In my last post, I talked about the remarkable career of Etienne-Gaspard Robert aka “Robertson”, who became famous in debunking the supernatural by revealing how ghosts and phantoms could be faked.  Remarkably, even today there are still places in the world where superstitious fear can be deadly, as two recent stories, here and here, demonstrate.  It seems to be ingrained in human nature to be credulous, and science-minded people must be vigilant in explaining away the supernatural before it gets out of control. Robertson himself had many experiences to drive this point home before he started his phantasmagoria.

One story that Robertson heard that clearly left a great impression on him was told to him by his early mentor, Monsieur Villette, an optical instrument maker.  Villette’s father was a maker of large concave mirrors, and at one point the local villagers became convinced that this mirror was producing excessive rain that was ruining their crops!

I share the account provided in Charles Dicken’s 1855 article* on Robertson, which was clearly adapted from Robertson’s own Memoires.

M. Villette used often to talk about his father’s mirror, which was described fully in the Journal des Sa vans for the year sixteen hundred and seventy-nine. He made four of the kind. The first was bought for presentation to the King of Persia; the second was sold to the King of Denmark; the third was presented to the King of France; and the fourth was that which brought its maker into trouble. These mirrors, of which the last was forty-three inches in diameter, concentrated the sun’s rays into so powerful a focus that they vitrified bricks and flints,  consumed instantly the greenest wood, and melted iron.

Robertson is not exaggerating about the melting of iron and the vitrification of bricks: a “solar furnace” in France can produce temperatures of 3500 °C at focus, as seen in the video below.

They had also, of course, their optical effects. The figure reflected from any concave mirror apparently stands out from its surface, just as the figure reflected from a convex mirror seems to be contained within it. When one of these instruments was presented to the King of France — Louis Quatorze — his majesty was requested to draw his sword and thrust towards the burmished surface. He did so; and because, at the same instant, his image appeared to leap forward and direct a thrust at his own face, the great monarch recoiled in alarm, and was so much ashamed of himself directly afterwards that he would see no more of the mirror for that day.

Though I giggle when I think of Louis XIV being scared by his own reflection, it was actually not an unreasonable response: the same illusion would probably startle most people today.  When standing in front of an ordinary planar mirror, the image you see of yourself is virtual: your image appears to lie behind and inside the glass.  The image produced by a concave mirror, however, is real: the image appears in real space, lying outside of the mirror**.  This is illustrated below.

Louis XIV versus his mirror image. Here “F” represents the focal point of the mirror, and “C” represents its center of curvature.  The red and blue lines are geometrical light rays, drawn to determine the position of the image.  A ray that approaches the mirror parallel will reflect through the focus, while a ray that passes through the focus will reflect parallel.

Louis would have been faced with an inverted image of himself floating in space.  The image grows in size as you approach the mirror; when Louis thrust towards the mirror, his image would not only have thrust back but would have seemed to have swelled in size as it did so.  no wonder he was startled!

Most of us don’t get to experience this sort of illusion, by virtue of the fact that human-sized or larger concave mirrors are quite uncommon (and expensive, if of good quality).  However, you may have come across the “mirage” toy at some point in your past, as shown below.

The mirage illusion. Photo via Grand Illusions website.

The pig seems to be hovering in space, but attempts to grab it fail — one is actually seeing a three-dimensional image of the pig, which lies below within a pair of concave mirrors***.

Louis XIV presumably understood that he had recoiled from an optical illusion; however, Monsieur Villette’s neighbors ended up having their own misguided notions about his mirror.

Now, it happened that while the last of  M. Villette’s mirrors was in his house at Liège, the autumn set in very rainy, and there was great difficulty about getting in the harvest, so that bread –the supply of which, in the good old improvident times, always became scanty as the season for a new harvest drew near –bread became very dear. The populace was soon convinced that M. Villette’s mirror caused the rain which spoilt the harvest. It was said in M.Villette’s family that certain Jesuits suggested this idea. At any rate, there soon were riots on the subject, and M. Villette’s house was surrounded by an angry mob, determined upon cheap bread and no optics. They proposed lowering the price of corn by breaking up the handiwork of the optician.

