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> I don't think this has anything to do with Miller's
> cognitive studies, which focused on memory, not on
> perception. Rather, it has to do with the Just
> Noticeable Difference (JND), a common measure in
> psychology. The JND varies a good deal depending on
> many factors, including the intensity of the stimulus.
> The lower the stimulus, the smaller the JND. The JND
> also varies a lot depending on which senses are
> involved. Your example of gray shading on graphs is
> just such an example of the JND. You might find that
> the shades needed varied according to the pixel
> density and how much area the gray covered on the page.
I've not heard of JND, but I do know that most (if not all)
human senses have a logarithmic response, not a linear
response. That matches your statement of "The lower the
stimulus, the smaller the JND." Scales that measure
sensory response are also logarithmic, for example decibels
and density (photographic, not chemical).