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c. 1790, unsigned



This is a very small microscope compendium commonly called a 'acorn microscope.'   It measures about 1 1/16 inches in maximal diameter, and 46 mm (a little more than 1 3/4 inches) high. View the images in sequence from left to right. When the top is unscrewed it reveals a round lens focused on a steel spike. Although this might initially seem like the entire instrument, two more instruments are hidden inside. Unscrewing the portion with the spike reveals a loupe-type magnifier, and by unscrewing the bottom plate a 'fleaglass' or 'vitrium pulicarium' comes out of the loupe, allowing the loupe or the fleaglass to be used. The loupe gives the widest and lowest power view. The fleaglass allows one to trap an insect, or a subject in a drop of pond water inside, and observe its behavior under magnification.   The lens on the top of the instrument aimed at the spike is held in place by a brass dome which in turn is held in place by a brass circlip. The lens for the loupe is held in by a circlip alone.

The innermost device (shown in the bottom three images above), consists of a cylinder which is approximately 18 mm high and 18 mm in diameter. It has a very slight taper, fitting inside the loupe perfectly. Both the top and bottom have narrow knurling around the edge to facilitate removal of those elements. About 6mm from the bottom of this little cylinder is a seam. When the bottom third of the cylinder is unscrewed, it reveals a flat glass in its bottom and a concave glass remaining in the upper portion, facing downward. This arrangement of a concave disk covered by a flat piece of glass is very reminiscent of the arrangement of the 'wet cell' type of live box supplied with my screwbarrel microscope, except that this is a single cell. The focal length of the optical element of this little cylinder is such that only an object between these two pieces of glass, is in focus. The lens for the fleaglass, like the lens on the top focusing on the spike, is held in place by a small brass dome, much like the lenses are held in the eyepiece components of a screwbarrel microscope.


The 'Acorn' microscope gets its name from the Acorn-like shape it has. They were apparently first made in the early 18th century and likely for about 100 years after that, into the early 19th century. They are found made of hardwood, ivory, or brass as this one is. The ivory and brass examples tend to be smaller than the wooden ones. More complex examples of small optical compendium exist and can contain additional components such as a telescope or screwbarrel microscope, but at the expense of increasing the length or width of the instrument. None of these instruments are signed by a known maker and so firmer dating is not feasible. One 'signed' example is signed 'B. Granger, Tettenhall, 1790' but this is likely the owner rather than the maker as it was passed down the Granger family for generations, and they were known to be attorneys, not instrument makers (JRMS, 1906, p 715).