Abstract: A slit image is projected through the optical device being measured and onto the optical input screen of a variable-gain photomultiplier tube. Before it is incident upon the input screen of the tube, the projected image is modulated to develop simultaneously a low-frequency image component and one or more higher-frequency image components. Electronic filters are employed to separate the electrical output signal of the photomultiplier tube into discrete electrical signals each representative of a different one of the image components, and one or more meters are used to indicate the relative amplitudes thereof. A phase-locked-loop detector and an amplifier are employed between each filter and meter to improve the signal-to-noise ratio of the system and increase the signal amplitude to a level suitable for driving the meter. In one embodiment, the overall gain of the system is manually adjusted until the meter reads 100 percent MTF for the low-frequency component.

Abstract: A microscope objective with correcting means comprising a front lens group arranged on the object side of a pre-determined airspace and rear lens group arranged on the image side of said airspace and arranged to correct aberrations, which will be caused by difference in thickness of the cover glass, by varying said airspace moving the rear lens group in respect to the front lens group so that a favourable image can be obtained for observation.