As a matter of fact, I believe I may say, that there is no
such thing in nature as a perfectly defined outline; since the diffraction
of the rays, in passing it, causes them to be projected upon it more or
less, according to the nature of the particular body, and the intensity of
the light. And I may remark, by the way, that I believe this circumstance
of the projection of a star upon the moon's disc at the time of an
occultation, is to be accounted for on this principle (though with all due
deference to higher authority); a phenomenon which is to this day
unexplained.
Of course every outline is rendered less defined by any motion of the eye
of the observer, however slight. Hence, perhaps, the comparative
indistinctness of outline commonly seen in pictures, compared with those in
nature; as the artist {64} would be apt to take advantage of this
circumstance, and give to his painting the same kind of effect the reality
would have to an eye wandering over it; thereby taking away the attention
from individual parts, and, as it were, forcing it to judge of the general
effect, which general effect is, perhaps, the main object in painting.
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