Abstract: An objective lens system particularly adapted for photography with a frontal aperture stop is provided. The lens system includes four lens components including a positive doublet. A diaphragm is provided in front of the first lens component and the lens system satisfies the following relationships;1.84 > N.sub.1 > 1.6 (1) 65 > V.sub.1 > 44 (2)0.7 > d.sub.1 + d.sub.2 /.SIGMA.d > 0.3 (3) 1.5 > .vertline.R.sub.3 .vertline./R.sub.4 > 0.5 (4)0.25 > .vertline.n.sub.3 - n.sub.4 .vertline. > 0.03 (5) 25 > .vertline.V.sub.3 - V.sub.4 .vertline. > 5 (6)0.0005 > n.sub.4 - n.sub.3 /r.sub.6 < 0.0035 (7) 1.0 > R.sub.5 /R.sub.7 > 3.0 (8)wherein, N, is the index of refraction, V is the Abbe number, D are axial distances along the optical axis and R is the radii of curvature. The subnumbers refer to consecutive measurements along the optical axis.

Abstract: A filter cassette is disclosed, for use in an air filtering system, which has a central supporting frame, inside of which an electrostatic filtering device is mounted, and on each side of which are carrier flanges adapted to receive separate removable filtering elements. The entire filtering cassette is designed to be removed from the air filtering system for servicing; and the filtering elements supported by the carrier flanges may be varied in different installations to suit the specific filtering needs.

Abstract: To prevent clandestine listening devices located in a key telephone subscriber station from utilizing the tip and ring leads of the subscriber station as a transmission path out of the subscriber's building, the subscriber station tip and ring leads are open circuited at the line card circuit during an on-hook or hold condition. Besides isolating the tip and ring leads from the subscriber station during hold and on-hook, the security line card circuit prevents high frequency signals from propagating down the telephone line when the subscriber station is on-hook. The security line card circuit allows the subscriber to exercise full supervisory control over the telephone lines and includes circuitry that controls the operation of audible and visual indicators of the associated key telephone.

Abstract: A temperature compensated pressure limited gauge is provided having a bourdon tube pressure sensing element. The coil is connected to a sealed cavity and has a predetermined quantity of fluid such as oil calibrated into the gauge. A solid cylinder or block of material is located within the cavity and has a relatively low volumetric thermal expansion characteristic relative to that of the fluid and of the gauge housing about the cavity. The dimensions of the block of material and housing cavity are such that their relative expansions are capable of compensating for any volumetric changes of the fluid throughout the applicable temperature range of the pressure gauge. Accordingly, a temperature compensation feature is provided that renders the pressure gauge independent of ambient thermal conditions. A fitting is included in the gauge to provide fluidic communication with the cavity.