An electron microscope is described, which includes an electron source, a condenser lens having either a circular aperture for focusing a solid cone of electrons onto a specimen or an annular aperture for focusing a hollow cone of electrons onto the specimen, and an objective lens with an annular objective aperture, for focusing electrons passing through the specimen onto an image plane. The invention also entails a method of making the annular objective aperture, using electron imaging, electrolytic deposition, and ion etching techniques. *Japanese manufacturing rights exclusively licensed to Japan Engineering Development Company. See page 23 for address. All other foreign patent rights available from NASA. The invention relates to laser apparatus for providing an intense source of coherent laser radiation which is tunable from the visible wavelengths through the infrared wavelengths. It can be used in absorption spectroscopy and in the detection and monitoring of pollution in the atmosphere. Selected radiation of the tunable laser is passed through the non-linear mixing crystal causing it to develop radiation of a third wavelength which is transmitted out of the optical cavity through the third reflector. *Japanese manufacturing rights exclusively licensed to Japan Engineering Development Company. See page 23 for address. All other foreign patent rights available from NASA. In the preparation of polyimide foams by the reaction of polyisocyanates and polyfunctional aromatic acid derivatives, flame retardant foamed products of improved cell quality, friability, and resiliency are obtained by conducting the reaction in the presence of an alkanolamine, such as 1-hydroxyethyl-2heptadecenyl glyoxalidine. Another aspect of the invention resides in the finding that polyimide foams of reproducible density above 1 lb/cu ft and below 6 lbs/cu ft can be obtained by, employing in the reaction at least 2% by weight of silozane-glycol copolymer as a surfactant which acts as a specific density control agent. A further feature of the invention is the production of polyimide foams into which reinforcing fibers such as silicon dioxide and carbon fibers may be incorporated. *Japanese manufacturing rights exclusively licensed to Japan Engineering Development Company. Please direct inquiries for manufacturing license in Japan to: Japan Engineering Development Company Room #1004; TBR Bldg. No. 10-2, 2-chome Nagata-Cho, Chiyoda-ku Tokyo, Japan 100 All other foreign patent rights available from NASA. SHOULDER HARNESS AND LAP NASA Case No. ARC-10519 Inventor: Albert P. Garavablia, et al. Ames Research Center Canada, Japan Address inquiries to: Attn: Patent Counsel Filed 8-22-72 Issued 6-3-75 Corresponding to U.S. Patent No. 3,887,233 The present invention provides a shoulder harness which is held by inertia reels so that the user is not unduly restrained and can freely move about, yet the reels will instantly lock when the user is subjected to deceleration forces. The shoulder straps preferably pass through openings in the seat itself so that they are substantially at shoulder height and there is little chance of the straps slipping off of the shoulders. The combined restraint of the present invention provides for slippage between the lap belt and the shoulder straps so that it readily adapts itself to users of different sizes. A system for controlling and stabilizing the attitude of an artificial earth satellite includes a gravity gradient member mounted in a gimbal arrangement to have two degrees of freedom. The angular deviation of the gravity gradient member and the satellite relative to the local vertical and the spacecraft angle command input signal selectively drive a plurality of inertial momentum wheels, one for each of the three spacecraft axes, provided to dampen the gravity gradient member librations. The gravity gradient member is controlled so that the equalibrium position of the longitudinal axis thereof is maintained in alignment with the local vertical in response to signals indicative of the rate of change of movement thereof with respect to the remainder of the satellite and a signal indicative of the angular deviation of the satellite position from the angle command. |