PEBIODICITY 186 PERISCOPE Journal." The most popular American reviews and magazines of our times are "Harper's Monthly Magazine," the "At- lantic Monthly," "Scribner's Magazine," "Century Magazine," "The Cosmopoli- tan," "The Metropolitan," "Munsey's Magazine," "McClure's Magazine," "American Magazine," "Everybody's," "American Review of Reviews," "The World's Work," etc. PERIODICITY, the disposition of cer- tain things or phenomena to recur at stated periods. It denotes the regular or nearly regular recurrence of certain phenomena of animal life, such as sleep and hunger. PERIOPHTHALMUS, a genus of Gohiidae, from the coasts of the Indo- Pacific, remarkable for their prominent retractile eyes, which enable them to see in the air as well as in the water, and for their strong ventral and pectoral fins, by the aid of which they can hop freely over the ground, when they leave the water, as is their habit at ebb tide, to hunt small crustaceans. The species are few in number; but P. koelreuteri is one of the commonest fishes of the Indian Ocean. PERIOSTEUM, a dense lining mem- brane covering the whole surface of bone, except the articulations, which have a thin cartilaginous layer. As long as a single portion of periosteum remains alive bone is capable of being reproduced. PERIOSTITIS, inflammatior of the periosteum, a painful ailment frequently brought on by sudden exposure to cold after being heated. PERIPATETIC, the name given to the followers of the Aristotelian philoso- phy. Aristotle partly adopted the re- sults of Plato, and made them available for the world. Both teachers admitted that science could only be formed from Universals, but Aristotle took the aew afterward called Nominalist, and con- tended that such Universals were noth ing more than inductions from particular facts. He thus made experience the basis of all science. In the Middle Ages, Albertus Magnus (1193-1280) did much to spread the Peripatetic philosophy, as well as the ethical and physical writings of Aristotle, and his pupil, St. Thomas Aquinas^ (1227-1274), the greatest of the Scholastics, was much influenced by them. The study of the works of Aris- totle was greatly revived in the 19th cen- tury, and those of St. Thomas Aquinas were specially recommended to clerical students by Pope Leo XIIL PERIPATUS, the sole genus of thcJ group Peripatidea or the order Onyco- phora. They are vermiform animals, in- distinctly segmented, with soft integu- ments. On each side of the body there are a number of short legs, terminated by a rudimentary jointed part, and a pair of hooked claws. The head bears a pair of simple annulated antennae, and a pair of simple eyes. They are vivipa- rous, nocturnal in habit, and are found in decaying wood. Several species are known, from the West Indies, the Cape of Good Hope, South America, and New Zealand. PERIPHERY. See CIRCUMFERENCE, PERISCOPE, an instrument for ob- servation from a concealed position. In its most elementary form it is a tube in each end of which are reflecting surfaces set parallel to each other at an angle of 45° with the axis of the tube. This form of periscope, with the addition of a simple lens, v^as much used for observation pur- poses in the trenches during the World War. Hood. (ik^oejEcTtve; (POPRISM Inmer tube (Ut)LEN& OuterTobe (b)i*o I GeARiN^ CnECTlN&P(?lSrt WHeei. Pop -—Rotating TUBE >^PiEce DIAGRAM OF PERISCOPE The principle of the periscope of a submarine is essentially the same — re- flecting prisms and several lenses are used, and a collecting eye piece is placed at the lower end. This type of periscope is protected by a casing tube and head. A system of gears is used to turn the inner tube, in order that it may be trained upon a desired point. Although the periscope in some form
