THE UNIVERSE. DISCOVERIES IN THE CELESTIAL SPACES. 327 



again, the varying brightness of which, however, as Arago 

 has shew* in an important memoir on the lustory of astro- 

 nomical discovery, ( 503 ) .was first recognised by Johannes 

 Phocylides Holwarda, professor at Franeker, in 1638 and 

 1639. Other phenomena of the same class were observed, 

 in the latter half of the 17th century ; stars of periodically 

 variable brilliancy were discovered in the head of Medusa, 

 in Hydra, and in Cygnus. In the memoir of Arago in 

 1842 above referred to, it is very ingeniously shewn, how 

 exact observations of the change of light of Algol might lead 

 directly to the determination of the velocity of the light of 

 that star. 



The use of the telescope now stimulated astronomers to 

 the observation of another class of phaenomena, some of 

 which could not escape the notice even of the unassisted eye. 

 Simon Marius described the nebula in Andromeda in 1612, 

 and in 1656 Huygens drew a sketch of the nebula in the 

 sword of Orion. These two nebulse may serve as types of 

 different states of condensation, more or less advanced, of 

 the nebulous cosmical matter. Marius, in comparing the 

 nebula of Andromeda with the light of a taper seen through 

 a semi-transparent substance, indicates very appropriately 

 the difference between it and the groups or clusters of stars 

 examined by Galileo in the Pleiades and in Cancer. As 

 early as the commencement of the 16th century, Spanish 

 and Portuguese navigators, though without the advantage 

 of telescopic vision, had observed and admired the two Ma- 

 gellanic luminous clouds which revolve round the southern 

 pole, and of which one, as we have already remarked, was 

 known as the "white patch/' or "white ox," of the Per* 

 skn astronomer Abdurrahman Sol, in the middle of the 



