This modified Marshall microscope has four lenses. A/B make up the eyepiece, C: Field lens; D: objective. Originally, Marshall instruments had the typical three lenses.
A: Eyelens: dia=3.4cm, f=4.2cm, biconvex. Possibly this lens was added by Culpeper.
B: Eyelens 2, dia=3.6cm, f=11, biconvex
C: Field lens: dia=5.1cm, f=19.5, biconvex
There are two remaining objectives with this specimen. #4 is attached to the microscope. #2 is shown here. Objective specs shown Right. All lenses are biconvex.
| Dia | f | Aperture | |
| #2 | 6mm | 3 | 2 |
| #4 | 7 | 20 | 4 |