Practical Optics and Polemical Purposes in Seventeenth-Century England James Everest UCL I, James Everest, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. 1 In memoriam Lisa Jardine 2 Contents Prefatory material 4 Introduction 7 Francis Bacon Dissolves Iron in Acid in the Dark 31 Thomas Hobbes Fires a Gun into a Lake 72 Robert Hooke Goes Back to Basics (and Draws the Same Conclusion) 115 Isaac Newton Waits for Robert Hooke to Die 163 Conclusion 208 Bibliography 213 3 Abstract What follows is a study of the prevalence and value of practical work in seventeenth-century English optics. I argue, firstly, that practical work – involving instruments and experiments – was a major aspect of the discipline at this time and, secondly, that a major purpose of this work was what I call ‘polemical’ in character. The first claim is directed at histories of seventeenth-century optics, which have tended to focus on the development of theories about light and vision, at the expense of the practical work that was such a prominent feature of the field. The second claim is directed at works on the ‘rhetoric of science’, which have tended to focus on a scientist’s deployment of various means, such as practical work, in a bid to persuade an audience that he or she is right about an aspect of the natural world, whereas my take on seventeenth-century natural philosophy is that practical work could serve another important function: it could act to persuade an audience that the ideas of a rival were wrong. The four chapters take the form of case-studies, examining in detail the practical work of Francis Bacon, Thomas Hobbes, Robert Hooke and Isaac Newton, each man the leading English optical philosopher of his generation. Each chapter supports the first claim of the thesis, by emphasising its subject’s practical work in optics; each supports the second, by highlighting the polemical purposes to which that practical work was turned. In addition, each chapter identifies the audiences before whom practical work was deployed, with the result that the four chapters can be read as a narrative, one that charts the expanding audiences for practical work in the seventeenth century. A prominent take on the increasingly public character of natural philosophy in this period stresses the quest for assent and the desire to manage dissent. The story told here, by contrast, emphasises the ongoing presence of a disorderly spirit of dispute. 4 Acknowledgements My thanks are due, firstly, to my supervisors, Lisa Jardine and Angus Gowland. I began the thesis at Queen Mary, University of London, and ended it at UCL, and have benefited from the intellectual atmosphere at both institutions. Meanwhile, staff at the Royal Society library provided a hospitable working environment for much of the research. Parts of what follows were read in draft form by Felicity Henderson, James Lancaster, Noah Moxham, Richard Serjeantson, Erin Webster and Clare Whitehead; I am grateful for their advice and encouragement. Waseem Yaqoob and Susan Everest went through the whole thing and provided valuable suggestions. Final and special thanks belong with my family: it is no hyperbole to say that without them the thesis would not have been completed. 5 Notes on Dates In seventeenth-century England, the new year officially began on 25 March, so anything dated, for example, ‘2 January 1663’ belongs to what we would call 1664. In addition, the calendar was 10 days behind the one used today, so ‘2 January’ in fact corresponds to what we would think of as 12 January. In what follows, I have silently updated the year where appropriate, but have maintained the original date of the month. Items that crossed national borders, such as letters, pose a particular problem, as Catholic Europe had already adopted the calendar that is now employed worldwide. I have followed the contemporary practice of providing both old- and new-style dating, e.g. ‘2/12 January’. Notes on Transcriptions When quoting from manuscript sources, I have preserved as many features of early modern scribal activity as possible, including contractions, superscript and ligatures. For the sake of clarity, however, I have dropped words or passages that are struck through in the original (except where they prove revelatory); similarly, I have not identified text that is a marginal or intralinear insertion as such. In both cases, my intervention is mentioned in the notes. 