From Sight to Light

The Passage from Ancient to Modern Optics

A. Mark Smith

• Publisher: University of Chicago Press
• Year: 2014
• ISBN:
  9780226174938
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

From its inception in Greek antiquity, the science of optics was aimed primarily at explaining sight and accounting for why things look as they do. By the end of the seventeenth century, however, the analytic focus of optics had shifted to light: its fundamental properties and such physical behaviors as reflection, refraction, and diffraction. This dramatic shift—which A. Mark Smith characterizes as the "Keplerian turn"—lies at the heart of this fascinating and pioneering study.       
           
Breaking from previous scholarship that sees Johannes Kepler as the culmination of a long-evolving optical tradition that traced back to Greek antiquity via the Muslim Middle Ages, Smith presents Kepler instead as marking a rupture with this tradition, arguing that his theory of retinal imaging, which was published in 1604, was instrumental in prompting the turn from sight to light. Kepler's new theory of sight, Smith reveals, thus takes on true historical significance: by treating the eye as a mere light-focusing device rather than an image-producing instrument—as traditionally understood—Kepler's account of retinal imaging helped spur the shift in analytic focus that eventually led to modern optics. 
           
A sweeping survey, From Sight to Light is poised to become the standard reference for historians of optics as well as those interested more broadly in the history of science, the history of art, and cultural and intellectual history.

Chapter 1

Introduction

First published by the University of Chicago Press in 1976 and reissued in 1981, David Lindberg’s Theories of Vision from al-Kindi to Kepler has stood to this day as the definitive study of pre-Keplerian optics and its historical evolution. The reasons are not far to seek. For one thing, that study is based on impeccable scholarship; Lindberg delved deeply and broadly into the appropriate primary and secondary sources available to him at the time. Crucial among these sources was a small group of late thirteenth-century “perspectivist” optical writers who drew heavily upon the Latin version of Ibn al-Haytham’s Kitāb al-Manāẓir (“Book of Optics”). Probably completed by 1030, this treatise was rendered from Arabic into Latin sometime around 1200 under the title De aspectibus and attributed to “Alhacen.”1

Another reason for the continuing success of Lindberg’s study is his gift for clarity and precision in both thought and articulation, a gift that is evident in the deceptively simple, linear way he framed his narrative. Picking up the threads of his analysis in Greek antiquity, Lindberg showed in admirable detail how they were unraveled, modified, and augmented over the succeeding centuries until Johannes Kepler finally managed to weave them together into a coherent whole at the turn of the seventeenth century. Moreover, the narrative itself is compellingly thesis driven; Lindberg argued clearly and forcefully that Kepler’s theory of retinal imaging in the Ad Vitellionem Paralipomena (“Supplement to Witelo”) of 1604 represented not a fundamental break with, but rather a continuation of the perspectivist optical tradition as it evolved over the later Middle Ages and Renaissance. Lindberg made no bones about it: “Kepler was the culminating figure in the perspectivist tradition”; and Lindberg’s analysis leads us systematically and inexorably to concur with this point.2

While thus emphasizing the centrality of perspectivist optics in the development of Kepler’s visual model, Lindberg also showed that the theoretical foundations of those optics are as sophisticated as they are systematic and coherent. In other words, pace T. S. Kuhn, perspectivist optics provided what amounts to a scientific paradigm during the later Middle Ages and Renaissance.3 Furthermore, Lindberg made it eminently clear that pre-Keplerian optics was not focused narrowly on the physical analysis of light and color but dealt more broadly with issues of visual perception. In order to be properly understood, then, pre-Keplerian optics had to be analyzed within this broader context because light theory and sight theory were interdependent before the seventeenth century—hence the focus on “vision” rather than “light” in the title of Lindberg’s study.4

That no one has yet offered a compelling alternative to Lindberg’s account is a testament to its coherence and persuasiveness.5 Yet over the past three decades a considerable amount of work, some of it revisionary, has been done on pre-Keplerian optics. New texts have been brought to light, Roshdi Rashed’s studies of previously unknown or little-known medieval Arabic sources being especially noteworthy in this regard.6 Old texts have been critically edited or reedited, translated, and closely analyzed. Abdelhamid I. Sabra’s work on the Arabic text of Ibn al-Haytham’s Kitāb al-Manāẓir and mine on its medieval Latin counterpart, Alhacen’s De aspectibus, serve as related examples.7 New interpretive avenues have also been opened. Katherine Tachau, for instance, has shown how deeply implicated perspectivist theory was in later medieval discussions of epistemology, and recent work on “practical” optics in the sixteenth century, particularly the study of mirrors and lenses, has shed light not only on the scientific milieu within which Kepler conducted his optical research but also on the conceptual and methodological basis of that research.8 In addition, the relationship between Renaissance art and perspectivist optics has been reexamined and, in some cases, reconfigured since Lindberg’s day.9

Although none of these developments, singly or collectively, calls for an outright rejection of Lindberg’s account, they do call for a significant revamping of it. That is what I propose to do in this study. I say “revamping” because Lindberg’s account will serve as the backbone of my own in terms of factual detail (or most of it), as well as basic lineaments. Accordingly, the cast of main characters will remain essentially the same, although the roles of some will change. Ptolemy, for example, will play a far more important part in my narrative, and Alhacen’s place in that narrative will be significantly altered by my linking him more tightly to Ptolemy and more loosely to Kepler than Lindberg did.

