Ricoh Innovations and Stanford University
 David G. Stork
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No convincing corroboratory documentation: Problems with conspiracy theories

Hockney suggests that early Renaissance artists would have kept the "secret knowledge" about mirrors to themselves because it was (in modern parlance) a trade secret, or might offend the church.

Here's a partial list of sources from fifteenth-century Europe that could conceivably have given us records about the concave mirrors required by the Hockney/Falco theory but as far as we know did not:

  1. Contemporary scientists: Early Renaissance scientists would have eagerly explored, discussed and disseminated information about concave mirrors. Such mirrors would surely have been a matter of great curiosity as they solved pressing problems of the time, many beyond those related to painting. Wouldn't the ownership and mastery of such a concave mirror have been a matter of pride? Such scientists (and art theorists) wrote extensively on optics and geometrical perspective (Scenographiae, sive Perspectiva; Perspectiva; Instituto artist perspectivae; La Pratica della Perspettiva...) but nothing on the use of concave mirrors, whose use was even trickier and surely needed more explanation. If concave mirrors were superior and in fact widely used, as claimed by Hockney, why not? Why is there no Istituto artist della riflessione? Even when Galileo had his troubles with the Inquisition, it was not over his optics and telescope but instead his theory about planets that he discovered with his telescope. No, the telescope was valuable to warfare and trade; Venetian merchants used them on towers to see which ships were approaching bearing what cargos, the better to do commerce
  2. Purported mirror makers: Purported makers of concave mirrors had every financial incentive to advertise and disseminate information about concave mirrors and their uses (even if they wanted to preserve trade secrets as to how such mirrors might be fabricated). And if the mirror makers were not spreading the word about concave mirrors, how did artists such as Lotto learn about such concave mirrors? Or obtain one? And who were these purported mirror makers... the famed Venetian glass makers? These glass makers advertised far and wide, and their products (if not their methods) were known and highly prized throughout Europe. Yet there seem to be no records of concave mirrors produced by Venetian glass makers or the Guild of St. Luke in Bruge, or indeed anyone, including any suggestion of the 'Arnolfini trick' about turning around a convex mirror. If not the Venetian glass makers, Guild of St. Luke, or any guild on record, then who?
  3. Patrons and subjects: Why wouldn't the patrons and subjects -- who sat for long periods for portraits or oversaw artists' budgets -- mention such newfangled devices, even in passing in personal letters? After all, the mirror setup included a large dark tent and very bright lights (see below), and was imposing. A concave mirror would have been an expensive proposition on an artist's budget sheet. While a poor illiterate model might not leave records, the wealthy, powerful and highly educated business leaders, church officials, royalty and political leaders recorded in portraits could and most likely would have left records. They wouldn't have to say 'Mr. van Eyck used a 5-inch concave mirror yesterday when painting my portrait' to show us they saw the complicated mirror system that imposed upon their household. Might not even one painter show his benefactor the wonderous images that have excited Mr. Hockney (perhaps to get a better commission), and wouldn't the patron remark upon this wonderous event? Not even once over several centuries?
  4. Artists and their families: Early 15th-century artists would have been even more elated than Mr. Hockney when learning of the image creating abilities of concave mirrors, and it is hard to accept that not one leaves us a record of discussing it, even allowing for some purported fear of the church or desire to keep a trade secret. No one ran through the streets of Bruges shouting "Eureka"? And were there no "leaks" from their family, curious friends, envious competitors or relatives?
  5. Conspiracy theorists: Even if we tentatively accept Mr. Hockney's claim that there was, in essence, a conspiracy to preserve "secret knowledge" about mirrors, there remains a problem. Why were there no contemporary writings in journals, letters, books, church records and so on about this secret mirror conspiracy? Nowadays there are writings about even the most secretive groups ("conspiracies," if you will), such as the Mafia or Al Qaeda. In the fifteenth century, there were writings about workings of the secretive Venetian glass manufacturers, the Knights Templar, and others. Are we to believe that for hundreds of years there was a conspiracy, when the mirror practitioners were regularly interacting with patrons, funders, subjects, church (Inquisition?) officials, family, friends and that not one of these outsiders leaves a record of a mention about the mirror conspiracy? Didn't they think it was odd? For the largest conspiracy of silence in history (regarding concave mirrors) to go on for centuries without anyone writing about it (until the 21st century) seems implausible.

