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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.