increase in gain but very little in the way of resolution. Two other tube developments that occurred but were not widely manufactured included optical relays to invert the image both in front and behind a Gen3 25mm tube. This allowed for significant increases in tube resolution and performance over the FO based third generation tubes and involved a lens assembly being housed in a container that matched the MX-9644 tube exactly. Related development for driving based devices included Fiber Optic expanders that magnified the image and could be retrofitted to driving scopes that also used this tube. Operation Operation of the device is through two controls located on the left side of the tube housing. The lower control is an on/off switch for the image intensifier tube and secondarily controls the gain of the tube, allowing the operator to control the brightness of the image. The upper control is an on/off switch for the illuminated reticle and secondarily allows the brightness of the reticle to be adjusted. Elevation and Windage adjustment is made through turrets located on the objective lens. Each click equals 1/4 moa. Day/night adapter The AN/PVS4 also has a Day/Night adapter that allows sighting in of the units during the day time or for daytime operation of the sight. Under extremely bright nighttime conditions, it may be necessary to use this adapter. The adapter consists of three hole sizes spaced evenly around the periphery These pass by three larger aperture holes and three dark glass lenses to allow for six different levels of light reduction into the scope. As a result, the AN/PVS-4 can be used in lighting conditions ranging from midday-sun in the desert, through to very-bright-moonlight and high levels of urban lighting. Under moonlight or lower conditions, the AN/PVS-4 can be used without the adaptor and the ABC or Automatic Brightness Controls will reduce the gain, although the image may be too bright for the reticle to show correctly. When used, the image of the AN/PVS-4 is reduced and appears somewhat triangular. Due to the small apertures and resulting light diffraction, focusing behavior differs from normal operation due to the increased depth of field. Limitations In December 1974, the final AN/PVS-4 Development Test recorded the various shortcomings found during field testing. The majority of the issues were addressed in final production. Issues not addressed were deemed permissible. Several issues noted were reliability, specifically with intensifier