German "Screw-Focus" Microscope, ca. 1650
This is the earliest microscope in this collection, most likely dating to the mid-17th century. It is formed of a small (4 cm long) cylinder made of bone (probably cow metatarsal), with a lens on top housed in a cylindrical wood ring covered by a bone disk. An opening at the side of the instrument allows an insect to be impaled on a steel spike, attached to a bone screw that can be turned in or out for focusing. The entire instrument measures merely 5 cm long.
The microscope in our collection has a matching cylindrical box made of wood (probably mahogany) and has an ivory base. As seen in the picture, the box exactly fits the small microscope and may be original, but this is not certain.
There are a few more examples of this microscope. One of them appears in Giordano (2012: # 6), where it is generally attributed to the 17th century. Another example was sold on eBay in 2017 and is currently in a private collection in the US. It may be attributed to a Dutch or German origin and may be tentatively dated to ca. the middle or second half of the 17th century.
Pocket-sized optical instruments such as this one became popular during this time. They were intended to satisfy the curiosity of the people in natural history, especially insects (thus the term "flea glass", or vitrum pulicarium in Latin). This was undoubtedly fueled by studies such as those by Francesco Stelluti (1577-1652), Federico Cesi (1585-1630) and Jan Swammerdam (1637–1680) about the the physiology of insects.