I myself have proved it to be of no small use, when in bed in the dark, to recall in fancy the external details of forms previously studied, or other… - Leonardo da Vinci

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I myself have proved it to be of no small use, when in bed in the dark, to recall in fancy the external details of forms previously studied, or other noteworthy things conceived by subtle speculation; and this is certainly an admirable exercise, and useful for impressing things on the memory.