222 POOR 



formulated by Tisserand. By an investigation of the path of a 

 comet through a planet's " sphere of activity," Tisserand 

 derived a function, n, of the comet's elements and of those of 

 the disturbing planet, which remains practically unaltered, how- 

 ever great the change in the individual elements. The action of 

 Jupiter, for example, even repeated at several very close appulses, 

 can cause but a very slight change in the numerical value of 

 this function. Schulhof found that the ;^'s for Lexell's and for 

 Brooks' comets differed, and that the comets, therefore, could 

 not be identical. 



This criterion, however, only holds if the comet has been 

 disturbed by a single planet, and in a later paper ^ Schulhot 

 discussed the possibility of the identity of the two comets, 

 assuming that there had been disturbances by both Jupiter and 

 Saturn. By assuming the identity of the two comets, he was 

 able to deduce, by means of the criterion, the most probable 

 orbit of the body between 1779 and 1886. He thus found 

 that, if these bodies are one and the same, its period must have 

 been about 32 years from 1779 to 1849, at which time it passed 

 close to Saturn, suffered large disturbances and had its period 

 increased to about 42 years. Thus the comet made three revo- 

 lutions in the 107 years to be accounted for, but three revo- 

 lutions of unequal duration. 



5. In the latter part of 1889 I took up this problem and 

 attempted to solve the question of identity between these two 

 remarkable bodies. I based my work on an orbit obtained by 

 the method of the variation of geocentric distances from ten 

 normal places between July 1889 and December 1890. The 

 results of this investigation ^ differed greatly from those obtained 

 by Chandler and left the matter in a very unsettled state. Mean- 

 while, Bauschinger undertook the determination of the defin- 

 itive elements of the comet's orbit, and upon the publication of 

 his results,^ I began a reinvestigation of the entire subject and 

 determined to do the work in as thorough a manner as possi- 



1 Btilletin Astronotnique, December, 1889. 



^ Astr. Jotir., No. 244. 



3 " Untersuchungen iiber den periocjischen Kometen 1889 V (Brooks)," i 'Iheil. 



