330 PERSONAL EQUATION PERSONS it is equally applicable to all scientific observa- tions where it is necessary to estimate very small portions of space or time. Where an event is of such a nature that it can itself be made to record the exact time when it took place, and simultaneously the time of its hap- pening is observed and recorded by a person, it has been found that, no matter how prac- tised and skilful the observer may be, he will always differ a little from the absolute truth. One observer will place it a little too early, another a little too late. It has also been found that these errors are habitual. An observer whose tendency is to place the time of an event too late will, on observing a great num- ber of repetitions of the same event, always place them too late. The habitual difference between the time noted by the observer and the true time is called the observer's absolute personal equation. Again, it has been found by experience that two observers equally skil- ful, using equally good instruments and observ- ing a great number of repetitions of the same event (for example, the transit of a star over the meridian), will constantly differ from each other by a small amount. If A habitually finds the time two tenths of a second too late, this two tenths of a second is A's absolute personal equation. If B habitually fixes the time three tenths of a second too late, then that amount is his absolute personal equation. The difference between these two absolute equations is called their relative personal equa- tion. Ordinarily the absolute personal equa- tion cannot be ascertained ; but as the differ- ence between two observers can be ascertained without deciding how much either of them differs from the exact truth, the relative per- sonal equation can always be found. The rela- tive personal equation of the same two ob- servers may vary according to the nature of the facts which they observe. Thus the tran- sit of a star over the meridian, or at least the process by which it is ascertained, occupies a considerable time, while the occultation of a star is instantaneous. Two observers in ob- serving transits may have a relative personal equation of a certain amount, while in observ- ing occultations it may be constantly of a dif- ferent amount. The causes of the phenomena of personal equation have given rise to much discussion. The most probable explanation seems to be as follows: The formation of every judgment requires time, and men of different organizations form judgments with different degrees of rapidity. Hence one person in ob- serving transits, for example, judges that a star is opposite one of the micrometric wires of his telescope almost at the instant that the fact occurs ; another requires a small fraction more of time to make up his mind. This small fraction of time is their relative person- al equation. The first recorded case of per- sonal equation occurs in the " Observations " for the year 1796 of Maskelyne, the astrono- mer royal of England. He says that in August, 1795, his assistant Mr. Kinnebrook began to record his observations half a second later than he should, and in 1796 about eight tenths of a second too late, and that it appeared to be im- possible for him to overcome the habit. Maske- lyne assumed that his own observations were correct, and discharged his assistant, although he says he was " diligent and useful." This was a case of personal equation, and at the pres- ent day astronomers place as much reliance upon the observations of Kinnebrook as upon those of Maskelyne. The subject has since been fully treated by Bessel in the u Konigsberg Observations " for 1822, and by Wolf in the Memoires de V Observatoire de Paris, vol. viii. PERSONS, or Parsons, Robert, an English the- ologian, born at Nether Stowey, Somersetshire, June 24, 1546, died in Rome, April 18, 1610. He was educated at St. Mary's Hall, Oxford, and Balliol college, of which he was succes- sively fellow (1568), tutor, bursar, and dean. Having become a Roman Catholic, he went to Padua in 1574, and studied medicine and civil law. He entered the society of Jesus in 1575, was sent to study in the Roman college, and there received holy orders. In 1580 he accom- panied Edmund Campian to England, and trav - elled about in various disguises ministering to his coreligionists. Campian having been im- prisoned in 1581, Persons soon afterward fled to the continent, opened a preparatory semi- nary for English youths at Eu in Normandy, became successively rector of the English col- lege at Rome and provincial of the English mis- sions, sent emissaries to the king of Scotland at Holyrood to enlist his sympathies in favor of his captive mother, and visited for the same purpose the courts of France, Rome, Portugal, and Spain. He chiefly resided in the Peninsula till 1594, and used his influence to found semi- naries for English students at Valladolid (1589), San Lucar (1591), Seville and Lisbon (1592), and St. Omer (1593). In 1594 he was reappointed rector of the English college at Rome, and re- tained that post till his death. He has been ac- cused by some writers of employing his credit at the courts of Roman Catholic sovereigns to foment conspiracies against Queen Elizabeth, an accusation supported by the tenor of his writings. His most important works are: "A brief Discours contayning certaine Reasons why Cath cliques refuse to goe to Church," printed in London, though dated at Douai (1580 ; also entitled " A Treatise on Schism ") ; De Perse- cutions Anglicana Libellus (Paris and Rome, 1582; English translation, Douai, 1582); "A Christian Directorie guiding men to Eternal Salvation" (part i., London, 1583; part ii., 1591 ; 2d ed. in modern English, 8vo, 1700, seve- ral times reprinted) ; " A Booke of Christian Ex- ercise appertaining to Resolution" (1584; "al- tered to the Protestant use," 1585, 1586, 1589, 1594, and 1609); Responsio ad Elisalethce Re- gince Edictum contra Gatholicos, claiming for the pope power to dethrone sovereigns and ab- solve subjects from their allegiance (Lyons,
