of the moon. Other experiments followed, and the average of all the results was that the light of the sun was about 300,000 times the average light of a full moon, both being viewed in the heavens at the same altitudes. The details will be found in Bouguer’s Traité d’optique. W. H. Wollaston in 1829 tried a series of experiments in which the ratio 801,072 was obtained; but the omission of certain necessary precautions vitiates the result (Phil. Trans. 1829). Bond (Mem. Amer. Acad. 1861, p. 295) adopted a different process. He formed the image of the sun on a silvered globe of some 10 in. diameter; the light of this image was reflected on to a small mercurial thermometer bulb, and then this second image was compared with a Bengal light so moved that the lights appeared to be equal. The same process was adopted with the full moon instead of with the sun. The result was that the sun’s light was 470,980 times that of the moon. Seidel long before this date had compared the light of the mean full moon with that of Jupiter in mean opposition; his result is 6430. So also this light of Jupiter was found to be ⋅4864 times that of Venus at her brightest; and Jupiter was found to give 8⋅2 times the light of α Lyrae. If, then, these numbers could be accepted with confidence, we should have the means of comparing the light received from the sun with that received from any of the stars. Adopting these precarious numbers on the authorities of Bond and Seidel we have the following results:—
| Sun’s light | = | 470,980 | that of | the full moon. |
| ,, | = | 622,600,000 | ,, | Venus at her brightest. |
| ,, | = | 302,835,000 | ,, | Jupiter at mean opposition. |
| ,, | = | 5,970,500,000 | ,, | Sirius. |
Lastly, Bouguer, by comparing the light of the full moon viewed at different altitudes with an artificial light, found that the atmosphere absorbs ⋅1877 of the light incident on it at the zenith of any place. Professor Pritchard, from photometric measures taken at Cairo, found this number to be ·157. At Oxford it was ⋅209. Thus Bouguer’s determination indicates an absorptive capacity in the atmosphere of Brittany just midway between those of Oxford and Cairo. Seidel at Munich expresses “surprise” at finding his own results so nearly accordant with Bouguer’s. Although rather outside the domain of photometry in the strict sense, a word or two may be said here about recent attempts to measure the heat received from the stars, the first being made with the “radio-micrometer” of C. V. Boys. (Proc. Roy Soc. 1890). This is an extremely delicate instrument for Very little Heat from the Stars. measuring radiant heat, and consists of a very light thermo-electric circuit (two tiny bars of antimony and bismuth soldered together at one edge, the outer edges being connected by a hoop of copper wire) suspended by a quartz fibre (a torsion fibre of the very greatest sensitiveness) in a strong magnetic field. A minute quantity of radiant heat falling on one of the junctions of the circuit sets up a current in the circuit, which thus rotates in the magnetic field until brought to rest by the torsion of the fibre. For use on the heavenly bodies the radiant heat is collected to focus by a reflecting telescope (an object-glass would absorb it), and when the telescope is pointed to the moon the varying radiation from different parts of the disk is beautifully shown. No heat comes from the unlit portion, and of the illuminated portion the maximum is obtained from near the limb. But when pointed to the brightest stars no indications were obtained, although the instrument is sensitive enough to detect the heat from a candle more than a mile off. It seems certain that indications of heat from the stars obtained by previous observers must be spurious It is also manifest that to obtain satisfactory results even more sensitive apparatus must be devised, and by using a radiometer and the powerful resources of the Yerkes Observatory E. F. Nichols succeeded in 1898 and 1900 in obtaining indications of heat from Arcturus and Vega, as well as from Jupiter and Saturn (Astrophysical Journ. xiii., 101), the heat received being comparable with that from a candle 6 m. away We may place alongside this result that obtained by W. J. Dibdin (Proc. Roy. Soc. April 1892), who compared candlelight with twenty-one stars ranging to the sixth magnitude, and found the light of a second magnitude star equal to that of a candle at 1260 ft. (H. H. T.)
PHRAATES (Phrahates; Pers. Frahāt, modern Ferhāt), the name of five Parthian kings.
