A FAMOUS ASTRONOMICAL PROBLEM 501 



four planets. Satisfactory causes were looked for in a possible ellip- 

 soidal form of the sun, in a In-pothetical ring of small planets bet^yeen 

 Mercun^ and Venus, in an assumed minute variation in the law of 

 gravitation from the Newtonian inverse square of distances, and in 

 other assumptions, but in vain. One hypothesis, that the finely divided 

 material which gives rise to the zodiacal light (by reflecting the sun's 

 rays) is the responsible disturbing mass, has been discussed several 

 times since the days of Le Verrier and as many times rejected, with 

 one exception. 



The exception is Professor Seeliger's recently published investiga- 

 tion. AVith great skill and with entirely reasonable assumptions as to 

 the form of space occupied by the zodiacal material, and as to the 

 density of the distribution of the material in this space, he establishes 

 that there is sufficient mass to account for the discrepancies in the 

 motions of all the four planets. 



The following table exhibits the results of Seeliger's theory in the 

 first column of figures, and the actual results of observation as deter- 

 mined by IS^ewcomb in the second column. The ([uantities in the 

 third column, which bear the sign ±:, are the " probable errors " as- 

 signed by Xewcomb to his results; and, for the benefit of non-mathe- 

 matical readers, we may explain that these " probable errors," deduced 

 from the observations themselves, are indications of the uncertainties 

 existing in the quantities to which they are attached. In this table 

 e and i are respectively the eccentricity of the orbit and the inclination 

 of the orbit plane to the ecliptic; and All, AO and A/ are respectively 

 the changes, per century, in the longitude of perihelion, in the longitude 

 of node and in the inclination of the orbit plane, unaccounted for by 

 the attractions of known masses, as in the second column, and produced 

 by the attractions of the zodiacal matter as computed by Seeliger. In 

 the last column are the differences between the Seeliger and !N"ewcoml3 

 numbers : in other words, a comparison of theory with actuality. 

 These differences are small. All are within the probable errors in 

 the third column ; with one exception, far within these probable errors. 



AVe can not ascribe tliis remarkable agreement between Newcomb's 



