The sensation is of a stepping into a light and the lifting of a dull grayness.  It's the ability to focus on something, to see and think the same thing.  It takes stamina; it's an athlete's work.  And it makes appear all things as possible: whatever appears in the mind can be made real.  It's short-lived, and thus a sensation.  The eye too often stops seeing, especially if the mind takes over.  And how beautifully then the mind wanders aimlessly to an exhaustion, back to a dull grayness.