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SCIENCE. 



[N. S. Vol. XII. No. 297. 



start from the spiral rather than from the 

 sphere of La Place's nebular hypothesis. 



Of Keeler's other contributions to science 

 two in particular deserve present mention : 

 his determination with the Lick telescope 

 of the motion in the line of sight of the 

 planetary nebulae and his demonstration of 

 the meteoric constitution of Saturn's rings. 

 The memoir which describes the first of 

 these investigations already ranks as a clas- 

 sic of astrophysical literature. From the 

 well-known principle of Doppler, the lines 

 in the spectrum of a moving luminous ob- 

 ject are displaced toward the violet or red 

 according as the motion is directed toward 

 or away from the observer. The spectrum 

 of the planetary nebulis consists of a small 

 number of bright lines, which under high 

 dispersion are widely separated from one 

 another, but not greatly weakened in inten- 

 sity. Keeler was the first to take advan- 

 tage of this fact by using in the powerful 

 spectroscope, designed by himself for the 

 Lick telescope, a closely ruled Rowland 

 grating. With the great dispersion of the 

 fourth order spectrum, he was able to meas- 

 ure the positions of the nebular lines with 

 an accuracy far surpassing that attained in 

 any previous observations of these faintly 

 luminous objects. The resulting velocities 

 of the nebulse in the line of sight were on 

 the average considerably smaller than the 

 extreme values, of which the greatest mo- 

 tion of approach was that of the nebula G. 

 C. 4373, 40.2 miles per second, while the 

 greatest motion of recession was 30.1 miles 

 per second, for the nebula N. G. C. 6790. 

 It was also found that the distance between 

 the Great Nebula of Orion and the Sun is 

 increasing at the rate of about 11.0 miles 

 per second. On account of the thorough 

 manner in which this research was planned, 

 the skill exhibited in designing the spectro- 

 scope for the Lick telescope, the care taken 

 in executing the measures and eliminating 

 possible sources of error, and the complete- 



ness of the discussion of the observational 

 material, Keeler's memoir on this subject 

 in Volume III of the Publications of the Lick 

 Observatory takes rank with the best ex- 

 amples of astrophysical literature. 



The spectroscopic demonstration of the 

 meteoric constitution of Saturn's rings is 

 perhaps the most striking of the many ef- 

 fective applications which have been made 

 of Doppler's fruitful principle. It has al- 

 ready been pointed out that the displace- 

 ment of a line is proportional to the velocity 

 of the luminous source. If an image of Sat- 

 urn is formed on the slit of a spectroscope 

 placed parallel to the planet's equator it is 

 evident that all the lines in the photograph 

 of the spectrum will be slightly twisted out 

 of the vertical position they would occupy 

 if the planet were not rotating on its axis. 

 The displacement due to the rotation in- 

 creases uniformly from the center of the 

 disk to the circumference, and the lines, 

 though inclined, remain perfectly straight. 

 If the rings were solid, forming a contin- 

 uous mass with the ball of the planet, it is 

 evident that the spectral lines would be di- 

 rect extensions of those due to the disk. 

 But Keeler found from a study of his photo- 

 graphs that in passing from the spectrum 

 of the disk to that of the rings the lines 

 were not only displaced as a whole, but 

 twisted in the opposite direction. In other 

 words, it appeared that the velocity of rota- 

 tion of the inner edge of the ring is greater 

 than that of the outer edge, a result evi- 

 dently incompatible with the existence of a 

 solid ring, but perfectly in harmony with 

 what must be true if the rings consist of 

 swarms of discrete particles. Careful meas- 

 urements of the photographs furnished the 

 first direct confirmation of the early theo- 

 retical researches of Maxwell, who had 

 shown mathematically that the rings could 

 not exist as solid bodies. 



Much more might be said of Keeler's 

 work, but this should suffice to indicate its 