There will be some changes in emphasis, as well. One such change centers on Lindberg’s eschewal of psychological and epistemological issues in order that his “investigation [not] get out of hand.” To be sure, he acknowledged, such issues were “often raised within the context of visual theory,” but they “were never its central concerns.”10 Long at odds with Lindberg over this point, I see these concerns as, if not absolutely central, then certainly integral to the formation of visual theory, and thus optics in general, from antiquity right up to the time of Kepler.11 In fact, I will argue that perspectivist optics was expressly designed with these concerns in mind. Thus, whereas Lindberg’s analysis effectively stops at the back of the eye, mine will trace the entire perceptual process from eye to brain, from the lowest-level apprehension of visible radiation to the conceptual and intellectual grasp of its object sources. Another change in emphasis involves reflection and refraction. I think Lindberg gave these two phenomena far shorter shrift than they merit because I am convinced that sixteenth-century efforts to understand reflection from concave mirrors and refraction through convex lenses played a crucial, perhaps determinative role in Kepler’s model of retinal imaging. I will therefore pay closer attention than Lindberg did to how these two phenomena, especially refraction, were understood and analyzed from antiquity to the seventeenth century.

This increased emphasis on reflection and refraction requires a somewhat more extensive discussion of ray theory and its mathematical underpinnings than Lindberg offered. Consequently, I have devoted significant portions of chapters 2 and 3 to showing how the foundations of that theory were laid in classical antiquity and how the ray geometry at its heart was used to explain a spectrum of optical phenomena from size perception to image formation in variously shaped mirrors. I have, however, tried to keep the ray-theoretical analysis in those chapters to a focused minimum aimed at providing the necessary background, and no more, for the discussion of spatial perception, reflection, and refraction in later chapters. I have also simplified that analysis as much as possible by taking a relatively superficial, descriptive approach to the mathematics on which it is based so as not to get entangled in the details of proof. I have therefore done my best to make at least the gist, if not the technical details, of that analysis accessible to any patient and attentive reader, no matter how math averse.

These are just some of the more salient revisions I will be making to Lindberg’s account. But the most marked difference between my account and his resides in our respective views of Kepler’s visual model and its relationship to perspectivist theory. Lindberg stressed continuity in that relationship. I, on the other hand, will emphasize discontinuity by arguing that Kepler’s theory of retinal imaging did represent a break, a radical break, with the perspectivist tradition. This break, I will show, occurred at two levels. First, Kepler’s theory of retinal imaging put the perspectivist visual model in jeopardy by severing the perceptual and epistemological link between eye and brain the perspectivists so carefully forged. As a result, the study of light was increasingly dissociated from the study of sight in post-Keplerian optics, as it evolved over the seventeenth century. The consequent shift of analytic focus from sight to light between roughly 1600 and 1700 constitutes what I call the Keplerian turn, and it is this turn, not Kepler himself, that is the ultimate focus of this book. Second, Kepler’s analysis of convex lenses, which was central to his theory of retinal imaging, depended on theoretical and methodological concepts that were nowhere to be found in the perspectivist sources available to him. In certain key respects, in fact, he succeeded in that analysis despite rather than because of what he could have gleaned from those sources. I am not, I hasten to add, suggesting that Kepler’s model of retinal imaging was a creatio ex nihilo. There is no question that he constructed that model on perspectivist foundations, but in the process he undermined those foundations in radical and ultimately lethal ways. In short, perspectivist optics served more as a foil than as a springboard for Kepler.

Significant though they are, these differences should not mask the fundamental points of agreement between Lindberg’s account and my own. Both have as an ultimate goal to explain Kepler’s theory of retinal imaging in proper historical context, although I will follow some of the ramifications of that theory into the seventeenth century, as they eventuated in the Keplerian turn. Both take perspectivist optics as a central component of that context. Both follow similar narrative paths, tracing the evolution of pre-Keplerian optics from Greek antiquity, through the Arabic Middle Ages, into the Latin Middle Ages and Renaissance. Both acknowledge Alhacen as a pivotal figure in the transition from classical Greek to medieval optics. Both recognize that pre-Keplerian optical theory was oriented toward the analysis of light and vision, not light alone. And both take a decidedly thematic or conceptual approach. At bottom, then, my aim here is not so much to supplant as to supplement Lindberg’s account.

At this point, I should briefly explain what I intend to do in the following chapters and how I propose to do it. To start with, I make no claims to writing a comprehensive, global history of optics from antiquity to the seventeenth century.12 My approach will be considerably more focused and thematic than that because, like Lindberg’s, my narrative path leads more or less directly to and through Kepler and is thus pretty restrictively goal oriented. Hence, I will pay scant attention, if any, to various bypaths along the way that do not loop back to the main track leading toward Kepler. This is not to say that such bypaths are uninteresting or insignificant; they are simply irrelevant to my purposes. The medieval “Arabic” optical tradition serves as a prime example.13 That it, like its medieval Latin counterpart, was firmly rooted in Greek sources is beyond question, as is the fact that several key Arabic thinkers—Alhacen foremost among them—were instrumental in the development of optics in the medieval Latin West. But to concentrate on these figures alone is to ignore some truly interesting and innovative optical work carried out in the medieval Muslim world.

