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4 LIVING PICTURES. Bodleian Library at Oxford Ptolemy in this treatise mentions that if a sector of a disc be coloured, the whole will appear of that colour when rapidly revolved, and if the sector be variously coloured at different distances from the centre, the disc will appear ringed. Alhazen, the great Arabian philosopher, refers to the subject about A.D. i lOO, as do others, including Leonardo da Vinci, who was born in 1452. Coming to later years, Newton, Boyle, and others mention the matter, but little practical investigation was done except the attempts by Segner, d'Arcy, and Cavallo to measure the duration of vision after the extinction of light. To conclude the references on this subject, it is only necessary to mention that the period of persistence is now accepted as (on the average) from -^-^ to _i_ of a second, subject to the degree of intensity, duration, and colour of the light received by the eye. Up to the end of the eighteenth century no progress was made in the application of the principle of per- sistence, and the character of last-century knowledge is well summed up in Abbe Nollet's " Legons de Physique," 1765, tome $, where he says: "When an object moves very rapidly before our eyes, we often attri- bute to it size and shape which it does not possess. A polyhedron revolved on its axis seems to us a sphere; as does also a circle revolved on one of its diameters," etc., etc. This statement merely implies the knowledge that one object may be seen in more than one place at once if it move fast enough; and here may be mentioned a very popular toy of thirty odd years ago, brought out by the Stereoscopic Company under the name of " The Optic Wonder." In this a piece of wire bent to the outline of one side of a vase or the like symmetrical figure was made to revolve rapidly round its vertical axis, and thereby gave the impression of a complete vessel. As an addition a glass rod, bent