Academic journal article Journal of the History of Ideas

Baroque Optics and the Disappearance of the Observer: From Kepler's Optics to Descartes' Doubt

Academic journal article Journal of the History of Ideas

Baroque Optics and the Disappearance of the Observer: From Kepler's Optics to Descartes' Doubt

Article excerpt


In the seventeenth century the human observer gradually disappeared from optical treatises. It was a paradoxical process: the naturalization of the eye estranged the mind from its objects. Turned into a material optical instrument, the eye no longer furnished the observer with genuine representations of visible objects. It became a mere screen, on which rested a blurry array of light stains, accidental effects of a purely causal process. It thus befell the intellect to decipher one natural object - a flat image of no inherent epistemic value - as the vague, reversed reflection of another, wholly independent object. In reflecting on and trespassing the boundaries between natural and artificial, orderly and disorderly, this optical paradox was a Baroque intellectual phenomenon; and it was the origin of Descartes' celebrated doubt - whether we know anything at all.


The human observer began to slip out of optics when Kepler discussed the concept of artificial observations in his Paralipomena Ad Vitellionetn1:

On 1602 21/31 December at 6h in the morning, through a . . . [camera obscura] and an instrument made for this purpose . . . the moon made an image of itself brightly upon the paper lying below, inverted in situation, just as it was in the heavens, gibbous. . . . You should not think that . . . the moon's ray was in the paper, for both the gibbous face and the spot in its middle were carried over to all parts of the paper . . . indeed, it was from moving the paper that the spot was first discovered.2

The observation, Kepler stressed, was not his. It was no one's. The image of the moon was not the culmination of a cognitive process. It did not require an observer; a piece of paper was enough, and the paper could be moved around without affecting the production of the image. Image production is the main concern of Ad Vitellionetn; being "The Optical Part of Astronomy," it is about the making of observations rather than their content. Earlier in the book Kepler established the legitimacy and efficiency of his main instrument of artificiosa observationes (the term he used in one of the subtitles), the camera obscura, by demonstrating that the image obtained through it is indeed that of the observed object.3 He went on to elucidate its underlying principle - the formation of an image on a screen behind a small aperture - by way of physical simulation. He set "a book on a high place to stand as a luminous body" and "a tablet with a polygonal hole" under it. He stretched threads from the book's corners grazing the edges of the hole and found that the four images of the hole were reproduced on the floor in reverse order. When this process was repeated from (ideally) every point of the book, "a narrow row of infinite figures" similar to the hole "outline the large quadrangular [reversed] figure of the book on the pavement."4

This was a novel solution to an age-old mystery, formulated already by Pseudo-Aristotle: "why does the sun penetrating through quadrilaterals form not rectilinear shapes but circles?"5 It was novel, indeed revolutionary, because it abandoned the fundamental assumption of all previous attempts to answer the question: that the pinhole image is a unique re -presentation of the sun.6 For Kepler's perspectivist predecessors, the circular image was not caused by the sun and by light; it was the true form of the sun or the perfect dissemination proper of light; the circularity of the image was a sign of its indubitable authenticity. "The spherical shape is associated with light," John Pecham characteristically explained, "therefore, light is naturally moved toward this shape."7 Even Francesco Maurolyco, who replaced this "natural association" with a geometrical account, preserved the essential similarity between source and image: the image cast through the aperture is composed of many images, not of the hole, but of the luminous body. …

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