In that interesting work _A Cycle of Celestial Objects_, Admiral Smyth says (p. 275), "Geminiano Montanari, as far back as 1670, was so struck with the celestial changes, that he projected a work to be intituled the _Instabilities of the Firmament_, hoping to show such alterations as would be sufficient to make even Aristotle--were he alive--reverse his opinion on the incorruptibility of the spangled sky: 'There are now wanting in the heavens,' said he, 'two stars of the 2nd magnitude in the stem and yard of the ship Argo. I and others observed them in the year 1664, upon occasion of the comet that appeared in that year. When they first disappeared I know not; only I am sure that on April 10, 1668, there was not the least glimpse of them to be seen.'" Smyth adds, "Startling as this account is--and I am even disposed to question the fact--it must be recollected that Montanari was a man of integrity, and well versed in the theory and practice of astronomy; and his account of the wonder will be found--in good set Latin--in page 2202 of the _Philosophical Transactions_ for 1671." There must be, I think--as Smyth suggests--some mistake in Montanari's observations, for it is quite certain that of the stars mentioned by Ptolemy (second century A.D.) there is no star of the 2nd magnitude now missing. It is true that Al-Sufi (tenth century) mentions a star of the _third_ magnitude mentioned by Ptolemy in the constellation of the Centaur (about 2° east of the star ε Centauri) which he could not find. But this has nothing to do with Montanari's stars. Montanari's words are very clear. He says, "_Desunt in Cœlo duæ stellæ_ Secundæ Magnitudinis _in_ Puppi Navis _ejusve Transtris_ Bayero β et γ, _prope_ Canem Majoris, _à me et aliis, occasione præsertim Cometæ_ A. 1664 _observatæ et recognitæ. Earum Disparitionem_ cui Anno debeam, non novi; _hoc indubium, quod à die_ 10 April, 1668, _ne_ vestigium quidem _illarum adesse amplius observe; cæteris circa eas etium quartæ et quintæ magnitudinis, immotis._" So the puzzle remains unsolved. Sir William Herschel thought that "of all stars which are singly visible, about one in thirty are undergoing an observable change."[322] Now taking the number of stars visible to the naked eye at 6000, this would give about 200 variable stars visible at maximum to the unaided vision. But this estimate seems too high. Taking all the stars visible in the largest telescopes--possibly about 100 millions--the proportion of variable stars will probably be much smaller still. The theory that the variation of light in the variable stars of the Algol type is due to a partial eclipse by a companion star (not necessarily a dark body) is now well established by the spectroscope, and is accepted by all astronomers. The late Miss Clarke has well said "to argue this point would be _enforcer une porte ouverte_."