208 ; COSMOS. 



taming the date of the sudden appearance of any star, was 

 more and more recognized ; the periodicity* (their re-appear- 

 ance after many centuries) was discussed ; and Tycho Brahe 

 himself boldly advanced a theory of the process by which 

 stars might be formed and moulded out of cosmical vapour, 

 which presents many points of resemblance to that of 

 the great William Herschel. He was of opinion that the 

 vapoury celestial matter which becomes luminous as it 

 condenses, conglomerates into fixed stars: " Coeli mate- 

 riam tenuissimam, ubique nostro visui et planetarum circuitibus 

 perviam, in unum globum condensatam, stellam efnngere." 

 This celestial matter, which is universally dispersed through 

 space, has already attained to a certain degree of condensation 

 in the Milky Way, which glimmers with a soft silvery bright- 

 ness. Accordingly, the place of the new star, as well as of 

 those which became suddenly visible in 945 and 1264, was on 

 the very edge of the Milky Way (quo factum est quod nova 

 s-tella in ipso galaxise margine constiterit). Indeed, some 

 went so far as to believe that they could discern the very spot 

 (the opening or hiatus] whence the nebulous celestial matter 

 had been drawn from the Milky Way. 3 All this reminds one 



* Cardanus, in his controversy with Tycho Brahe, went 

 back to the star of the Magi, which, as he pretended, was 

 identical with the star of 1572. Ideler, arguing from his 

 own calculations of the conjunctions of Saturn with Jupiter, 

 and from similar conjectures advanced by Kepler on the 

 appearance of the new star in Ophiuchus in 1604, supposes 

 that the star of the Magi, through a confusion of ao-r^p with 

 acrrpov, which is so frequent, was not a single great star, but 

 a remarkable conjunction of stars, the close approximation 

 of two brightly shining planets at a distance of less than a 

 diameter of the moon. Tychonis Progymnasmata, pp. 324- 

 330 ; contrast with Ideler, Handbuch der mathematischen und 

 technischen Chronologic, bd. ii. s. 399-407. 



3 Progymn., pp. 324-330. Tycho Brahe, in his theory of 

 the formation of new stars from the Cosmical vapour of the 



