76 Things not generally Known. 



our colossal telescopes. Struve assumes for Herschel's 20-feet 

 reflector, that a magnifying power of 180 would give 5,800,000 

 for the number of stars lying within the zones extending 30 

 on either side of the equator, and 20,374,000 for the whole 

 heavens. Sir William Herschel conjectured that 18,000,000 of 

 stars in the Milky Way might be seen by his still more powerful 

 40-feet reflecting telescope. Humboldt's Cosmos, vol. iii. 



The assumption that the extent of the starry firmament is 

 literally infinite has been made by Dr. Gibers the basis of a 

 conclusion that the celestial spaces are in some slight degree 

 deficient in transparency ; so that all beyond a certain distance 

 is and must remain for ever unseen, the geometrical progres- 

 sion of the extinction of light far outrunning the effect of any 

 conceivable increase in the power of our telescopes. Were it 

 not so, it is argued that every part of the celestial concave 

 ought to shine with the brightness of the solar disc, since no 

 visual ray could be so directed as not, in some point or other of 

 its infinite length, to encounter such a disc. Edinburgh Re- 

 view, Jan. 1848. 



STAKS THAT HAVE DISAPPEARED. 



Notwithstanding the great accuracy of the catalogued po- 

 sitions of telescopic fixed stars and of modern star-maps, the 

 certainty of conviction that a star in the heavens has ac- 

 tually disappeared since a certain epoch can only be arrived at 

 with great caution. Errors of actual observation, of reduction, 

 and of the press, often disfigure the very best catalogues. The 

 disappearance of a heavenly body from the place in which it 

 had been before distinctly seen, may be the result of its own 

 motion as much as of any such diminution of its photometric 

 process as would render the waves of light too weak to excite 

 our organs of sight. What we no longer see, is not necessarily 

 annihilated. The idea of destruction or combustion, as ap- 

 plied to disappearing stars, belongs to the age of Tycho Brahe. 

 Even Pliny makes it a question. The apparent eternal cosmical 

 alternation of existence and destruction is not annihilation ; 

 it is merely the transition of matter into new forms, into com- 

 binations which are subject to new processes. Dark cosmical 

 bodies may by a renewed process of light again become lu- 

 minous. Humboldt's Cosmos, vol. iii. 



THE POLE-STAB FOUR THOUSAND YEARS AGO. 



Sir John Herschel, in his Outlines of Astronomy, thus shows 

 the changes in the celestial pole in 4000 years : 



At the date of the erection of the Pyramid of Gizeh, which precedes 

 the present epoch by nearly 4000 years, the longitudes of all the stars 



