Question: What is refraction?

In the 1800's Birge Harrison coined this term to describe a "most elusive but most important quality". He may have used another word or phrase for this purpose but he could no find one in English to describe it.

As he defined it, refraction is "that intimate effect of one mass of color or value upon it adjoining mass which results in the 'lost-edge', and a general diffusion of tone, thus giving to pictures their atmospheric quality."

Harrison suggests that this effect is mainly a "visual fact whose operation is due to the imperfect construction of the lens of the human eye." What he means by this is that we only see sharp edges and intense colors at the center of our focus, and that all other edges and colors must be softened. Unlike a camera, our eye blurs all that we see in our peripheral view leaving only what we are see at the center of our vision in vibrant focus.

The radius of our exact vision is so limited, that "at a distance of six feet from the eye it would hardly be possible for any human being to enumerate accurately the spots on a target four feet in diameter, while holding the gaze rigidly fixed on the bull's-eye. Beyond the radius of twelve inches from the centre the image begins to blur, and this blur increases rapidly, until out of the tail or the eye on either side we get only an indefinite consciousness of things rather than any genuine vision of things themselves."

Take a look at this painting by James McNeil Whistler in 1864 titled Caprice in Purple and Gold: The Golden Screen. Although the overall affect seem universally crisp and in focus, compare the edges and color intensity at the center of focus to that just behind the subject.

In summary Harrison says the following about refraction:

"Now any interesting picture motive generally has a focus, or centre of interest on which the artist's eye rests with especial pleasure; and in view of the visual limitations just described it is evident that this portion will appear much more definite in outline than the outlying regions of the composition; which will become more and more blurred as they recede, with the softened or lost edge everywhere. This is refraction; and as the eye sees it, so, without question, the hand should paint it."