The forty (40) is the objective lens diameter in millimeters. This is a variable scope because you can vary the magnification of the scope from three to nine, stopping anywhere in between. You would describe this scope as a "three to nine by forty." Some scopes have a 3x magnification range such as a 3-9. Some have 4 or more as in a 4-12.

Physics Tools for optics - Find image distance and magnification for two lens system Use this simple tool to solve physics problem related to two lens system. You can use this tool find position of the final image formed by the combination of the two lenses, and magnification of the final image formed by the combination of the two lenses. Resolution is a somewhat subjective value in optical microscopy because at high magnification, an image may appear unsharp but still be resolved to the maximum ability of the objective. Numerical aperture determines the resolving power of an objective, but the total resolution of the entire microscope optical train is also dependent upon the ...