ASTRONOMY. 1 83 



be supposed to be 2.8 iiiillimetors, the entire discrepancy in the secular 

 acceleration of the moon's motion disappears. And on tlie same hy- 

 pothesis, the density of the medium surrounding the earth and moon is 

 eqnivalent to the density of air divided by 3,700,000,000,000. Dr. von 

 Oi)polzer's paper will be found entire in No. 2573 of the Astronomische 

 Nachrichten. ( The Nation. ) 



THE PLANETS. 



Mercury. — Denning, in a recent number of the Observatory, gives an 

 account of certain markings detected by himself npuu the surface of 

 Mercury, in November, 1883, from which he deduces a rotation period 

 of about twenty five hours. The value given in our textbooks, and 

 provisionally accepted, though with much reserve, is twenty-four hours 

 five minutes, depending upon certain observations of Schroeter and 

 Harding, at Lilienthal, early in the century. Scliiai)arelli, also, has ob- 

 served markings on the planet several times during the past two years, 

 and says that " the rotation period, as usually adopted, is not exact; in 

 fact, is very far from the truth"; but he does not say whether he finds 

 the period greater or less than that assigned by Schroeter. A memoir 

 upon the planet Mercury is expected soon to appear from his pen, and 

 will probably add considerably to our present knowledge of the planet. 



The photometry of Saturn^s ring.* — In his paper, Professor Seeliger en- 

 ters into some investigations with the view of pointing out the knowledge 

 that he thinks may be obtained with regard to the construction of 

 Saturn's ring by means of photometrical observations of the amount 

 of light reflected from it at different times. Were the ring a body of 

 continuous surface, the a]>parent intensity of its illumination would 

 (unless, indeed, assumptions were made of an altogether improbable 

 kind as to its structure) be very different in different relative positions 

 of the Sun as well as the earth. Changes of this nature are not, as a 

 matter of fact, indicated by observation, the apparent brightness of the 

 ring being always nearly the same, and the amount of light received 

 from it would seem to depend entirely upon the proportion of the whole 

 surface which is turned toward the earth, or upon the angle of eleva- 

 tion of the earth above the plane of the ring. Heuce Zollner concluded 

 that Lambert's law of photometry was not applicable to this case. But 

 Professor Seeliger shows that, under certain plausible assumptions, the 

 observed effects are consistent with that law, the extent of application of 

 which can hardly, he thinks, be overestimated. Maxwell pointed out, 

 twenty-five years ago, from purely mechanical considerations, that the 

 ring could not be a compact solid or fluid mass, but nuist consist of a 

 number of separate discrete particles similar to those which compose a 

 meteoric stream. On this supposition, the observed photometric condi- 

 tions admit of a simple explanation, though their full significance can- 

 not be worked out until more accurate observations have been made 

 with regard to the variation iu intensity of the light of the ring at dif- 



* Astron. Nachr., No. 2612. 



