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The Phi phenomenon, also called the persistence of vision, is a term used for an optical fact that an image seen by they eye is retained for a split second upon the retina of the eye after the image is withdrawn. This phenomenon fueled a number of inventions during the 19th century.

The concept of the persistence of vision was demonstrated by an English physician named John Ayrton Paris in 1824. He invented an apparatus called thaumatrope. Thaumatrope consisted of a disc with two images, one on each side, and two strings, which served as handles. When the handles were turned, the disc spun and the two images combined together into one.

Another apparatus, called the phenakistiscope, or stroboscope, was invented by a Belgian scientist named Joseph Plateau and independently by Professor Simon Ritter Von Strampfer. Phenakistiscope, invented by Joseph Plateau, consisted of a disc mounted on a spindle. The animation was observed through the slots with the images facing the mirror. The stroboscope, invented by Von Stampfer, had two discs, one with holes and another one with images, so it was possible to see images without the mirror when the two disks were spun simultaneously.

The third invention, called the zoetrope, was invented by William Horner and patented in 1834. It was made up of a rotating cylinder on a stand. Holes were made on the sides of the cylinder and the images were placed inside the cylinder. As the cylinder rotated, the images came to life.

Then came the praxinoscape. It was invented by a French inventor named Emile Reynaud in 1878 and was similar to the zoetrope. The major problem of the zoetrope was the fact that not enough light entered the cylinder, which made images hard to see. Reynaud fixed that problem by placing a candle in the middle of the cylinder and using mirrors instead of holes. Praxinoscape also had surrounding images to make the experience more complete.

Another similar apparatus was commonly referred to as the “magic lantern film”. The inventor of the magic lantern is unknown. A lamp was placed inside the body of the magic lantern and a reflector reflected the light from the lamp towards the condensing lens, which focused the light onto the slide being projected. A roll of slides was placed above the body and a handle on the bottom was used to roll the slides down. A lens in front of the slides magnified each slide so the audience could see it. Some slides had to be doubled or tripled to get the right amount of frames per second.

The sixth invention based on the phi phenomenon was the Cathode Ray Tube (CRT). The first CRT was invented by a German scientist named Karl Ferdinand Braun, in 1897. The Cathode Ray Tube consisted of a glass tube, blocked off at each end, with outlets coming out at each end. This tube was connected to a vacuum. Two electrodes, one on each side of the tube, were connected to the opposite charged wires. The cathode was on the left, while the anode was on the right. A fluorescent screen, known as the cathode-ray oscilloscope, was placed in between the electrodes. This screen would emit light when struck by electrons, as well as keep the electrons closer together. When the two plates were charged with appropriate electric charges, the electrons emitted from the negatively charged electrodes went towards the positively charged electrode. Different colors could be achieved by filling the tube with different gases.

The Cathode Ray Tube was further improved by many different scientists, engineers, and inventors and became what is known today as the television and the monitor. These two apparatuses have an electron gun at the end. The electron gun shoots out electrons, which pass through the intensity control, passes through the horizontal and vertical deflection yokes, and reaches the fluorescent coating. This results in a dot, referred to as a pixel, or a pel, to appear on the screen. The electron beam starts at the upper-left corner towards the upper-right corner. Upon reaching the right side, the trace is blanked out and the electron beam is repositioned to the left side of the screen, this time one line below from the first trace. This continues until the beam has reached the bottom part of the screen, at which point it starts the whole process all over again. A typical type it takes for the electron beam to complete the entire field (the whole screen) is approximately 1/60 of a second, about 63 microseconds for a single sweep. With the improvement of televisions and monitors, color displays came about. These devices use three electron guns, Red Green and Blue (RGB). All of them target different pixels, and a Shadow Mask is placed in between them and the fluorescent coating to ensure that they hit the right pixel.

In conclusion, the instruments based on the persistence of vision have come a long way. It all started with a simple, two-frame thaumatrope and developed into a short film projector called the magic lantern film. Because of the Phi phenomenon, the people of today can enjoy cartoons and movies.


Optical Toys. November 20, 2001 http://web.inter.nl.net/users/anima/optical/index.htm.
Exhibit of Optical Toys. November 20, 2001 http://phywww1.ncssm.edu/gallery/collections/toys/opticaltoys.htm.
US patent 64,117. November 20, 2001 http://home.hccnet.nl/c.lucassen/patents/64117.htm.
History of the Cathode Ray Tube. March 01, 2002 http://inventors.about.com/library/inventors/blcathoderaytube.htm
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