228 COSMOS. 



motion, remained in all its branches amenable to mathemat- 

 ical treatment. This advantage gives to the elementary 

 works on theoretical astronomy a great and entirely peculiar 

 charm. In them is reflected what the intellectual labors of 

 later centuries have achieved by the analytical methods ; 

 how configuration and orbits are determined ; how, in the 

 phenomena of planetary motion, only small oscillations about 

 a mean condition of equilibrium can take place ; how the 

 planetary system, from its internal arrangement, works its 

 preservation and permanence by the compensation of ^:?er- 

 turbations. 



The examination of the means of forming a general con 

 ception of the universe, the ex])lanation of the complicated 

 celestial phenomena, do not belong to the plan of this work. 

 The physical description' of the universe relates to what fills 

 space, and organically animates it, in both spheres of urano- 

 logical and telluric relations. It adheres to the consideration 

 of the discovered laws of nature, and treats of them as ac- 

 quired facts, as immediate results of empirical induction. In 

 order to carry out the work of the Cosmos within the appro- 

 priate limits, and not with too great extension, it must not 

 be attempted to establish theoretically the connection of phe- 

 nomena. In this Hmitaticn of the plan laid down beforehand, 

 I have, in the astronomical volume of Cosmos, applied so 

 much the more care to the individual facts and their arrange- 

 ment. From the consideration of universal space, its tem- 

 perature, the degree of its transparency, and the resisting 

 medmm which fills it, I have passed on to natural and tele- 

 scopic vision, the limits of visibility, the velocity of light, ac- 

 cording to the difference of its sources, the imperfect meas- 

 urements of luminous intensity, and the new optical means 

 of distinguishing direct from reflected hght. Then follows 

 the heaven of fixed stars ; the numerical statement of its 

 dclf-luminous suns so far as their position is determined ; their 

 probable distribution ; the changeable stars which reappear 

 at well-defined periods ; the proper motion of the fixed stars ; 

 the assumption of the existence of dark cosmical bodies, and 

 their influence upon the motion of the binary stars ; the 

 nebulous spots, in so far as these are not remote and very 

 dense swarms of stars. 



The transition from the sidereal part of uranology — from 

 the heaven of the fixed stars to our solar system, is merely 

 a transition from the universal to the particular. In the 

 class of binary stars, self-luminous cosmical bodies move about 



