Superintendent of Signalling of the Pennsylvania Railroad (PRR). They were introduced in 1915 as a replacement for semaphore signals on the Main Line between Paoli and Philadelphia as an effort to reduce the maintenance required by semaphore signals as well as visibility problems caused by the new overhead electrification project. The original system used rows of four lights. The system was later reduced to use rows of three lamps, surrounding a common center. This reduced the "sail" effect of the inordinately large and tombstone shaped background of the four-light variant. The original installation made use of lamps positioned in front of a free standing black sheet-iron backing, but shortly thereafter, the new circular background was fitted to the then reduced 3 lamp per row device and directly to the backing on a framework referred to as a "spider." Each position lamp unit is equipped with a 12 volt, 6 candlepower bulb mounted in front of a parabolic mirror that increases the relatively weak bulb's intensity. To avoid phantom indications the design makes use of a special inverted toric lens (i.e. a single clear Fresnel lens mounted step sides outwards) with a portion of the lens steps painted black. A light yellow tinted conical glass with frosted tip was chosen, as this color was determined to have the highest visibility under fog conditions based on empirical studies at Corning at that time. A standard high position light consists of two heads; the bottom head can remain dark unless it is needed. In addition to the high position light signals the PRR developed a dwarf position light, as with many railroads, these dwarf signals are also referred to as a "pot," a tradition carried over from the 19th century revolving "Pot Type Signal." Four plain white lamps are able to display four low-speed aspects each with two lamps. In 1930, close clearances of the Philadelphia Suburban Station complex spurred development of the pedestal-type position, which consisted of two position dwarf signals in a common cast backing. PRR type position lights were used throughout the vast PRR system as well as the Long Island Rail Road (LIRR), a PRR subsidiary, and the Norfolk and Western, which was one-third-owned by the PRR. US&S was the sole supplier of classic position light equipment as this manufacturer's factory was formerly located on the four track mainline of the P.R.R. in Swissvale, Pa. In 1954, the PRR