Magnifiers: a closer look (XXI)
Did antique engravers use magnifiers? A controversial discussion

First of all, it should be emphasized that most of this is about strong personalities and individual opinions. So we do recommend to look at the literature yourself, possibly use a complementary ChatGPT dialogue - and then judge by yourself. Nevertheless we feel that there might be a clear tendency and moreover a moderate compromise.

The most colorful "antique magnifiers - yes!" advocate appears to be Robert Temple. Robert Temple [Temple 1999] has international reputation and ample self-confidence. At the same time he might be characterized as an enfant terrible and red rag towards academic archaeologists.

The following quotation, taken from his best-selling book "The Crystal Sun" might help to illustrate his personal flavour amidst the discussion:
"My own measurements are more precise, but merely the same ..."

Other authors like Kaspar Paasch [Paasch 1999] are following the more sober, scholarly path and tend to avoid final conclusions, keeping all doors open ("further research needed").

In "Crystal Sun" Robert Temple is primarily featuring the so-called Nimrud lens (fig. 1). One strong argument for antique magnifiers' use is his referral to the prominent scientist Sir David Brewster who appears to have acknowledged the use of magnifiers in ancient times.

Fig. 1: The so-called Nimrud lens (also Layard lens) in the British Museum. It is dating back to ancient Nimrud (today situated in Iraq), excavated by Austen Henry Layard in 1850. It is made of quartz and dated back to the 8th entury BC.
Image source user:geni License: CC-BY-SA.

When looking at fig. 1 there can be no doubt that the Nimrud lens will be considered as very poor craftswork by nowadays opticians. Even Robert Temple's practical magnification demonstration (table 37 in his book) gives an impression of nebulous wavyness, reminding of a very strict verdict aired by Richard Greef:

"Those who do not want to believe in what specalists in optics are thinking (about those antique "lenses") are advised to perform a practical check: simply go ahead and try to do some fine work through such a quartz crystal lens! No chance!" (The original text of this quotation is written in German language, rather sluggishly translated by us - still trying to grasp its essence)
Source: Richard Greef, 1921, quoted by [Gloede 1986]

The original German text by Greef is as follows:
"Wer aber den optischen Fachleuten immer noch nicht glauben will und es besser weiß, der versuche doch mal, durch eine solche Linse aus Bergkristall feine Arbeiten auszuführen. Es wird ihm auch nicht für einen einzigen Augenblick möglich sein."

One crucial point in the discussion of the optical properties of the Nimrud lens is its oval shape. This oval shape is resulting in different curvatures for the x and y directions. Without any further refinement those differences will cause focus problems.

Robert Temple is referring to the technical specifications as used by professional opticians, characterizing the Nimrud lens in their specific terms as "+4D sph / +4D cyl". Transferred to plain wording this means that the diffraction values of the lens are not homogeneous, depending on the direction of measurement, varying between values of 4 and 8 diopters. Thus the long axis of the lens is delivering a diopter value of 4. In the short axis there is an overlay of additional 4 diopters which is commonly called a cylindrical deviation. In the eyes of a modern optical technician such a lens might be used to correct for astigmatism, i.e. a highly complex individual refractive eyesight problem.
It goes without saying that such an asymmetric lens will not be of general benefit as a magnifier for every users. Instead it should be considered as kind of special.
It appears that Robert Temple noticed this point. But, instead of simply acknowledging the obvious failure of the Nimrud lens as a simple magnifier he went one step further claiming that the lens should have been a kind of medical device, custom-fitted for an individual with astigmatic eyesight problems!
We are not astonished that Robert Temple is continuing on this basis that the lens should have been made for some kind of noble person ...

Furthermore he is assuming that the lens should have been originally in a noble metal fitting as the clam-shell type fracture marks could indicate a brute removal of the respective fitting by metal thieves.

In "Crystal Sun" virtually every glass lens in any museum (and there appear to exist hundreds of this kind) appears to be an optical instrument, whereas most archaeologists tend do declare all those objects as ornaments or kind of decorative fun, a few as potential burning glasses.

Finally, dear readers, it is depending on your judgement, whether you tend to lean towards Paul Temple (antique optical lenses: yes!!!) or his opponents (antique optical lenses: no!!!).

But we will present our own resume and compromise in the next issue of the magazine.

Literature

Wolfgang Gloede: Vom Lesestein zum Elektronenmikroskop. p. 10-11. Berlin 1986

Paasch, Kasper: The history of optics: From ancient times to the middle ages.
DOPS NYT 14(1999) p. 5-8.

Temple, Robert: The Crystal Sun. Century (Ed.), 2000.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
The Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.

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