1. Phraates I., son of Priapatius, reigned c. 175–170 B.C. He subdued the Mardi, a mountainous tribe in the Elburz (Justin xli. 5; Isid. Charac. 7). He died young, and appointed as his successor not one of his sons, but his brother Mithradates I. (Justin xli. 5).
2. Phraates II., son of Mithradates I., the conqueror of Babylonia, reigned 138–127. He was attacked in 130 by Antiochus VII. Sidetes, who, however, in 129 was defeated and killed in a great battle in Media, which ended the Seleucid rule east of the Euphrates (see Seleucid Dynasty). Meanwhile the kingdom was invaded by the Scythians (the Tochari of Bactria), who had helped Antiochus. Phraates marched against them, but was defeated and killed (Justin xlii. 1; Johannes Antioch, fr. 66).
3. Phraates III., “the God” (Phlegon, fr. 12 ap. Photius cod. 97 and on some of his coins), succeeded his father, Sanatruces, in 70 B.C., at the time when Lucullus was preparing to attack Tigranes of Armenia, who was supreme in western Asia and had wrested Mesopotamia and several vassal states from the Parthian kingdom. Naturally, Phraates declined to assist Mithradates of Pontus and Tigranes against the Romans (see Tigranes). He supported his son-in-law, the younger Tigranes, when he rebelled against his father, and invaded Armenia (65 B.C.) in alliance with Pompey, who abandoned Mesopotamia to the Parthians (Dio. Cass. xxxvi. 45, 51; Appian, Mithr. 104; Liv. Epit. 100). But Pompey soon overrode the treaty, he acknowledged the elder Tigranes, took his son prisoner, occupied the vassal states Gordyene and Osroene for the Romans, and denied the title of “king of kings,” which Phraates had adopted again, to the Parthian king (Plut. Pomp. 33, 38; Dio. Cass. xxxvii. 5 seq.). About 57 Phraates was murdered by his two sons, Orodes I. and Mithradates III.
4. Phraates IV., son of Orodes I., by whom he was appointed successor in 37 B.C., after the death of Pacorus. He soon murdered his father and all his thirty brothers (Justin xlii. 5; Plut. Crass. 33; Dio Cass. xlix. 23). He was attacked in 36 by Antonius (Mark Antony), who marched through Armenia into Media Atropatene, and was defeated and lost the greater part of his army. Believing himself betrayed by Artavasdes, king of Armenia, he invaded his kingdom in 34, took him prisoner, and concluded a treaty with another Artavasdes, king of Atropatene. But when the war with Octavianus Augustus broke out, he could not maintain his conquests, Phraates recovered Atropatene and drove Artaxes, the son of Artavasdes, back into Armenia (Dio. Cass. xlix. 24 sqq., 39 seq, 44, cf. li. 16; Plut. Antonius, 37 seq.). But by his many cruelties Phraates had roused the indignation of his subjects, who raised Tiridates II. to the throne in 32. Phraates was restored by the Scythians, and Tiridates fled into Syria. The Romans hoped that Augustus would avenge the defeat of Crassus on the Parthians, but he contented himself with a treaty, by which Phraates gave back the prisoners and the conquered eagles (20 B.C., Mon. Anc. 5, 40 sqq.; Justin xlii. 5); the kingdom of Armenia also was recognized as a Roman dependency. Soon afterwards Phraates, whose greatest enemies were his own family, sent five of his sons as hostages to Augustus, thus acknowledging his dependence on Rome. This plan he adopted on the advice of an Italian concubine whom he made his legitimate wife under the name of “the goddess Musa”; her son Phraates, commonly called Phraataces (a diminutive form), he appointed successor. About 4 B.C. he was murdered by Musa and her son (Joseph. Ant. xviii. 2, 4).
5. Phraates V., or Phraataces, the younger son of Phraates IV. and the “goddess Musa,” with whom he is associated on his coins Under him a war threatened to break out with Rome about the supremacy in Armenia and Media. But when Augustus sent his adopted son Gaius Caesar into the east in order to invade Parthia, the Parthians preferred to conclude a
