Magnifiers: a closer look (XXV)

Being caught between the chairs - a two lens (!) Steinheil reading glass Many decades ago a flourishing local optical industry delivered a wide range of optical instruments to Munich's citizens: e.g. binoculars made by Rodenstock, reading glasses by "Sendlinger Optische Werke" or, as shown in fig. 1, a hand magnifier by the famous Steinheil company.

Fig. 1: Reading glass made by the famous Steinheil company, in its original protective housing. Probably dating back to the 1960s. Ca. 1.8 fold magnification. Equipped with two-lens aplanatic optics - see below. Overall length 18.2 cm, height in the lens area 16.5 mm, free lens diameter 62 mm. The lens fittings and handle are made of aluminium. The handle is coated with black textured paint. Weight 164,8 g.

The signature "STEINHEIL MÜNCHEN" (fig. 2) reminds of older products by the same company, in particular the formerly shown splendid, true triplet 3x Bakelite folding mangifier.

Abb. 2: Signature on the handle of the Steinheil reading glass shown in fig. 1. The precision of the engraving appears to be perfectly flawless, almost sterile. Apparently the dots on the big "Ü" were slightly modernized with respect to those on older Steinheil instruments.

The Steinheil magnifier is different from most 20th century reading glasses as it houses a two glass lens optical system. When seen from the side only the flat surfaces of those two plano-convex lenses are visible:

Fig. 3: The Steinheil reading glass optics cage as seen from the side. The lenses are permanently encapsulated in an aluminium cage (without threads).

There is a simple trick to assess the number of lenses in such a closed optical system: a spot light beam can be directed onto the outer optical surface. It will result in a characteristic reflection pattern:

Fig. 4: The reflection pattern on the Steinheil magnifier when illuminated by a spot light. Four distinct light source reflection images are visible here. They can be interpreted as reflections from the back and front sides of two separate lenses.

When moving the spot light source we can assess two pairs of light source images and will learn that the reflection images of those two pairs move in a coordinated manner, thus signalizing which of them belong to a common lens. When compared to ordinary single lens readings glasses the benefits of this system will become apparent. Straight text lines will remain straight and free of colour fringes. The best visual image impression will be achieved when this magnifier is held closer to the eyes than a typical reading glass, in a similar manner that can be seen on many Sherlock Holmes portraits. Those spot light reflex patterns can be studied also in more complex optical systems. Sometimes the optical coatings of the various lens surfaces will become apparent. But don't worry: this will not be a topic for the magazine issues 282 to 300!

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