ob- 

 jects no longer interfere with the pictures of the 

 mind, they become more vivid and distinct ; and 



