Magnifiers: a closer look (XX)
An alternative approach: magnifying without glass!

As discussed in our most recent journal issues, antique craftsmen might have been able to perform miniature work without glass lenses (e.g. when being shortsighted by nature).

It appears that optical aids were widespreadedly used much later, in the 14th century. When thinking about the needs of elderly people it becomes clear that the use of magnifying glasses is highly improbable in antiquity. And there is virtually no archaeological evidence for the systematic use of magnifiers, e.g. in antique Roman mints (and no trace of spectacles either!).

In any case a craftsman's use of a magnifier is seldom depicted, even in much later times. Our best example with an engraver is quite modern - from the 19th century:

Fig. 1: Detail from Buch der Erfindungen (Book of inventions, 1876 - volume 1, p. 491) with the caption "engraver's workshop" (in German: "Ein Kupferstecher-Atelier").
This is apparently a thoroughly professional magnifier use, with the magnifying glass kept close to the eye, at an estimated object distance of less than 10 cm. The instrument shown here might be made up a two-lens system in horn fitting.

So the basic question is still remaining: if the antique craftsmen had no magnifiers at all, how on earth were they able to perform such impressive miniature work? Well there is an alternative - without glass! We are demonstrating this by experiment - but you should check yourself.

As a demonstration object we chose a German 1 Cent coin. The date number on this coin is rather tiny, merely 0.65 mm in height, just readable by means of the bare eye, but easily resolvable under a dissecting microscope (fig. 2).

Fig. 2: The backside of a German 1 Cent coin as seen under a dissecting microscope. In reality its size is much smaller - 16 mm in diameter!

For a typical, average observer this coin will probably look less clear, like simulated in fig. 3:

Fig. 3: Simulation of the visual appearance of the 1 Cent coin as perceived by a typical observer. Depending on age and individual visual acuity the date number might be just readable (or not readable).

So, let's assume for a moment that we were antique coin engravers. In order to engrave the numbers of the date as shown in fig. 1 a minumum optical resolution as the one below would be highly desirable:

Fig. 4: Simulation of a slightly better optical resolution, appropriate for fine detail engraving work.

How should we proceed in order to reach this higher resolution? Is it possible to enhance our visual capabilities without a glass lens? In fact, it is!

Just have a look at the following demonstration, making use of a so-called object micrometer, commonly used to calibrate and assess tiny distances under the microscope:

Fig. 5: A common object micrometer (or "calibration slide"), easily available via internet. The dimensions of this slide are 76 mm x 26 mm (3 inch x 1 inch). The cross marks on the right side of the slide have central dots measuring 0.15 mm and 0.07 mm respectively.

We were able to see the 0.15 mm dot by means of the bare eye - but not the one with 0.07 diameter. This is in accordance with the typical resolution capability of the human eye, known to be ca. 0.12 mm.

Well, there are clever people apart from the scientific world and they might have the appropriate ideas, like the one published in the "Expedition" magazine in winter 1981 with the title "Close Work without magnifying lenses?".
One of the readers of the magazine had pointed out that a simple pinhole might help as a tool in order to perform miniature work [Gorelick 1981, see link below].
We are not astonished that this hint was brushed aside by the professionals: it was answered that an ophthalmologist had denied the usefulness of such a pinhole, arguing that the pinholes used in ophthalmology were routine and didn't provide a noticable resolution advantage at all! Moreover, some kind of stand would be needed to use it.

But the layman's hint was correct. You see, ophthalmic pinholes and pinhole spectacles have much bigger holes than the ones needed for this particular challenge. As a consequence they are not delivering.
But an appropriately small pinhole will perform the task.

Fig. 6: Pinhole spectacles like the one shown here are sold via internet. But please note that the pinholes on this one are 1.5 mm in diameter - by far not small enough to enhance detail resolution.

All of us do know that a small diaphragm, when used in photography will provide an enhanced depth of field, thus allowing to picture an object at closer distance - still sharply resolved. That's exactly what an isolated, very small pinhole is doing: it allows the user to approach an object more closely, closer than the normal minimum observation distance while preserving detail and sharpness. So the pinhole is performing the same job as a convex lens, allowing the user to watch an object from a closer point of view, thus enhancing the angle of view which is defining the opical resolution!

It goes without saying that this experiment is no proof that ancient engravers and craftsmen actually used pinholes but it is definitely a proof that the visual acuity can be markedly enhanced when looking through a pinhole - an this is in fact what magnification does.

Fig. 7: For the practical experiment we used a tiny pinhole, mounted in a blackened wooden support. The pinhole shown here is merely 0.5 mm in diameter. It is by no means a comfortable tool. Moreover the field of view is very modest. But we were able to see the 0.07 mm dot on the calbration slide when looking through it and we got a much better clearer visual impression when looking at the date of the 1 Cent coin as well!

Somply give it a try - pinholes with variable diameters are separately sold in context with experimental pinhole cameras. They are not expensive and so you can easily check our statement that a pinhole can perform a similar magnifying job as a glass lens!

In the next magazine we will sum up our findings, the pros and cons for magnifier use in antique times.

Image source and literature

Source of fig. 1: Das Buch der Erfindungen, voume I, p. 491 (illustration #371),
Leipzig and Berlin 1876.

Gorelick, Leonard; Gwinneth, A. John: Close Work without magnifying lenses?
Expedition, Philadelphia 23(1981) p. 15.

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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