Perry Flashlight indicator

Perry Flashlight Indicator

This steam indicator of the mirror-reflecting type was invented by Prof. John Perry, F.R.S., and described by him before the Physical Society in 1891. The application of a mirror as an indicator had been in 1885 by Messrs. Clarke and Low. The indicator is furnished with an elastic diaphragm whose bulging under the varying steam pressure causes a concave mirror to move in a horizontal plane. A beam of light is thrown upon the mirror and reflected on a dark screen placed at a distance from the engine, as shown in the photograph adjacent. Persistence of vision keeps the image on the screen so that it can be traced off by hand or else be photographed