Magnifiers: a closer look (XV)
Oldie beating true triplet?

In the July issue we had presented the theory behind an advanced focal length measurement method (making up an almost universal micro-optics magnification measurement method!).

The respective practical procedure had been explained in August.

Some measurement results had been presented in September, October and November.

Those among you who bravely followed our discussion about true and false triplets might have ended up with the conclusion that any triplet magnifier should perform better than any non-triplet magnifier. But, as mostly in life, the practical reality is more complex:

Fig. 1: Archaic folding magnifier. Looks like wood but is actually made of some kind of bone. Single bi-convex lens, ca. 11 mm free diameter. We measured a magnification of 12x. Length (when folded) 50 mm, height 10.5 mm, weight 6.5 g. The glass is delivering a strong greenish fluorescence under UV light (365 nm).
Date of production? - difficult to tell.
But, tardigrade-wise, this magnifier is well able to detect a tardigrade in a dry moss sample!

Now, what is the verdict? Primitive? Collectible? In any case the feeling and handling is very agreable. The instrument is sliding smoothly in any pocket, even the smallest pocket. And there are no indications that the folding mechanism is jamming or becoming loose.

In fact, this rather primitive instrument is actually well able to resolve the 10 µm ruling of our object micrometer, whereas a classical 10x magnifier will not be able to do this!

As explained already previously 10x magnifiers cannot help us in order to crack this 10 µm threshold. The reason behind is not product quality or lens finish but shere magnification. In this peculiar situation 10fold is slightly too low for our eyes. As a consequence the primitive 12x magnifier is the winner in this contest - though by a small margin, but still a clear winner. Funny enough it might turn out to be superior to any high quality 10x magnifier - admittedly in very few situations where brute magnification is crucial.

So we do have reasons to stick to those archaic magnifiers and to have a closer look at some of them. Moreover we will be able to learn about forgotten crafts and materials!

When looking around in the internet you will notice that almost every black vintage magnifier will be marked as "Bakelite" - but the reality might be much more complicated. Just have a look at fig. 1. On the basis of the surface texture one could assume that the housing of this instrument might be made of wood. But microscopic inspection and spectral analysis (revealing so-called amine bands) tell a different story: this magnifier is made of horn. On second thought it will become clear that horn is much better suited in order to confine a glass lens - as it is softening when heated in boiling water and enclosing the glass lens safely when shrinking back (hardening) at room temperature.

On the other hand the biological nature of the horn material can turn out to be a disadvantage in the long run: the horn has a kind of memory of its former state, thus not respecting the geometry imposed by human craftsmen, instead bending and cracking back to its primary form. As a consequence cracks and deformations are a good indicator to discern horn from fully synthetic material.

Let's have a look at one of those mysterious "Bakelite" items. A simple method for the study of black magnifier housings is the use of UV light. The instrument shown in fig. 2 can serve as an example:

Fig. 2: A huge black folding magnifier with iris. We were able to measure a similar magnification at both lenses, namely 2.65x, summing up to a combined 5.3x. So the instrument is closer to a reading glass - not so much a loupe for tardigrade studies!
Housing length (when folded) 9.5 cm, housing height 2.55 cm, free lens diameter 40 mm. Weight 73.7 g. Estimated date of production: 1920-1930. In order to illustrate the enormous size of this magnifier we placed a modern "14x" folding magnifier in front of it.

When looking at the huge magnifier under 365 nm UV flashlight an impressively colorful image is arising:

Fig. 3: The magnifier as shown in fig. 2, under UV light. The outer parts and lens fittings of the magnifier appear in a brownish color, the diaphragm in blue and the glass in green color! On the basis of this image we think that the outer housing is made of so-called Vulcanite (see below) and the iris of some natural material (horn).

The following GIF animation is illustrating how UV light might help us in order to discern some of those archaic magnifier materials:

Fig. 4: Gif animation with four overlay images. Obviously the various UV effects are signalizing different materials.

Our huge magnifier is marked in the GIF animation as "Vulcanite", i.e. a synthetic caoutchouc based material which was widely used in the 1920s and 1930s. For those products liquid caoutchouc was mixed with sulphur and compressed at high temperatures, ending up as a solid, non-flexible, extremely durable material.

On the other hand Bakelite is a completely synthetic resin produced by chemical reaction of phenol and formaldehyde - so it shouldn't be mixed up with "half-natural", caoutchouc based Vulcanite. But both are showing a similar brownish fluorescence in the above GIF (with the Bakelite looking darker), whereas natural materials like horn and tortoise shell normally look blueish under UV. Cellulose nitrate, sometimes used to imitate tortoise shell is showing blue fluorescence as well.

Literature

Fa. Josef Eschenbach: Katalog Lupen / Lesegläser / Fadenzähler. 32 pages plus price list. Nürnberg 1923.
[Annotation: this is a catalogue full of caoutchouc based magnifers]

© 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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