Commentary Magazine

Newton's Optical Writings, by Dennis L. Sepper

The Vision Thing

Newton’s Optical Writings.
by Dennis L. Sepper.
Rutgers. 217 pp. $42.00.

Can there be more than antiquarian interest in scientific works that have long been superseded? To judge by their actions, most contemporary scientists would answer no. Few modern-day chemists have read the works of Antoine Lavoisier, the founder of their subject. Few physicists have read Isaac Newton. Masterworks of Discovery, however, a series of “guided studies of great texts in science,” answers with an emphatic yes.

Harvey Flaumenhaft, the general editor of the series, argues the importance of understanding where we are by the light of where we have been. Science feeds on questions, and in the normal course, he writes, “new questions are built from the answers that were given to the old questions”; but then

the old questions are . . . no longer asked. . . . Progress thus can lead to a kind of forgetfulness, making us less thoughtful in some ways than those whom we go beyond.

As a result, he warns, we may easily become prisoners of second-hand or ready-made opinions:

Only by actively taking part in discovery—only by engaging in rediscovery ourselves—can we avoid both blind reaction to the scientific enterprise and blind submission to it.

In Newton’s Optical Writings, one of the early volumes in the series, Dennis L. Sepper also emphasizes the pleasures of encountering first-rate work of any time or place. He compares scientific art to literary art. Great poems, he writes,

not only stand up to intensive scrutiny, they positively invite it and reveal remarkable density and texture to those who take them to heart. Great scientific works are much the same.

One might add that historical works like this one also provide the layman with a rare practical chance to understand work “on the frontier”; to see real science in action; and to follow the plot when much is in doubt and much at stake.



Sepper’s guidebook to Newton’s optics is straightforwardly organized. It contains a brief account of Newton’s life; a description of prevailing theories of optics in the 17th century and the controversies aroused by Newton’s findings; appendices with technical details and suggestions for further reading; and, interleaved with commentary, two of Newton’s works that bracket his scientific career: his letter to the Royal Society (1672) and the Opticks (1704). The letter is Newton’s first publication. Sepper calls it the first great instance of a now-familiar genre—the journal article. The Opticks is Newton’s last major scientific work.

In Newton’s day, optics was an exemplary mathematical science. One of its great achievements was the discovery of a law for refraction, which permits us to explain the workings of things like lenses in telescopes, microscopes, and the human eye. This law, which Sepper explains in detail, amounts to a way of quantifying light’s refrangibility, its tendency to bend when passing from one medium, such as water, to another, such as air.

Newton’s letter begins with an account of simple observations of refraction that pose a complex puzzle. He examines the image projected onto a screen by sunlight after it has passed through a circular hole in a window shade and then been refracted by a glass prism. Newton goes out of his way not to discuss the most striking aspect of this image—that the seemingly colorless beam of sunlight has been somehow transformed into a rainbow’s band of colors. Instead, he calls our attention to the image’s shape, an oval five times longer than it is wide.

Now, received theories of optics in the 17th century implied that the image resulting from such a procedure would be circular, because a prism would bend all the rays composing the initial, circular beam of light by the same amount. Newton’s account, proceeding like a detective story, arrives at a radical explanation. The oval is formed because sunlight is not a homogeneous substance but is composed of rays of many different kinds, each following the law of refraction but each having a different refrangibility.

This conclusion transforms optics from a geometric account of the patterns of light rays to an experimental investigation of the physical nature of light. Light becomes a substance. Along the way, Newton proposes an equally radical theory of color.

Sepper discusses not only the logic of Newton’s argument, but also the look and feel of his scientific experimentation. If we attempt to reproduce Newton’s observations, what we see comes as a surprise. The puzzling image with which Newton’s discussion begins is not the oval which he reports and which, in strict logic, his theory implies, but is actually pear-shaped. Do we say that he cooked the books or that he merely exercised his judgment?

Newton seems to have decided that the elongation of the image was a significant phenomenon requiring explanation, while the deviation from a strict oval shape could be disregarded, at least provisionally. While that decision has justified itself by its scientific fruitfulness, Newton’s idealized description of the image as an oval did not have entirely happy results. Sepper suggests that some of the scientific opposition to Newton’s optical findings can be explained by this inaccuracy, which made it impossible to reproduce the experiment precisely as described.



While Newton’s letter is an elegant miniature, his Opticks is shot in cinemascope: deepening and broadening the experimental account of optical phenomena and color, and outlining a speculative theory that would integrate these explanations within the framework he had earlier used with such brilliant success to account for gravitation and planetary motion. The conclusion of the Opticks presents a research program, an extended sequence of questions that suggest how optical phenomena could be used to study the ultimate constitution of matter. It also provides a glimpse of how Newton’s “Method of Analysis”—a process of inductive argument “from Experiment and Observation” that, he believed, freed scientists from reliance on untestable “Hypotheses”—might permit progress toward a unified account of the fundamental laws of nature.

Newton’s conclusion, presented in a famous passage that takes seriously the interrelatedness of all knowledge, is appropriately grand:

[I]f Natural Philosophy . . . by pursuing this Method, shall at length be perfected, the Bounds of Moral Philosophy will also be enlarged. For so far as we can know by Natural Philosophy what is the first Cause, what Power he has over us, and what Benefits we receive from him, so far our duty toward him, as well as that toward one another, will appear to us by the Light of Nature. . . .

With clichés ruling our own public discussions of science—from the boosterism that confuses science with the pursuit of useful technology, to self-satisfied displays of feeling good about feeling bad about pollution and the atomic bomb—Sepper’s unpretentious guidebook is timely. It invites discussion that is serious and, by summoning us back to Newton’s texts, helps us to recall something we knew, or should have known, all along. Great scientific works are of enduring interest for their passionate attention to the phenomenal world, for their daring and imagination, and, not least, for chronicling the exploits of great heroes of the life of the human mind.

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