by Douglas Allchin
Few scenes are quite as inspiring as a single thread of light entering a prism and radiating into the magnificent color of a spectrum. The image is a classic icon in optics: the image itself seems to demonstrate self-evidently the composition of white light from its component colors, commonly denoted as 'roygbiv'. For many, the image comes from Isaac Newton, whose work on optics--though often not as highly profiled now as his work on dynamics and gravitation--was well known among his contemporaries.
But Newton was certainly not the first to work with spectrums produced by refraction through glass. The colors from light passing through glass spheres or pieces of broken glass was widely known in the Middle Ages. In the century before Newton, books on "Natural Magick"--which documented so many of nature's fascinating phenomena--also described the colorful effects from a "three square cristall prisme" or "Fools Paradises."
Nor was Newton the first (or last!) to propose a theory of color. René Descartes, for example, had already argued that all color is apparent, not a real property of the things observed. Several historians have recently reexamined Newton's work--highlighting how the notions of color that seem simple to us were not always so "black and white."
Newton pinned his argument on one particular experiment he selected from a series of trials that he made in 1666 while still a student at Cambridge. In his 44th trial, he isolated a beam of light emerging from one prism and cast it through a second prism, noting that no further color appeared. The reason, he proposed in his 1672 publication, was that he had separated a primitive color from the rest. There is no evidence from his notebooks, however, that Newton saw this trial as crucial when he first performed it.
Robert Hooke--no stranger to optics with his own work using the microscope--replicated Newton's results. But he disagreed with his conclusion. For Hooke, the colored rays were produced by the prism. Once produced, they clearly maintained their color and specific refrangeability. Hooke noted other experiments where one could produce colors without refraction, indicating that color had other sources than white light. Hooke criticized Newton especially for basing his conclusions on a single experiment, rather than on many. Surely, our conclusions should be based on induction from a wealth of experience, he claimed, not from solitary events.
Others also found Newton's results interesting and replicated his procedure with two prisms, sometimes adding variations of their own. In Liège, a group of English Jesuits found that violet rays, when refracted, could produce some red. Later, Edmé Mariotte in the 1670s passed a violet ray through a second prism and saw red and yellow tinges. He reported the simple observation (conclusion?): colors were mutable.
In correspondence, Newton charged that the group had used bad prisms: they must be made of good quality (Venetian) glass--free of veins, bubbles and subtle color. Newton himself had used flint glass, containing lead oxide. But why should his results about "ordinary" light have depended on such extraordinary glass? Newton also suggested that the Liège group had used concave, rather than convex prisms. They acknowledged that this was true--but again, why should this have mattered?
In his letters, Newton turned from the primary "demonstration" to additional factors. In the 6th of his series of 1666 trials, he had observed that the spectrum cast by a tiny circular beam of light was oblong, not circular itself. Newton insisted that the image must be at least five times as long as it was wide for one to observe the effect properly. One needed to use prisms with large angles and high refractive power. Newton himself had switched to equilateral triangular prismatic vessls filled with water (again, with lead oxide). But why? Here, Newton was adding careful stipulations to the types of observations that he tried to present as self-evident.
Much later, in the 1720s (after Newton's 1704 Opticks), Giovanni Rizzetti, in Venice, specificed his own conditions for what constituted a good experiment with two prisms:
care is to be taken that the second prism is not too distant from the first, nor the slit, through which the light of one colour is transmitted from refraction at the first prism to the second, is too narrow.These, of course, were exactly the opposite of Newton's criteria. And the conclusions that Rizzetti reached were likewise quite different. For him, pure yellow light transformed itself, upon a second refraction, into red, green and indigo--with the yellow itself disappearing.
Newton's proposed conclusion depended very much on a certain type of prism used in a certain type of way. His results were not "universal" in the sense that they applied to all prisms under all conditions or uses. One could only "see" the primitive colors through a narrow window of experimental contingency.
For us, of course, the "right" way seems self-evident. But why do we privilege particular prisms or particular conditions of observations? If we appeal to the results from those very instruments or methods, then we use our ultimate conclusion as a foundational premise--thus arguing in a circle.
For Shaffer, Newton's prisms were far from "transparent." The ability to draw conclusions from them involved many assumptions or aspects of instrumentation that cannot be found in the observations themselves. The evidence did not speak for itself, as illustrated in the historical debates. Shaffer notes further that "the resolution of such disputes masks the process by which agreement is accomplished." When the controversy ended--and the prisms had become transparent--the vestiges of the prism's problematic status disappeared and the real science was thereby disguised.
Can we expect the observations of a prism to speak for themselves to students? Here, the history underscores the need for us to articulate the hidden layers of reasoning. What makes a good prism, and why? And how do we resolve the question, when the effects of the prism are themselves part of the problem?
An important scientific achievement of experimentation in Renaissance art was an abandonment of Aristotle's long influential notion of color. Aristotle had claimed that yellow, red, violet, green and blue fit between white and black in a 7-color scale from light to dark and that the colors were mixtures of the two extremes. As in music, simple ratios of white and black in combination yielded colors pleasing to the senses.
