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      4. “An Instrument of Use to take the Draught, or Picture of any Thing,” in Robert Hooke, Philosophical Experiments and Observations (London, 1726). Courtesy University of Michigan Special Collections.

      So, how did this container-like space get there? That is, how did Locke’s image of the mind’s dark room arrive in his mind, if the mind contains nothing but sensory materials and its own reflections? One possibility is that Locke is merely describing the mind’s native structure, the facilities that make it possible to accept and to sort ideas in the first place. This would be to understand the “dark room” as something innate to the mind itself—even, perhaps, an innate idea, something lurking there even before the mind thinks it. This, however, poses problems; the Essay was after all begun in an effort to dispel the doctrine of innate ideas. A less radical but related possibility would be to accept the “dark room” as a mere metaphor describing powers already existing; the dark room would in this sense be one metaphor among others, used instrumentally to describe a set of affordances the mind would have had anyway. But Locke himself has already described the understanding as an emergent property, growing stronger and more evident as the mind becomes stocked with ideas (see Exhibit 1). That is, it remains to be seen how a mind could include some things while excluding others, much less how it could make the distinction between “in” and “out,” without the more categorical ideas of rooms, closets, and containers of all sorts. In light of Locke’s imperative to trace ideas back to their originals, his insistence that it would help us correct our understanding if we chased our conceptual vocabulary back to its haptic grounding, we should perhaps rather entertain a third possibility. Perhaps Locke’s whole system, the room that seems to evade the system of categorical distinctions that it enforces, is itself a function of its own emergent logic. Perhaps the dark room is not a mere metaphor among others but something more intimately felt than this, a structuring condition and space to be dwelled in. Locke after all did not invent the idea of the dark room; he witnessed it at work, or, in other words, experienced it. For this dark room, even as a private image of the mind to itself, had a very public circulation, at once as an optical gadget, and as a space where the intellect might be put on display.

      Had we timed it right, we might have caught Locke, possibly already carrying an imperfectly figured room in his head, seated at a public assembly, setting eyes on a technical device that was making the rounds of the Royal Society.⁵ This device was the camera obscura, and it would come to offer the organizing center of Locke’s epistemology, the very place where a working relationship with the environment is hardened up into a stable structure.⁶ The device is simple; it is essentially a pinhole camera, and can be produced in any conveniently sized, sufficiently dark room—the meaning, after all, of “camera obscura.” Light passing through a tiny hole or conveniently sized lens is projected, dimly, on the wall opposite, where a strikingly clear but inverted epitome of the world is made to appear.⁷ Over the course of the late seventeenth century, camera obscuras appeared in a wide variety of models and formats; to the pinhole was added a lens, and to the dark room a mirror and later a semitransparent screen of oiled paper. By the turn of the eighteenth century, the camera obscura had become a familiar sight; in the year of Locke’s death alone, encyclopedist John Harris offered a detailed account of the device for popular use, a “Mr. Marshall” began selling a version of the camera obscura in his shop on Ludgate Hill, and Newton’s Opticks demonstrated its employment in optical experiments.⁸

      The device had its practical applications. When Robert Hooke presented a design for a portable camera obscura to the Royal Society, it was exactly with an eye to its “use to take the draught of a picture of anything.”⁹ This was not his first; he had previously pitched two such devices, first in 1668, and again in 1680. But his last was his most ambitious. Hooke was trained as a draughtsman, having among other things served as apprentice in the studio of the painter Peter Lely; as his sketchbooks and illustrations attest, he was himself brilliantly accomplished in capturing the outlines of things (see Exhibit 10). The goal of the portable object, as Hooke delivered it, was to unfold the technique of drafting into its components; it was to take the art of design out of the head and put it in a tool. The purpose of Hooke’s “picture box” was explicitly to make possible the accurate depiction of objects in the field by people untrained in the art of drawing, rendering one stage or aspect of the brokerage of images an automatic process.

      But the camera obscura had philosophical attractions, too. First, it models the work of the eye, which condenses a world of light into a picture on the retina.¹⁰ Several things had to happen for this to be possible. For one, people had to begin thinking of the eye as a lens combined with a receptive surface—cornea and retina. Prior to early seventeenth-century thinkers, including Johannes Kepler and René Descartes, theory was likely to think of the eye as a mere container for an absorptive “vitreous humour”;¹¹ but after Kepler and Descartes, the new science was most apt to think of the eye as an instrument, working by focusing rays of light on its back surface.¹² From this shift was developed an epitome theory of vision, that is, the eye as a machine for capturing compact but precise pictures of things; Hooke, for instance, describes the interior globe of the eye as a “microcosm, or a little World,” perfectly answering, point for point, the visual field it confronts. This is quite explicit. “When a Hemisphere of the Heavens is open to its view,” Hooke concludes, the eye “has a Hemisphere within it self.”¹³ For every point in the field of view, there is a corresponding point in the back of the eye. Locke agreed with Hooke that the critical juncture between mind and world was “far from being a point”; rays of light “strike … on distinct parts of the retina,” where they “paint” a “figure”—the bigness of which Locke is at some trouble to estimate.¹⁴ These are theories of vision made possible, as Svetlana Alpers and others have noted, by experiments conducted with the camera obscura. Locke’s theory of optics, like Hooke’s, depended upon practical modeling; in this way, experiments with scopic gadgets affected abstract theories of vision.¹⁵

      But this is only the first move the camera obscura makes possible, and Hooke’s design, which manages to slice the camera neatly in half while leaving its operator (Hooke himself?) neatly intact, additionally implies the strange doubling that must occur for the object to make epistemological sense. By isolating images from objects, the camera obscura offered a model for the mind separate from matter, in which a metaphorical eye—the “eye of the understanding”—presides over sensory images.¹⁶ Locke is quite clear about the similarities between eye and mind; “impressions made on the retina by rays of light” produce isomorphic “ideas in the mind.”¹⁷ It is from the senses, Locke therefore insists, that the “white paper” of the mind derives its “vast store,” and all the “endless variety” of ideas that have been “painted on it.”¹⁸ Images from the eye fall on a surface in the mind, upon which the judgment or understanding

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