Take the tenth-century mathematician Ibn Sahl (d. ca. 1000) associated with the ʿAbbāsid court in Baghdad. In the course of analyzing curved mirrors and lenses in his On Burning Instruments, he provided an elegant and ingenious mathematical demonstration of the focusing property of hyperboloidal lenses based on what amounts to the sine law of refraction.14 As far as we know, Ibn Sahl’s version of that law predates the earliest European version, generally attributed to Willibrord Snel, by more than six centuries. So too, as far as we know, it would take well over six centuries before anyone in Europe undertook an equivalent analysis of hyperboloidal lenses.15 Why, then, not include Ibn Sahl as an integral part of my narrative? Because, not having been translated into a European language until very recently (by Roshdi Rashed), his On Burning Instruments remained unknown in the Latin West. There is, in short, no evidence whatever that Ibn Sahl had anything to do with the sine law arrived at by Snel. Examples like this abound of brilliant and innovative Arabic thinkers, such as Kamāl al-Dīn al-Fārisī (d. 1318), whose optical work developed along byways that never looped back to my main track and to whom I will therefore give little more than lip service.16

One unavoidable consequence of my taking this tack is that the resulting narrative will have a definite “Western” slant. That such a slant smacks of Edward Said’s “Orientalism” hardly needs saying.17 After all, as far as the development of modern optics goes (via Kepler), some of the most innovative and forward-looking Arabic thinkers play little or no part whatever in my narrative. I can thus be accused of treating the development of modern optics Eurocentrically, as a uniquely “Western” phenomenon to which the “West” can claim exclusive proprietary rights.18 One of the most prominent recent critics of such a culturally isolationist view of science is George Saliba, who poses the rhetorical question, “Whose science [is] it . . . anyway?”19 His answer, of course, is that science belongs to everyone, to every culture; any claim to ownership within a particular culture is thus hegemonic. In principle I agree with Saliba; it would be difficult for me not to, having spent over a quarter of a century closely studying Alhacen’s optics. But in historical practice I find his open stance problematic because it is based on what I view as an unwarranted absolutism according to which a given “science” or scientific concept remains temporally and culturally constant or atomic.

Let us go back to the sine law for a moment. If we take that law as factually determinate, and if we think in terms of temporal priority alone, then the sine law obviously belongs to Ibn Sahl and, by extension, the Arabic “East.” But to take the sine law in that way, as a brute scientific fact, or at least a declaration of scientific fact, is naively reductionist. As the French phrase les faits sont faits sums it up nicely, facts may be facts, but they are also constructed. Indeed, as recent sociologists of scientific knowledge would have it, they are socially constructed.20 Consequently, if we place Ibn Sahl’s and Snel’s versions of the sine law in their respective “marketplace of ideas,” they take on an entirely different cast. Whereas there appear to have been no buyers in Ibn Sahl’s marketplace, there was a brisk trade in Snel’s.21 It was therefore in the “West,” not the “East,” that the sine law became historically significant and meaningful because it was there that it became communal and therefore fruitful.

The same holds for the evolution of modern optics over the sixteenth and seventeenth centuries. It may well be that certain key ideas, laws, and concepts that contributed to that evolution were anticipated by Arabic or, for that matter, Indian, Chinese, or Mesoamerican thinkers. And it is certainly the case that there was a lively cross-cultural marketplace of commodities and ideas between the Latin “West” and Arabic “East” throughout the Middle Ages and Renaissance. The fact remains, though, that it was in Europe that those ideas, laws, and concepts were eventually assimilated, refined, channeled, and combined in such a way as to form the basis for what most of us today would characterize as modern optics. Any claim to the contrary strikes me as historically perverse. Furthermore, to contend that the evolution of modern optics over the sixteenth and seventeenth centuries happened in Europe is not to give Europe proprietary rights to that science or to accord Europe cultural exceptionalism or superiority for having developed it. I therefore strongly resist any charge of being trapped, whether wittingly o...

Table of contents

• Cover
• Title Page
• Copyright Page
• Contents
• Preface
• chapter 1 Introduction
• chapter 2 The Emergence of Optics as a Science: The Greek and Early Greco-Roman Background
• chapter 3 Ptolemy and the Flowering of Greek Optics
• chapter 4 Greco-Roman and Early Arabic Developments
• chapter 5 Alhacen and the Grand Synthesis
• chapter 6 Developments in the Medieval Latin West
• chapter 7 The Assimilation of Perspectivist Optics during the Later Middle Ages and Renaissance
• chapter 8 The Keplerian Turn and Its Technical Background
• chapter 9 The Seventeenth-Century Response
• Bibliography
• Index
• Notes