DISCOTERIES IN THE CELESTIAL SPACES. 713 



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 cannot 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 matter beyond the outermost circle of the planetary 

 bodies, and beyond the orbit of any comet. In the same period 

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

 suddenly appeared in Cassiopea, Cygnus, and Ophiuchus, 

 David Fabricius, pastor at Ostell in East Friesland (the 

 father of the discoverer of the sun's spots), in 1596, and 

 Johann Bayer at Augsburg in 1 603, observed in the neck 

 of the constellation Cetus, another star which again dis- 

 appeared, whose changing brightness was first recognised by 

 Johann Phocylides Holwarda, 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 discove- 

 ries.* 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 manner 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. 



* Annuaire du Bureau des Longitudes pour I'an 1842, pp. 312-353 

 (Etoiles changeantes ou periodiques). In the seventeenth century 

 there were recognised, as variable stars, besides Mira Ceti (Holwarda, 

 1638), a Hydra (Montanari, 1672), /3 Persei or Algol, and x Cygni 

 (Kirch, 1686). On what Galileo calls nebulae, see his Opere, t. ii. p. 15, 

 and Nelli, Vita, vol. ii. p. 208. Huygens, in the Systema Saturninum, 

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

 nebulae generally : " Cui certe simile aliud nusquam apud reliquas 

 tixas potui animadvertere. Nam ceterse nebulosse olim existimatse 

 atque ipsa via lactea, perspicillis inspects, nullas nebulas habere com- 

 periuntur, neque aliud esse quam plurium stellarum congeries et fre 

 quentia." It is seen from this passage that the nebula in Andromeda, 

 which was first described by Marius, had not been attentively considered 

 by Huygena any more than by Galileo. 



