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for aerodynamic testing of airflow. This optical system detects changes within a test area medium (air) and records the changes in the form of an image on a screen. The image is formed by refraction and scattering from what is introduced into the test area, which are areas of varying refractive index. This film was made in 1958. Schlieren (from German; singular "Schliere", meaning "streak") are optical inhomogeneities in transparent material not necessarily visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses devoid of these inhomogeneities. These inhomogeneities are localized differences in optical path length that cause light deviation. This light deviation can produce localized brightening, darkening, or even color changes in an image, depending on which way the ray deviates. Schlieren flow visualization is based on the deflection of light by a refractive index gradient The index gradient is directly related to flow density gradient. The deflected light is compared to undeflected light at a viewing screen. The undisturbed light is partially blocked by a knife edge. The light that is deflected toward or away from the knife edge produces a shadow pattern depending upon whether it was previously blocked or unblocked. This shadow pattern is a light-intensity representation of the expansions (low density regions) and compressions (high density regions) which characterize the flow. Opening: exterior shot of the Illinois State Museum, Springfield, IL. Titles: "Schlieren A Shell Film" (:06-1:09). Parallel wires. Glass is waved in front of the wires. A new system was created: the Schlieren system was created to find streaks in glass. We see the Schlieren system and how it works - moving the glass by hand in front of the lines. Testing glass in front of the lines. Color Schlieren is shown (1:10-3:07). A high speed wind tunnel experiment. Men observe and photograph. Different colors mean different things. Schlieren system is used to observe. The principal of the Schlieren system is explained and shown with a light. A man adjusts a mirror and positions it. A beam of light is shown in diagram. A camera is put into position by a man. A diagram shows what will happen with the lens. A man looks through the camera lens. Two shadowy hands make a shadow dog on the wall. A man adjusts a camera as another person enters the room in a birdseye pov shot. A man puts a black line on a piece of wood (3:08-7:18). An animated diagram explains air density. A shockwave is explained and shown in an animated diagram. Bent light is shown. A color filter is held by two hands. It is then placed in front of a camera lens. Animation shows bent light. Red shows increasing density. A light experiment shown in animation (7:19-10:34). Flow patterns are compared at different sound speeds. Schlieren shows a flow pattern. A model of an airplane is shown in an airflow at nearly twice the speed of sound. It is moved around on a stick. Schlieren system can be invaluable (10:35-13:56). Temperature variations are shown as air warmed by shadowy hands is pictured. Air cooled by a mouth, licking ice cream, is shown in shadow. A bottle has a colorless heavy gas and the Schlieren system shows it. Flames can be studied under the Schlieren system (13:57-14:58). Sugar put into water under the Schlieren system - sugar sinks to the bottom of the glass. A spray is shown under the Schlieren system. The effect of pouring clear liquid into another (14:59-16:20). A shockwave is shown slowed down in the Schlieren system. Warm water on the left, cold on the right shown under the Schlieren system. The two waters meet and this can be clearly seen (16:21-18:02). A person plays a clarinet and under the Schlieren system, air can be seen pouring out of the holes. A squirt gun shoots water onto the instrument (18:03-18:17). Fade to black, no end credits (18:18-18:19). We encourage viewers to add comments and, especially, to provide additional information about our videos by adding a comment! See something interesting? Tell people what it is and what they can see by writing something for example: "01:00:12:00 -- President Roosevelt is seen meeting with Winston Churchill at the Quebec Conference." This film is part of the Periscope Film LLC archive, one of the largest historic military, transportation, and aviation stock footage collections in the USA. Entirely film backed, this material is available for licensing in 24p HD, 2k and 4k. For more information visit http://www.PeriscopeFilm.com