191 1.] MOLECULES IN THE TAIL OF HALLEY'S COMET. 255 



tion agrees with what would be shown by particles leaving the 

 nucleus and travelling in hyperbolic orbits away from the sun, the 

 sun being in the full or the empty focus according to the speed of 

 recession. 



Although the general fact is thus evident, to measure the reces- 

 sion directly is to obtain both an observatioral proof of it and also 

 something approaching an exact value of the velocity at a given 

 time and place. Accordingly I determined to do this in the case of 

 Halley's comet at its recent apparition. At my disposal were the 

 two hundred photographs taken of it at the Lowell Observatory 

 between April i8 and June 6. To obtain trustworthy results the 

 photographs to be compared must not be separated by too long an 

 interval, since with time a general commingling of the various par- 

 ticles takes place which not only renders particular decipherment of 

 different outbursts impossible but entirely alters the actual speeds. 

 In the case of Halley's comet this difficulty \\as enhanced bv the 

 unusual uniformity of the tail. Irregularities, bunches or knots 

 were rare : the tail presenting as a rule, a remarkably orderly deport- 

 ment, dishearteningly same. Among the many plates, however, I 

 was able to select a pair taken seriatim capable of recognition and 

 measurement. Some of these handles to ir.vestigation were in the 

 nature of bunches of matter, some of abrupt changes in its direction 

 looking like promontories along the general line of the tail. I chose 

 four of the more salient excrescences and selecting identical features 

 of them in the two negatives measured their respective distances 

 from the nucleus on the two plates. The first plate was exposed 

 from 9h 23m to gh 53m and the second from loh om to loh 53m, 

 so that the one followed directly on the other. 



\\'hen the angular amounts of the changes in place of the several 

 knots were corrected for differential refraction and then reduced to 

 speeds, account being taken of the distance of the comet from the 

 earth and of the inclination to the line of sight of the respective 

 positions along the tail, the results came out as follows : 



From these measurements the fact emerges unmistakablv that a 

 repulsive force directed away from the sun acted upon the particles 

 on the tail. 



