J.R. sends me a find from a shed in Victorville, a vintage concave glass parabolic mirror framed in a crude wood box at 14-inch diameter. What was it used for?
I’m no scientist, but research tells me that a concave mirror collects light rays after it “sees” a reflection, and the image it sends back is “virtual” or inverted (upside down).
Its brother, the convex mirror, is ubiquitous. We use convex mirrors in rear view mirrors, and the image it sends takes a little getting used to, as the cars behind look smaller and farther away. If you are close to my age, you hate those bathroom mirrors that magnify one’s face. Those are convex.
A simple experiment with a teaspoon will show you the difference between concave and convex reflections. The interior bowl reflects as a concave mirror, and the exterior as a convex. Thus, you’ll see a right side up image in the outer bowl, and an upside down image in the center bowl. Don’t ask me why; that’s too much math.
Thus the question remains, what was the concave mirror at 14 inches used for, and why it is in a wooden frame? The last question is easy. The concave mirror is thin at the center and wider at the edges, so the frame is a later addition for protection.
The first question (what it was used for) is harder because these mirrors throughout history were used in all sorts of optics, as well as in philosophical metaphysics, and as a thing of utility and beauty as well in interior lighting.
None other than Leonardo da Vinci drew plans in his “Codex Atlanticus” in 1503-1505 for machinery (called a Mirror Lapper) to make a concave mirror. He again returned to the problem in 1515, and a model of the same was made in 1957 for the Science and Technology Museum in Milan.
In another application of these concave mirrors, a fan of da Vinci in County Offaly, Ireland, in 1840 began work on the world’s largest reflecting telescope in the world in his Birr Castle, Demesne, a 400-year-old manse, where, from 1845-1915, scientists came from all over the world to discuss the “leviathan” that discovered the spiral nature of some galaxies.
The Third Earl of Rosse, inventor, had a like-minded wife, Lady Mary, who was an early pioneer in the then new medium of photography, which also used concave lenses. Meanwhile, their son Charles was out back building a steam turbine. On family nights, the three (probably early nerds) worked on the Lord’s massive telescope, and they no doubt ate popcorn over a copy of da Vinci’s 1492 math studies of concave mirrors of constant and parabolic curvatures.
From the insanely scientific to the mystical: The great 17th-century alchemist Athanasius Kirchner, in his 1617 book “Ars Magna Lucis et Umbra,” illustrates in a beautiful woodcut of an alchemist at work. In that illustration, a concave mirror hermetically reflects the divine light into the eyes of the alchemist, who then pours the light into the polis.
By the 18th century, concave and convex mirrors were used for beauty and utility indoors, as they amplified candle power and showed off that you were rich enough to afford such a mirror. A series of etchings by John Gwynn (1713-1786) shows the great concave mirror installed at Vauxhall Gardens on the South Bank of the Thames, in a grand rotunda.
And of course, every early American 1950s house where I grew up partook of the “Ethan Allen Colonial Style “ of home décor, and we all had a convex mirror, usually with a fake eagle on top. The real ones can be quite valuable: I just appraised one for $20,000.
Farther afield, the ancient practice of Feng Shui uses concave mirrors to receive and contain harmful energy in a room. Convex mirrors are not “good Feng Shui” because they push the energy out, like a death ray. These are called Bagua mirrors.
So, what was J.R.’s mirror used for? This mirror was used for military purposes, probably from World War I, for a search light. (Such mirrors were used in World War II as well, but they were usually aluminum, not glass.) I cannot put the mirror as earlier, because the “silver” is not “real” silver as in early mirrors. I do not see lichen-like spots associated with the degradation of the real silver back.
But alas, JR’s mirror back is painted. Wartime mirrors usually had some government branding on the reverse. The value is $400.
Dr. Elizabeth Stewart’s “Ask the Gold Digger” column appears Mondays in the News-Press Life section.
Written after her father’s COVID-19 diagnosis, Dr. Stewart’s book “My Darlin’ Quarantine: Intimate Connections Created in Chaos” is a humorous collection of five “what-if” short stories that end in personal triumphs over present-day constrictions. It’s available at Chaucer’s in Santa Barbara.