Under certain circumstances, a flow of heat through a system can give rise to acoustic oscillations, converting some of the heat to work. Natural vibrators maintained by heat flows have been studied since the 1770s. Some of the best-known examples come from acoustics: the “singing flames” first investigated by Byron Higgins in 1777, the Sondhauss tube and the Rijke tube. Most experimenters in cryogenics have observed the “Taconis oscillations” that occur when a tube, closed at the top, is inserted into a liquid-helium dewar. A group at Tsukuba has studied such oscillations quantitatively. Oscillations driven by heat also occur on a very large scale, in certain classes of variable stars.

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Perhaps the simplest optical instruments are the Keplerian (or astronomical) telescope and the compound microscope since they consist of only two converging lenses. Physics textbooks at a variety of levels describe the operation of both instruments in remarkably similar ways: The objective lens forms a real image which is then magnified by the eyepiece.1 We describe a set of activities designed to demonstrate to students that there actually is a real image between the lenses. These activities are primarily intended for liberal-arts physics courses, but they also may be useful for other physics, astronomy, and perhaps even biology courses. The telescope and the microscope discussed are intended to illustrate principles, not to be practical instruments.2

In this paper the construction of both mechanically and optically compensated zoom lenses is discussed. Particular attention is paid to the Computer Optics Inc. Infrared Optically Compensated Zoom Lens.Zoom lenses are important for a variety of commercial and military applications. Indeed, live broadcast television would be hard pressed to operate without zoom lenses capable of framing the big picture at the wide angle setting and then magnifying into the telephoto position to show the fine detail of the specific action to the viewer. For the military, zoom lenses are used in targeting, guidance and surveillance systems as well as other equipment. Sample of industrial uses are those for machine vision, security systems and photography. Of particular interest to our discussion here is the extension of zoom lens technology into the infrared. We have completed such an extension using optical compensation techniques to produce a 3-5 micron continuous zoom lens. It is our belief that in the next few years we will see a certain percentage of the infrared lens systems move to zoom lenses for the ease of switching between situational awareness and detailed analysis without losing the frame between these two extremes. The relatively high index of materials in the infrared when compared to the visible opens up the possibility of making well corrected zoom lenses using the technique of optical compensation.For the purposes of this paper, a zoom lens will be defined as a compact optical system whose focal length may be varied while keeping its image plane fixed in space. Recent developments in electronics, directed towards automatic focusing,

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