With his Ad Vitellionem Paralipomena, Quibus Astronomiae Pars Optica Traditur (Supplement to Witelo, in Which Is Expounded the Optical Part of Astronomy), published in 1604 and his Dioptrice, published in 1611, Johannes Kepler (1571–1630) initiated a new direction in the history of optics. One of the major changes that he introduced was to change the concept of how the eye actually sees on a physiological level. The predominant belief, before Kepler, was that the image perceived by the eye was formed in the crystalline humour, what we call the lens. Kepler, based on the models of the eye’s anatomy as presented by Johannes Jessenius (1566–1621) in his Anatomia Pragensis (1601) and especially that of Felix Platter (1536–1614) presented in his De corporis humani structura et usu. (1583), hypothesised in his Astronomiae Pars Optica that the eye was like a camera obscura with a lens which formed the perceived image on the retina at the back of the eye.
We now know that Kepler was right but, back at the beginning of the seventeenth century, people had difficulty accepting this hypothesis, not least because the image cast on the retina by the lens would be inverted. Kepler was well aware of the problem and surmised quite correctly that the brain turned the inverted image upright. The first person to develop and extend Kepler’s work on the physiology of the eye was the Jesuit scholar, Christoph Scheiner (1573–1575).
Scheiner is best known as the astronomer, who had a major dispute with Galileo (1564–1642) as to who first observed sunspots with a telescope and how they were to be interpreted. Actually Thomas Harriot (c. 1560–1621) was the first but as he didn’t publish the others couldn’t know that. The first to publish sunspot observations was Johannes Fabricius (1587–1616) with his De Maculis in Sole observatis, et apparente earum cum Sole conversione, Narratio, etc. Witebergae, Anno M.DC.XI. It appears that neither Scheiner nor Galileo were aware of Fabricius’ publication. Galileo got the interpretation right, showing them to actually be on the surface of the Sun, as opposed to Scheiner, who initially thought they were small satellites orbiting the Sun. Having won that dispute, Galileo abandoned his sunspot observations, whereas Scheiner continued his programme of solar observations, using a special helioscope, designed by his fellow Jesuit, Christoph Grienberger (1561–1636), which enables the observer to track the movement of the Sun whilst observing,
publishing the results in his Rosa Ursina sive Sol. (Bracciano, 1626–30), which remained the best volume on solar observation until the nineteenth century.
Scheiner was also an inventor, who invented the pantograph, an instrument which could duplicate plans and drawings to an adjustable scale, about which he published Pantographice seu Ars delineandi res quaslibet per parallelogrammum lineare seu cavum” (Rome 1631).
However, it is Scheiner the physicist that interests us here and his research into the theory of vision, which he published in his Oculus, hoc est: Fundamentum opticum ( The Eye, that is The Foundation of Optic) (Innsbruck, 1619).
The first major change that Scheiner introduced was a new image of the eye that was anatomically much more accurate that the preceding images of Andreas Vesalius (1514–1564) published in his De Humani Corporis Fabrica Libri Septem (On the Fabric of the Human Body in Seven Books) in 1543 and that of Felix Platter (1536–1614) presented in his De corporis humani structura et usu. (The Structure of the Human Body etc.) from1583, which had guided Kepler’s work.
What was important for Kepler’s was Platter’s emphasis on the retina in the process of visual perception:
The principal organ of vision, namely the optic nerve, expands through the whole hemisphere of the retina as soon as it enters the eye. This receives and discriminates the form and colour of external objects which together with the light enter the eye through the opening of the pupil and are projected on it by the lens.
Schner’s anatomical model radically changed the structure of the eye in comparison to the Vesalian- Platter model. Scheiner moved the crystalline humour into its correct position directly behind the iris and perhaps just as importantly moved the optic nerve from its central position to the side letting it enter the eye nasally. He also abandoned the concept that the cornea was spherical, stating instead that it was either a parabolic or hyperbolic spheroid. His crystalline humour has portion of a sphere on both sides the back having a smaller radius than the front. Altogether Scheiner’s diagram, or as he calls it effigies, is fairly close to the modern medical image of the eye.
His descriptions of his dissections and the resulting anatomical model of the eye take up the first part of Book I of his Oculus.
Moving on Scheiner tells us “Book I part 2, in 14 chapters, brings forth wonderful yet well-tested experiences on behalf of the teachings immediately following” He distinguishes in his work between experiences (experientiae) and experiments (experimenta):
Experiment, for Scheiner, becomes a way to further develop and refine one’s cognitive state of having experience or being experienced, a refinement necessary to establish true first principles for a science.
This keeps Scheiner in line with his commitment to Aristotelian philosophy as a Jesuit. What he describes are things that he has witnessed under controlled conditions with many repetitions i.e. experienced.
His experientia prima concerns variations in the pupil, firstly observing, the already known fact that the pupil contracts when exposed the bright light and dilates when the light is dimmed. He then shows that light variation are not necessary to provoke changes in the pupil. Holding a needle close in front of the eye he demonstrates the pupil contracts when he moves the needle closes and dilates when he moves it away.
In his second experientiae, he demonstrates crossed rays when viewing an object through a small hole H in a thin sheet of oblique material:, which he designates DEFG.
In total there are eleven experientiae in Book I part II of which two describe individuals with vision defects, one of which concerns a man with a cataract. Interestingly some of Scheiner’s experientiae and the devices he developed for them are still used in modern ophthalmology, even with his name attached.
Book II starts with the elimination of various parts of the eye as contenders for the seat of visual faculty based largely on the results of his experientiae in the previous book. He also introduces and discusses various aspects of optics, refraction, the nature of rays (or species) of light, colour, etc.
In Book III, part 1 he concludes that the retina is the seat of vision, both by process of elimination and owing to the fact that the substance and position of the retina are appropriate for receiving visible rays. Following this he reconstructs the visual cone using the retinal theory, thus allowing that theory to accommodate Euclid’s axioms. The crucial benefit of the retinal theory of vision is that, unlike the traditional visual cone model, it combines seamlessly with the burgeoning science of dioptrics—the science for understanding the effects of burning lenses, eyeglasses, and telescopes—particularly as developed by Kepler (1611). A good portion of Book III, part 2 is therefore spent resolving outstanding questions about experiences with eyeglasses, telescopes, vision disorders, and other matters.
In his Rosa Ursina (1626–1630), Scheiner underscores the retinal theory in that he describes by taking eyes from dissected cattle and humans, removing the sclera and uvea from the back of the eyeball and thus exposing the inverted image visible on the retina. Unfortunately, Descartes (1596–1650) is usually given credit for being the first to carry out this experiment, which he repeated eleven years later and published in LaDioptique as an appendix to his Discours de la Méthode pour bien conduire sa raison, et chercher la vérité dans les sciences, in 1637, which we will look at in a later post in this series.
That Descartes got credit for something Scheiner had done and published well before him shows that Scheiner’ optical investigation did not have that much of an impact but they were read and did have some influence in the gradual acceptance of Kepler’s optical theory.
[1] This is taken, as is most of the information on Scheiner’s optical studies, from Baker, T. (2024). Christoph Scheiner’s The Eye, that is, The Foundation of Optics (1619): The Role of Contrived Experience at the Intersection of Psychology and Mathematics. In: Schickore, J., Newman, W.R. (eds) Elusive Phenomena, Unwieldy Things. Archimedes, vol 71. Springer, Cham., which you can access online here