Pocket drum microscope, (by Martin, but not signed) in green leather case; with accessories

This example dates from about 1770, and differs from the earlier instruments of this form in having adjustment for the condensing lens and in the form of the eyepiece.

There are two objectives, of 0.22in. and 0.33in. focus respectively. The lenses constituting the eyepiece are mounted in a separate inner tube, and consist of a field-lens of 1 3/8in. and a double eye lens of having an equivalent focal length of 5/8in. The field-lens is distant 2in. from the object lens and 1 1/2in. from the lens nearest the eye.