Students of the history of optics looking for a decent introduction to the field have hitherto had two main options: David C. Lindberg's Theories of Vision from Al-Kindi to Kepler (1976) and Olivier Darrigol's A History of Optics from Greek Antiquity to the Nineteenth Century (2012), which, in pursuing the story beyond Kepler, is, by necessity, sketchier on the earlier period. A. Mark Smith's book (first published in 2015, now available in paperback) seems destined to be a new ‘go-to’ resource, even if his stated intention is to supplement, rather than supplant. He describes his work as a ‘revamping’ of David Lindberg's book (p. 4): as he writes, the broad narrative – the change in the focus of optics that provides his title – remains more or less the same, although there are some significant shifts in emphasis.
The most obvious change lies in the additional attention that Smith pays to the classical roots of the tradition within which thinkers before Kepler largely operated. Lindberg's book covered the whole of ancient optics in an opening chapter on the ‘background’ to the work of the ninth-century Arab scholar al-Kindi. Smith, by contrast, splits things up into two chapters covering, first, the emergence of optics as a science and, second, the ‘flowering’ of Greek optics, with Ptolemy featuring prominently in the latter. Perhaps a more profound bit of ‘revamping’, however, comes with the focus that Smith brings to issues that we might call ‘psychological’ or even ‘epistemological’. He has argued elsewhere that the fundamental framework for approaching classical or medieval work in optics is an understanding of how thinkers in those periods approached cognition. Placing the optical work he examines here within this framework certainly leads to a richer account, even if discussions of what classical and medieval philosophers thought about psychology or epistemology do not always make for the easiest reading, even in Smith's elegant paraphrases.
Some revamping that Smith does not particularly trumpet is an approach that sits squarely in the historiographical tradition that has risen to prominence since the publication of Lindberg's book. In his introduction, he references some key texts in the shaping of the perspective that is known as ‘the social construction of scientific knowledge’. He does so in a bid to defend himself against charges of orientalism in the creation of what is an avowedly Western-oriented narrative. Taking as an example Ibn Sahl and Willibrord Snel's independent discoveries of the sine law of refraction in the tenth and seventeenth centuries, he emphasizes the importance of the contemporary ‘marketplace of ideas’:
Whereas there appear to have been no buyers in Ibn Sahl's marketplace, there was a brisk trade in Snel's. It was therefore in the ‘West’, not the ‘East’, that the sine law became historically significant and meaningful as it was there that it became communal and fruitful (p. 9).
Smith's account is unapologetically teleological, with the work of Johannes Kepler standing at the telos. That seems fair enough, although readers with a particularly historical cast of mind may wish he had done a bit more to distinguish the seventeenth-century optics present in the final chapter of the book from the ‘modern optics’ of the title and the content of ‘any modern textbook’ mentioned in the preface (p. ix). More disconcerting is the emphasis placed on Kepler's place in the narrative. The bit of revamping that Smith particularly emphasizes in his introduction is his differing conception of Kepler, whose work in his eyes is marked not by continuity with the tradition of perspectivist optics (pace Lindberg), but by ‘a radical break’ (p. 5), to the extent that the shift from sight to light is described throughout the book and particularly in the final two chapters as the ‘Keplerian turn’.
The use of this term sits somewhat awkwardly with Smith's stress elsewhere in the book on the ‘marketplace of ideas’. To be fair, he takes pains at the beginning of his final chapter to make it clear that he is not claiming that Kepler's analysis of retinal imaging was ‘directly and causally responsible’ for developments in seventeenth-century optics (p. 373); the argument here is that Kepler was, more or less, the right man in the right place at the right time. But, if that is the case, it is difficult to see what value there is in associating the turn so closely with him, to the extent that it bears his name. Could we not just call it the turn from sight to light?
At root, I suppose, this is a general point: the modern historiographical emphasis on the social construction of scientific knowledge does not sit particularly well with an account that seeks to highlight the distinctive and innovative features of an individual's thought. Perhaps A. Mark Smith will supplement his excellent book with a consideration of the issue and a robust account of why the ‘Keplerian turn’ does, in fact, deserve to be known as ‘Keplerian’.