Mr. Ennis speaks of Capella as "blue" (classing it with Rigel), and comparing its colour with that of Vega![350] But the present writer has never seen it of this colour. To his eye it seems yellowish or orange. It was called red by Ptolemy, El Fergani, and Riccioli; but Al-Sufi says nothing about its colour. Of β Ursæ Minoris, Heis, the eminent German astronomer said, "I have had frequent opportunities of convincing myself that the colour of this star is not always equally red; at times it is more or less yellow, at others most decidedly red."[351] Among double stars there are many cases in which variation of colour has been suspected. In some of these the difference in the recorded colour may possibly be due to "colour blindness" in some of the observers; but in others there seems to be good evidence in favour of a change. The following may be mentioned:-- η Cassiopeiæ. Magnitudes of the components about 4 and 7½. Recorded as red and green by Sir John Herschel and South; but yellow and orange by Sestini. ι Trianguli. Magnitudes 5½ and 7. Secchi estimated them as white or yellow and blue; but Webb called them yellow and green (1862). γ Leonis, 2 and 3½. Sir William Herschel noted them white and reddish white; but Webb, light orange and greenish yellow. 12 Canum Venaticorum, 2½ and 6½. White and red, Sir William Herschel; but Sir John Herschel says in 1830, "With all attention I could perceive no contrast of colours in the two stars." Struve found them both white in 1830, thus agreeing with Sir John Herschel. Sestini saw them yellow and blue in 1844; Smyth, in 1855, pale reddish white and lilac; Dembowski, in 1856, white and pale olive blue; and Webb, in 1862, flushed white and pale lilac. On October 13, 1907, Nova Persei, the great new star of 1901, was estimated to be only 11·44 magnitude, or about 11½. When at its brightest this famous star was about zero magnitude; so that it has in about 6 years faded about 11½ magnitudes in brightness; in other words, it has been reduced to 1/40000 of its greatest brilliancy! CHAPTER XVII Nebulæ and Clusters In his interesting and valuable work on "The Stars," the late Prof. Newcomb said-- "Great numbers of the nebulæ are therefore thousands of times the dimensions of the earth's orbit, and most of them are thousands of times the dimensions of the whole solar system. That they should be completely transparent through such enormous dimensions shows their extreme tenuity. Were our solar system placed in the midst of one of them it is probable that we should not be able to find any evidence of its existence"! Prof. Perrine thinks that the total number of the nebulæ will ultimately be found to exceed a million.[352]