330 cosmos. 



pie. In our own time, the intersecting orbits of the small 

 planets between Mars and Jupiter, the interior comets, which 

 were first proved to be such by Encke, and the swarms of 

 falling stars associated with definite days (since we can not 

 regard these bodies in any other light than as such cosmical 

 masses moving with planetary velocity), have enriched our 

 views of the universe with a remarkable abundance of new 

 objects. 



During the age of Kepler and Galileo, our ideas were very 

 considerably enlarged regarding the contents of the regions of 

 space, or, in other words, the distribution of all created mat- 

 ter beyond the outermost circle of the planetary bodies, and 

 beyond the orbit of any comet. In the same period in which 

 (1572-1604) three new stars of the first magnitude suddenly 

 appeared in Cassiopeia, Cygnus, and Ophiuchus, David Fa- 

 bricius, pastor at Ostell, in East Friesland (the father of the 

 discoverer of the sun's spots), in 1596, and Johann Bayer, at 

 Augsburg, in 1603, observed in the neck of the constellation 

 Cetus another star, which again disappeared, whose changing 

 brightness was first recognized by Johann Phocylides Holwar- 

 da, professor at Franeker (in 1638 and 1639), as we learn 

 from a treatise of Arago, which has thrown much light on the 

 history of astronomical discoveries.* The phenomenon was 

 not singular in its occurrence, for, during the last half of the 

 seventeenth century, variable stars were periodically observed 

 in the head of Medusa, in Hydra, and in Cygnus. The man- 

 ner in which accurate observations of the alternations of light 

 in Algol are able to lead directly to a determination of the 

 velocity of the light of this star, has been ably shown by the 

 treatise to which I have alluded, and which was published in 

 1842. 



The use of the telescope now excited astronomers to the 



* Annuaire du Bureau des Longitudes pour Van 1842, p. 312-353 (Eto~ 

 ties Changeantes ou Piriodiques). In the seventeenth century there 

 were recognized, as variable stars, besides Mira Ceti (Holwarda, 1638), 

 a Hydrse (Montanari, 1672), (3 Persei or Algol, and x Cygni (Kirch, 

 1686). On what Galileo calls nebula?, see his Opere, t. ii., p. 15, and 

 Nelli, Vila, vol. ii., p. 208. Huygens, in the Systema Saturninurn, re- 

 fers most distinctly to the nebula in the sword of Orion, in saying of 

 nebulas generally, " Cui certe simile aliud nusquam apud reliquas fixas 

 potui animadvertere. Nam cetera? nebulosas olim existimatae atque ipsa 

 via lactea, perspicillis inspectas, nullas nebulas habere comperiuntur, 

 neque aliud esse quam plurium stellarum congeries et frequentia." It 

 is seen from this passage that the nebula in Andromeda, which was 

 first described by Marius, had not been attentively considered by 

 Huygens any more than by Galileo. 



