Monitor Readers Assist in Prism Reform. To err is editorially human, we show again and again with Ideas page logo. WHAT'S IN AN ICON?

THE idea was innocent enough. Our newspaper designer, trying to capture the spirit of the Monitor's Ideas page in a graphic element, decided on the image of light traveling through a refractive prism, being broken into its spectrum of colors. It was an eye-catching icon that identified the page. So far so good.

Artists, it turns out, aren't always experts on optics as well. Almost immediately after the logo was first printed on Jan. 3, sharp-eyed readers began to call and write in, pointing out that the prism represented a physical impossibility. A tapered shaft of ``white'' light penetrated to the center of the prism (see logo below) and burst forth in a rainbow of colors - except the colors were in the wrong order and bent or refracted in the wrong direction.

No problem. We reversed the triangular prism to change the angle of refraction. But that wasn't the end of it.

``The 1/9/89 logo has the spectrum reversed,'' wrote Kemp Maples of Wellesley, Mass., as did many others making the same point. Several readers attached drawings or photos to illustrate their point: One sent a diagram from Isaac Newton's ``Opticks,'' published in 1704; another provided the cover illustration from a Pink Floyd rock album.

A quick check with a physics book or two proved them right. Undaunted, we reversed the order of the color spectrum, but somehow the purple snuck in below the red. When we corrected the color swap, the entire spectrum flipped back to its original (wrong) position.

That mistake was fixed. All seemed correct, until C.M. Larsen of Ithaca, N.Y., wrote to remind us that the white light shouldn't narrow as it enters the prism, but should be depicted as a beam of uniform width ``so that there will then be an angle of refraction at the bottom of the ray, as well as at the top, where the ray enters the prism.''

Score one more for the readers. Another change made. At last, we believe the icon (at the upper left of the opposite page) is a reasonable depiction of the workings of a prism. We no longer offend nature's order or the memory of Newton, who discovered the properties of a prism.

As Mr. Larsen points out, Newton's use of a prism to discover the properties of light is actually a nice metaphor for the Ideas page:

``On the page of [Newton's] notebook describing the experiment of passing a beam of sunlight through a triangular glass prism, he wrote the conclusion that the sunlight had been broken up into its component colors - and he then crossed out that conclusion! But then he wrote the conclusion again - and again crossed it out! At last he wrote the conclusion a third time, and let it stand.''

Larsen continues, ``What was going on in Newton's mind as he recorded those notes shows a key element of scientific inquiry: the search for possible alternatives. Could it possibly be that the colors were created by the action of light on the glass, or of the glass on the light - and therefore the colors were not present in the white light to begin with? Newton wondered, and did other experiments to rule out such alternatives? possibilities, before he was satisfied. Your logo therefore seems to me to be an excellent one to express the search for significant ideas.''

And how does a prism change light from white to a spectrum that runs from red to purple?

Unlike air, the glass prism bends the different colors of light (already part of the white light) by different amounts as they pass through it. This refraction occurs most vividly in nature in the form of rainbows.