Finally, there seems to be no compelling way to reconcile the existence of fifteenth-century records for convex mirrors and for concave mirrors of short focal length (e.g., burning mirrors), with the absence of records for long-focal length mirrors (concave or convex) that are required by the Hockney/Falco theory. Indeed, here is a partial list of optical devices, some quite obscure, culled from a few books on history of optics and art from 1400 to the mid-nineteenth century, and conspicuous by its absence is any mention of the use of concave mirrors for projecting and tracing images as given by the Hockney/Falco theory. If there was a conspiracy to keep hidden the "secret knowledge" described by Mr. Hockney, that conspiracy must have been astoundingly stealthy! (Or our historians have an acute and amazingly specific tunnel vision.)

Device Purpose or description
anamorphic mirrors and lenses deliberately creating and undoing drastic distortions in art
astrolabe primitive "computer" for predicting positions of planets but which was also used for surveying and estimating sizes of buildings
bacolo of Euclid bars for estimating the height and width of a distant object, such as a building
bussola device for measuring horizontal and vertical bearing angles
camera lucida "light room," a prism for simultaneously viewing a scene and a drawing surface
camera obscura "dark room," a dark room with a lens or hole to cast a dim image on the opposite wall
kaleidoscope tube of plane mirrors for creating abstract image patterns
magic lantern early slide projector
megalographs paper with small holes which cast large shadows onto walls
microscope seeing small objects
multiplying spectacles eyeglasses with faceted lenses for producing multiple images
myriorama image that can be cut and reassembled numerous ways
optigraph mirror linked to a pen for copying images or drawing from nature
pantograph mechancial device for enlarging or reducing images
Brunelleschi's peephole-mirror device showing a three-dimensional scene in two dimensions
periscope two mirrors in a tube for extending the user's visual reach
perspective box box whose inside walls hold distorted images which when viewed through a peephole appear in proper perspective
perspective glass tube with faceted lens making multiple drawings appear as one
perspectograph or automatic perspective machine mechanical system for transferring an image from a flat surface to another possibly curved surface, such as a curved wall or ceiling
phenakistoscope or phantasmoscope or "stroboscope" or zoescope slotted disk with sequence of images, which when rotated reveals a primitive "movie"
physionotrace device used for tracing portraits
radio astronomico sliding crossed sticks for measuring the height and width of a building
shadow lantern candle and rotating silhouettes that cast shadows on the translucent walls of a small box
spectacles primitive eyeglasses
telescope seeing distant objects
thaumatrope card with a partial image on each side which, when rotated, shows the full image
zoetrope or wheel of life slotted cylinder with sequence of images, which when rotated reveals a primitive "movie"
zograscope or optical diagonal machine convex lens and mirror giving a magnified view of a flat image

There are also works or devices so obscure that we do not know their name, such as in a seventeeth-century Bohemian painting, "Mercury and Venus." If the small flanged metal dial at the center is turned horizontally, then Mercury and Venus appear belly-to-belly, if the dial is turned vertically they appear instead back-to-back.

Summary: As for the written record of the purported use of concave mirrors in the fifteenth century and later, the silence is deafening. There are numerous parties who had every reason to discuss, learn, and disseminate information about concave mirrors -- or about a mirror conspiracy itself -- yet we have little if any such corroboratory written evidence. In every specific case of which I am aware, the record describes mirrors that could not have been used for any of the paintings put forth by Hockney and Falco. Hockney and Falco give us no corroboratory evidence from fifteenth-century Europe that a concave mirror was ever used for producing an image (rather than burning). Scholars and historians of science and technology have uncovered numerous obscure devices and would never have missed records for the concave mirror method, had such records existed.

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