In contrast with the above, the same astronomers have discovered a star in Perseus which seems to vary from about the 6th to the 7th magnitude in the very long period of 7½ years! It is now known as X Persei, and its position for 1900 is R.A. 3{h} 49{m} 8{s}, Dec. N. 30° 46', or about one degree south-east of the star ζ Persei. It seems to be a variable of the Algol type, as the star remained constant in light at about the 6th magnitude from 1887 to 1891. It then began to fade, and on December 1, 1897, it was reduced to about the 7th magnitude. On the night of August 20, 1886, Prof. Colbert, of Chicago, noticed that the star ζ Cassiopeiæ increased in brightness "by quite half a magnitude, and about half an hour afterwards began to return to its normal magnitude."[328] This curious outburst of light in a star usually constant in brightness is (if true) a very unusual phenomenon. But a somewhat similar fluctuation of light is recorded by the famous German astronomer Heis. On September 26, 1850, he noted that the star "ζ Lyræ became, for a moment, _very bright_, and then again faint." (The words in his original observing book are: "ζ Lyræ wurde einen _Moment sehr hell_ und hierauf wieder dunkel.") As Heis was a remarkably accurate observer of star brightness, the above remark deserves the highest confidence.[329] The variable star known as the V Delphini was found to be invisible in the great 40-inch telescope of the Yerkes Observatory on July 20, 1900. Its magnitude was, therefore, below the 17th. At its maximum brightness it is about 7½, or easily visible in an ordinary opera-glass, so that its range of variation is nearly, or quite, ten magnitudes. That is, its light at maximum is about 10,000 times its light at minimum. That a sun should vary in light to this enormous extent is certainly a wonderful fact. A variable discovered by Ceraski (and numbered 7579 in Chandlers' Catalogue) "had passed below the limit of the 40-inch in June, 1900, and was, therefore, not brighter than 17 mag."[330] The late Sir C. E. Peck and his assistant, Mr. Grover, made many valuable observations of variable stars at the Rousden Observatory during many years past. Among other interesting things noted, Peck sometimes saw faint stars in the field of view of his telescope which were at other times invisible for many months, and he suggested that these are faint variable stars with a range of brightness from the 13th to the 20th magnitude. He adds, "Here there is a practically unemployed field for the largest telescopes." Considering the enormous number of faint stars visible on stellar photographs the number of undiscovered variable stars must be very large. Admiral Smyth describes a small star near β Leonis, about 5' distant, of about 8th magnitude, and dull red. In 1864 Mr. Knott measured a faint star close to Smyth's position, but estimated it only 11·6 magnitude. The Admiral's star would thereupon seem to be variable.[331]