Emphasis mine.  I love the turn of phrase Dickens used to describe the situation!  Fortunately, there were sensible people in authority in the region, who acted quickly:

A sensible prelate governed Liège, who put down the rioters by force of arms, and afterwards, as neither the rain, nor the superstition as to the cause of it, showed signs of abatement, issued this proclamation:–

“Joseph Clement, by the grace of God Archbishop of Cologne, Prince-Elector of the holy Roman Empire, Arch-Chancellor for Italy and Legate of the Holy Apostolic Chair, Bishop and Prince of Liège, of Ratisbon, and of Hildesheim, Administrator of Bergtesgade; Duke of the two Bavarias, of the Upper Palatinate, Westphalia, Enguien and Bouillon, Count Palatine of the Rhine, Landgrave of Leuchtenberg, Marquis of Fanchimont, Count of Looz, Horne, &c.

“To all who see these presents, greeting.

“A most humble remonstrance having been made to us whereupon we learn that a rumour has spread over our town of Liège and its environs, to the effect that Nicholas François Villette, resident for the last fifteen or eighteen years in our said town, has attracted by his burning mirror the rains with which not only these lands, but the lands of our surrounding neighbours, are chastised for their iniquities, we consider ourselves obliged by the care we should have of our flock to declare, as hereby we declare, that this is an error sown by ignorant or evil-disposed people, or even by the spirit of evil, which by diverting in this wise our people from the idea and the assurance that it is for its sins that it is chastened, causes it to attribute to a mirror that which comes from God.

“We declare, therefore, that this mirror produces, and can produce, only effects purely natural and very curious, and that to believe that it can attract or beget the rains, and so to attribute to it the power of opening or shutting heaven, which can belong only to God, would be a very blameable superstition.  And we command the curates and the preachers in all parts of our diocese into which such an error may have crept, that they use what power lies in them for its removal.

“Given in our consistory of Liège, under the signature of the administrator of our Vicariat-General in spiritualibus, and under our accustomed seal, this twenty-second of August, seventeen hundred and thirteen.

“L.F.,

“Bishop of Thermopylæ, Administrator of the Vicariat-General of Liège.

J.F. Choriste, pre P. Rollin.”

We have here the very curious situation that it was a religious leader of the area who severely put down the notion that the mirror had some sort of supernatural effect on the weather.  Though religion certainly has spread its share of misinformation through the centuries, even still today, it is worth noting that it also played a role in beating down superstition. Christian monotheism directly contradicted, in the church’s view, the idea that people could possess supernatural powers.  Indeed, I’ve discussed this argument before on this blog: in 1584, Reginald Scot argued against the existence of witchcraft using a beautiful collection of rational and theological arguments.

The troubles of Monsieur Villette highlight a dangerous aspect of human psychology that all rational people must be constantly on guard against: faced with natural disaster, it is apparently human nature to blame someone or something tangible for the problem, no matter how preposterous the explanation.  Villette’s troubles are a darkly humorous illustration of why science education is so important for the general public.

To end, I give Charles Dickens the last word:

Ignorant as we are, we surely have improved a little on the good old times ! Yet we have no great reason for boasting. Foolish thought as it was to take a mirror for the source of some of nature’s grandest operations, it is a good deal more foolish to take nature for a mirror, and some are to be found even in these days who

Do yet prize
This soul, and the transcendent universe,
No more than as a mirror that reflects
To proud self-love her own intelligence.

***************************

*Charles Dickens, “Robertson, Artist in Ghosts,” Household Words, No. 253 (Saturday, January 27, 1855), p. 553-558.  Link here.

** A nice video demonstrating this sort of illusion can be seen here.

***A good quality version of this device can be bought at the Grand Illusions store, but their description of the illusion as a “hologram” is incorrect!  It is nevertheless a wonderful shop that I have purchased a number of clever science toys from.