The retinal image and the theory of lenses (Ad Vitellionem paralipomena)
An optical book of 1604 in which vision is explained entirely as optics, and the eye is described as an instrument that can be calculated. Conceived as a supplement to Witelo's Perspectiva, its author, Johannes Kepler, shows that the pupil gathers the light radiating from every point of the object onto the crystalline lens, which refracts it and paints on the retina a real, reversed and inverted image that Kepler calls a pictura. How this picture turns into visual sensation he left to others.
The same book gives the first correct mathematics of the camera obscura, the behaviour of light in media and the rules for constructing images; with them one can also calculate the lenses of spectacles, which by then had been worn for three centuries.
Kepler completed the theory of lenses in another, later work. From there also comes the two-convex-lens design of the telescope, which is still called Keplerian.
Why this date
exact: Ad Vitellionem paralipomena, quibus astronomiae pars optica traditur, Frankfurt, 1604 (MacTutor; Smith 2022). The theory of the retinal image is set out in chapter 5. The plan as a supplement to Witelo took shape earlier: according to the DSB (Gingerich), by 1602 that task had already taken second place to the broader programme of Astronomiae pars optica. The theory of lenses and of the telescope was completed later, in the Dioptrice (Prague, 1611), written within six months of Galileo's Sidereus nuncius (1610); the axis value is 1604, the year of first publication.
What it grew from
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Book of Optics and experiment as argument
influenced · Confidence: high
Kepler wrote the book as a supplement to Witelo, responding to Risner's edition of 1572 in which Witelo's Perspectiva was printed together with Ibn al-Haytham's De aspectibus, and his theory of the retinal image grew inside that perspectivist tradition as a solution to the problem of vision which the tradition itself had posed.
Dispute: Historians differ on whether this is continuity or rupture. Lindberg (1976) sees the theory of the retinal image as the culmination of the medieval perspectivist tradition. Smith (2022) stresses the opposite: for Ibn al-Haytham and Witelo the crystalline lens itself senses the rays falling on it along the perpendicular, whereas for Kepler it refracts the light and forms a real image on the retina, so that Kepler undermined the very scheme he formally set out to supplement. The continuity holds on either reading; only its sign changes, continuation of the tradition or rupture with it.
Link sources
- Smith A. M. «Optics to the Time of Kepler», Encyclopedia of the History of Science (ETHOS) (2022) Verified 20 Sep 2026
Whatever the case, it saw its first printing in 1535 and its second in a tandem edition with Alhacen's De aspectibus published by Friedrich Risner in 1572.
- Smith A. M. (2022) Verified 20 Sep 2026
This, most likely, was the edition of Witelo's Perspectiva to which Kepler was responding in the Paralipomena of 1604.
- Smith A. M. (2022) Verified 20 Sep 2026
In doing so they brought Alhacen into the mainstream of medieval and Renaissance optical thought.
- Richards R. J., Lindberg D. C. «Theories of Vision from Al-Kindi to Kepler», Journal of the History of the Behavioral Sciences, 380 (1979), 380 Verified 20 Sep 2026
The final chapter of Lindberg's study is devoted to what he regards as the culmination of the medieval perspectivist tradition, Kepler's theory of the retinal image.
- Richards R. J. (1979), 380 Verified 20 Sep 2026
Kepler, building on the views of Witelo, Pecham, and Porta, came to appreciate the refractive use of the lens in forming an image on the retina.
- Norton S. D. «Abu 'Ali al-Hasan ibn al-Hasan ibn al-Haytham», Science and Its Times (Encyclopedia.com) (2001) Verified 5 Oct 2026, quotation assembled from fragments
the most important and influential work on optics between the time of Ptolemy...and Johannes Kepler
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
- Smith A. M. «Optics to the Time of Kepler», Encyclopedia of the History of Science (ETHOS) (2022) Verified 20 Sep 2026
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Latin translations from Arabic in Toledo and Spain
influenced · Confidence: medium
Kepler wrote his Supplement to Witelo, that is he was answering the perspectivist tradition of Bacon, Witelo and Pecham, and that tradition grew on the Latin De aspectibus, the translation of Ibn al-Haytham that appeared by no later than 1200 in the Spanish translating milieu opened up by the capture of Toledo.
