No less than 3748 variable stars had been discovered up to May, 1907. Of these 2909 were found at Harvard Observatory (U.S.A.) chiefly by means of photography.[340] The star 14. 1904 Cygni has a period of only 3 hours 14 minutes, which is the shortest period known for a variable star. A very interesting discovery has recently been made with reference to the star μ Herculis. It has been long suspected of variable light with a period of 35 or 40 days, or perhaps irregular. Frost and Adams now find it to be a spectroscopic binary, and further observations at Harvard Observatory show that it is a variable of the Algol (or perhaps β Lyræ) type. The Algol variation of light was suggested by MM. Baker and Schlesinger. The period seems to be about 2·05 days.[341] The northern of the two "pointers" in the Plough (so called because they nearly point to the Pole Star) is about the 2nd magnitude, as Al-Sufi rated it. It was thought to be variable in colour by Klein, Konkoly, and Weber; and M. Lau has recently found a period of 50 days with a maximum of "jaune rougeâtre" on April 2, 1902. The famous variable star η Argus did "not exceed the 8th magnitude" in February, 1907, according to Mr. Tebbutt.[342] This is the faintest ever recorded for this wonderful star. It is stated in _Knowledge_ (vol. 5, p. 3, January 4, 1884) that the temporary star of 1876 (in the constellation of Cygnus) "had long been known and catalogued as a telescopic star of the 9th magnitude with nothing to distinguish it from the common herd." But this is quite erroneous. The star was quite unknown before it was discovered by Schmidt at Athens on November 24 of that year. The remark apparently refers to the "Blaze Star" of 1866 in Corona Borealis, which _was_ known previously as a star of about the 9th magnitude before its sudden outburst on May 12 of that year. This "new star" of 1866--T Coronæ, as it is now called--was, with the possible exception of Nova Persei (1901), the only example of a _nova_ which was known to astronomers as a small star previous to the great outburst of light. It is the brightest of the _novæ_ still visible. It was the first of these interesting objects to be examined with the spectroscope. It was observed by Burnham in the years 1904-1906 with the great 40-inch telescope of the Yerkes Observatory (U.S.A.). He found its colour white, or only slightly tinged with yellow. In August and September, 1906, he estimated its magnitude at about 9·3, and "it would seem therefore that the Nova is now essentially of the same brightness it was before the outburst in 1866." It shows no indication of motion. Burnham found no peculiarity about its telescopic image. A small and very faint nebula was found by Burnham a little following (that is east of) the _nova_.[343]