When Renaissance artists began to mix pigments, however (rather than rely on "pure" natural ones), they soon found that they had to separate the chromatic colors from black and white. This then prompted them to search for the basic or primary colors from which all others could be made--though they did not meet with much immediate success. Alberti, in Italy, proposed four primaries: red, green, blue and yellow. Leonardo seems never to have been entirely definitive, proposing six at one time, eight at another--and including black and white in each set. Camillo Leonardi, in a 1502 work on gems, adopted three: red, yellow and green. Guiseppe Arcimboldo, in Prague, proposed instead five: red, yellow, blue, green and brown. The set of three primaries that we now recognize eventually appeared independently from four thinkers between 1601 and 1613 (one was a professor of medicine, another a physician-naturalist; one taught mathematics; and the last, finishing medical studies, drew his knowledge from dyers; their diverse professions speak to the position of the natural sciences in the early 17th century as growing out of other fields). The early 17th-century discovery of the primary colors was marked most dramatically in a painting by Pierre Paul Rubens in 1611: Juno and Argus--a virtual celebration of red, yellow and blue. The artists' separation of the black and white scale from the chromatic scale and the identification of primary colors would both later be critical for Newton.
By the early 1600s, the context for thinking about color among natural philosophers had changed dramatically. Earlier, Medieval scholars had noted the colors of rainbows and prismatic spectrums and distinguished between the apparent colors of insubstantial phenomena and the real colors of solid bodies. In the early 1600s, the new mechanical philosophies allowed natural philosophers to think of all colors instead as affected by light, as apparent. The problem shifted to explaining not artists' pigments, but how color was a property of light.
It was in this context that Newton pursued his various investigations into color in 1666. His experiment with refracted light, highlighted above, was key in advancing his conclusion that white light could be decomposed into several colors. But Newton was also making a subtly different and also much bolder corresponding statement: that different colors of light, when combined, could create another color of light. White light was to be viewed as a compound, or mixed, color.
In announcing his results, Newton's seized on the metaphor with painters' primaries, which had just been widely introduced and adopted in England (during the early 1660s). Newton drew especially on the language of Robert Boyle, whose 1664 work "Touching Colours" discussed the mixing of colors by "painters, dyers and other artificers." Newton took Boyle's terms 'primary','simple' and'`primitive' used to describe pigments and applied them to the properties of his spectral rays of colored light.
The metaphor made Newton's concept readily understandable, but it also generated some confusion. For Newton, there were (originally) an infinite number of primaries, not the three of the painters: how, then, could they be truly primary? In addition, many of Newton's "primaries," such as green, could be produced by a combination of other colored lights--in this case, from yellow and blue. 'Primary' seemed an inppropriate term from the existing context of artists' colors. Newton, of course, was trying to denote that certain rays of light were irreducible in color, though he had cast his ideas in a way that was readily interpreted in terms of color mixing. Finally, Newton's scheme contradicted the common sense notion that a combination of colors produced black, not white. Newton, in response, appealed to the other artists' notion that the white-black scale was separate from the colored or chromatic one--that white light was bright, not merely colored. (The difference between additive and subtractive mixing of colors only emerged with Helmholtz's work in 1852.)
In responding to these various challenges, Newton did not distance himself from artists' colors. Rather, he endeavored to make the connections even stronger. Between his original lectures on color in 1671-72 and his published Opticks in 1704, for instance, he streamlined his notion of "an indefinite variety of Intermediate gradations" of color, emphasizing instead only seven spectral colors (the roygibiv "primaries"). The number seven--much closer to three--facilitated general acceptance of his theory, though it was based on a misleading comparison. At the same time, however, Newton's shift in emphasis also made it easier for others to contend (as Shaffer has noted, above) that any of his basic colors could still be divided into yet other colors. Newton further promoted the comparison between his colors and artists' colors by constructing a color-mixing circle, showing how colors (such as pigments) would combine to form other colors. Indeed, artists often adopted his scheme and adapted it into the well known color wheel over the next century.
Newton's color-mixing circle had transformed the linear spectrum into a circle. Newton may have seen colors as cyclical. He certainly saw them as musical, much as Aristotle had. At first, Newton split his spectrum into five principal colors. But the number did not fit his conception that colors, like notes of music, expressed harmonies. A spectrum of colors, like a musical scale, he imagined, must have seven steps to make a full octave. (Note, here, the converse use of the color term 'chromatic' applied to musical scales that include all their accidentals, or half-steps.) To arrive at the requisite seven "notes," then, Newton inserted orange and indigo into his initial scheme, each addition representing a narrow "half-step" appropriately spaced in the spectral "scale." The roygbiv designation so familiar today thus not only reflects an arbitrary division of the spectrum, but also one shaped by a musical notion of octaves and the diatonic scale.
Newton's Opticks nevertheless became one of the most widely read scientific works in the eighteenth century, and certainly the most influential in optics. Artists soon found Newton's scheme unworkable, though, particularly in relation to complementary colors, and thus abandoned a conception that they had helped shape. The scientific ambiguities inherent in Newton's adaptation of artists' conceptions, by contrast, persisted for well over a century and a half. When the trichromatic theory of color vision ultimately emerged in the mid- to late nineteenth century, revising Newton's color theory, art was once again at the forefront of the science of color. On this occasion, it helped inspire and guide another artistic revolution: that of the Impressionists.