Dispute: Who translated De aspectibus into Latin is unknown. Smith cautiously allows for Gerard of Cremona's involvement but puts a question mark after his name and speaks of at least two translators; no historian asserts the whole chain, from the Spanish translations to Kepler, in a single sentence: it holds through separate links, each attested on its own. The connection therefore runs not through a named translator but through the Spanish translating milieu and the Latin corpus that came out of it.
Link sources
- Smith A. M. «Optics to the Time of Kepler», Encyclopedia of the History of Science (ETHOS), Carnegie Mellon University Libraries (2022) Verified 20 Sep 2026
The opportunity came in the twelfth century as Christian powers gained control of Muslim territory in the Iberian Peninsula and Sicily. Emblematic of this power shift was the capture of Toledo in 1085 and with it the sudden acquisition of libraries containing a wealth of Arabic texts
- Smith A. M. (2022) Verified 20 Sep 2026
Consisting of Roger Bacon (d. 1292), Witelo (d. 1280?), and John Pecham (d. 1292), this group laid the foundations for the science of geometrical optics (Latin perspectiva) in the medieval Latin West.
- Smith A. M. (2022) Verified 20 Sep 2026
This, most likely, was the edition of Witelo's Perspectiva to which Kepler was responding in the Paralipomena of 1604.
- Smith A. M. «Alhacen. A Critical Edition», Preface (2001) Verified 5 Oct 2026
By no later than 1200, and perhaps somewhat earlier, this treatise appeared in Latin under the title De aspectibus.
- Smith A. M. (2001) Verified 20 Sep 2026
there were at least two translators at work on the Latin text, one of them (Gerard of Cremona?) hewing as faithfully as possible to the Arabic original
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- Smith A. M. «Optics to the Time of Kepler», Encyclopedia of the History of Science (ETHOS), Carnegie Mellon University Libraries (2022) Verified 20 Sep 2026
Node sources
- Smith A. M. «Optics to the Time of Kepler», Encyclopedia of the History of Science (ETHOS), Carnegie Mellon University Libraries (2022) Verified 5 Oct 2026
In 1604 Johannes Kepler published a treatise with the unprepossessing title, "A Supplement to Witelo" ( Ad Vitellionem Paralipomena ).
- Smith A. M. (2022) Verified 5 Oct 2026
Kepler, who proposed the theory of retinal imaging in chapter 5 of the Paralipomena , was convinced that the eye operates like a camera obscura .
- Smith A. M., (pictura) (2022) Verified 5 Oct 2026
In so doing, it projects a real, reversed and inverted image (or "painting" [ pictura ] in Kepler's terminology) of the objective source on the retinal screen.
- MacTutor History of Mathematics, «Johannes Kepler» Verified 5 Oct 2026
Kepler did some work on optics, and came up with the first correct mathematical theory of the camera obscura and the first correct explanation of the working of the human eye, with an upside-down picture formed on the retina.
- MacTutor History of Mathematics, «Johannes Kepler» Verified 5 Oct 2026
Kepler wrote a study of the properties of lenses (the first such work on optics) in which he presented a new design of telescope, using two convex lenses (Dioptrice, Prague, 1611).
- Gingerich O. «Kepler, Johannes», Dictionary of Scientific Biography (via Encyclopedia.com) (1973) Verified 5 Oct 2026
Kepler's subsequent work applied the idea of the light ray to the optics of the eye, showing for the first time that the image is formed on the retina.
- Gingerich O. (1973) Verified 5 Oct 2026
Kepler intended at first to publish his optical analyses merely as Ad Vitellionem paralipomena, but by 1602 this 'Appendix to Witelo' had taken second place to the broader program of Astronomiae pars optica.
- Gingerich O. (1973) Verified 5 Oct 2026
Nevertheless, he had set forth the essential background by which the formation of images with lenses could be explained, and so he was able to complete his Dioptrice within six months after he had received Galileo's Sidereus nuncius (1610).
- Kepler J. «Optics: Paralipomena to Witelo and the Optical Part of Astronomy», W. H. Donahue, Santa Fe: Green Lion Press (2000) Verified 5 Oct 2026
Thus vision is brought about by a picture of the thing seen being formed on the concave surface of the retina. That which is to the right outside is depicted on the left on the retina, that to the left on the right, that above below, and that below above.
- Smith A. M. «Alhacen. A Critical Edition», Preface (2001) Verified 5 Oct 2026
Finally, at the divide between Perspectivist and modern optics, Johan Kepler's Ad Vitellionem paralipomena (»Emendations to Witelo«) of 1604 has finally appeared in a critical English translation.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
Further reading
Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.