PRE-ACHROMATIC OBJECTIVE IN BLACKENED IVORY MOUNT

Circa 1730s

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The objective shown here to the left contains a single biconvex lens in a blackened ivory mount. According to Dr Joe Zeligs, it probably dates from the 1730s and may have belonged to an early form of Culpeper microscope, perhaps by Mathew Loft or one of his contemporaries.This lens is marked with two dots and also the number 2 on the threaded end, indicating it is the second to highest power of a set for its associated microscope. When viewed under a stereo microscope, this lens has some scratches and scuffs on both surfaces.

The front aperture is smaller than the lens diameter, and the back of the lens is also stopped down by a greater amount by the rear casing that screws into the front to hold the lens in place. This was a common method to reduce aberrations, but also one that reduces the resolving power of the lens.

This pre-achromatic lens produces fair images. As expected there is chromatic aberration, spherical aberration, and also pincushion distortion. In addition, the field is not flat, so when detail near the center of the lens is in focus, features outside of that area are not and vice versa.