6 Introduction 1. Practical Optics In 1772, the dissenting clergyman and practising natural philosopher Joseph Priestley published a book on optics, The History and Present State of Discoveries Relating to Vision, Light, and Colours, a sequel to the hugely successful History and Present State of Electricity, his first foray into the field now thought of as the ‘history of science’. In the opening lines of the earlier work, he outlines his intentions: “I flatter myself that I shall give pleasure [...] and I hope the work will be of some advantage to the science [of electricity] itself”1. What kind of pleasure does he have in mind? It is, he claims, not unlike “that of the sublime [...] For an object in which we see a perpetual progress and improvement is, as it were, continually rising in its magnitude; and moreover, when we see an actual increase, in a long period of time past, we cannot help forming an idea of an unlimited increase in futurity; which is a prospect really boundless, and sublime”2. What kind of advantage, meanwhile, does he think will accrue from such a sublime prospect? “Great conquerors”, he writes, “have been both animated and also, in a great measure, formed by reading the exploits of former conquerors. Why may not the same effect be expected from the history of philosophy to philosophers?”3 When it comes to the history of natural philosophy, he concludes, “an intimate knowledge of what has been done before us cannot but greatly facilitate our future progress, if it be not absolutely necessary to it”4. 1 Priestley, History of Electricity, i. 2 Ibid., ii. 3 Ibid., v. 4 Ibid., vi. 7 Priestley is, of course, conscious that a history of all the various branches of natural philosophy would be an immense project, “perhaps more than any one man ought to undertake”. Here, accordingly, he limits himself to “that branch which has been my own favourite amusement”, the study of electricity, with the hope that his book will encourage “other persons to do the like for theirs”5. His choice of subject-matter is, however, unquestionably exciting: “Few branches of Natural Philosophy [...] can boast such a number of discoveries, disposed in so fine a series, all comprised in so short a space of time, and all so recent, the principal actors in the scene still living”6. The literate public seems to have agreed: the book was a commercial and critical triumph. Five years later, Priestley was back with his sequel, The History and Present State of Discoveries Relating to Vision, Light, and Colours. As we have seen, he had previously considered the study of all the branches of natural philosophy to be beyond the scope of any one man’s labour, but success in his first venture seems to have prompted a change of heart: “like the fox with respect to the lion, a nearer view has familiarized it to me, and I now look upon it not only without dread, but with a great deal of pleasure; considering it not only as a very practicable business, but even as an agreeable amusement”7. The new book is devoted to optics, a field of activity that, Priestley suggests, offers “a number of the finest gradations in the discoveries of different persons, a view of the greatest and happiest exertions of human genius, and the labours of those who are the most celebrated for their philosophical pursuits”8. Unfortunately, his readers were less enthused a second time around: although 5 Ibid., vii. 6 Ibid., viii (mispaginated vii). 7 Priestley, History of Light, Vision and Colours, iii. 8 Ibid., x. 8 favourably reviewed, the work was a commercial flop and his project for histories of the remaining branches of natural philosophy died with it9. A disaster for Priestley, The History and Present State of Discoveries Relating to Vision, Light, and Colours is a valuable source for historians seeking an insight into the priorities of early modern natural philosophers undertaking work in optics10. Perhaps as a consequence of Priestley’s conception of history as an aid to future progress, the book is overwhelmingly concerned with the recent past. It is divided into six ‘periods’: the first a brief account of developments up to ‘the revival of letters in Europe’; the second a description of work dating to the sixteenth and early seventeenth centuries; the third a survey of the discoveries of René Descartes and his contemporaries; the fourth an overview of later seventeenth-century activity; the fifth a study of the work of Isaac Newton; and the sixth an account of developments since. He concludes by sketching some directions that future research could take. The obvious (because most recent) point of comparison is A. Mark Smith’s From Sight to Light, a history of optics from antiquity to the seventeenth century, published in 2014. Unlike Priestley, Smith devotes equal weight to each of the periods in his narrative, constructing a tale that moves from Greek and Roman science, to developments in the Arabic world, to the medieval Latin West and the Renaissance. Like Priestley, however, Smith writes with one eye turned to the present, not with the goal of furthering the progress of optics, but with that of explaining the modern manifestation of the discipline.