THE HEAVENS FOR MAY 



401 



Optical Resolution of the Ring of 

 Saturn into its Component Satel- 

 lites or particles. 



BY PROF. DAVID TODD, AMHERST COLLEGE 

 OBSERVATORY, MARCH 1 8, K)12. 



I do not need to call to mind here 

 the progress of telescopic research on 

 the ring of Saturn, from the time when 

 Galileo first observed it and Huygens 

 first divined its nature as a true ring, 

 near the middle of the seventeenth cen- 

 tury, thereby explaining all the ap- 

 pearances that had baffled his predeces- 

 sors and showing' how the ring might 

 disappear and reappear in a complete 

 cycle of phases from absolute invisi- 

 bility to the amplest widening. Cassini 

 later showed that the ring was divided 

 into two parts unequal in breadth, the 

 inner one the brighter and broader ; 

 while Encke similarly found a division 

 in the outer ring, though still more dif- 

 ficult to recognize. About the middle 

 of the eighteenth century Bond and 

 Dawes discovered the broad dusky ring 

 inside the inner ring of Huygens, and 

 near the end of the same century 

 Keeler provided the first observational 

 proof by means of the spectroscope 

 that the ring is composed of clouds of 

 satellites or particles revolving around 

 Saturn, in full accord with the har- 

 monic law of Kepler. There was still, 

 however, the possibility that the ring 

 might be gaseous in composition. 



Hence the desirability of visualizing 

 if possible the separate particles of 

 which the ring had been mathemati- 

 cally and spectrosopically proved to be 

 composed. 



The writer has for many years ob- 

 served Saturn at every favorable op- 

 portunity, and with the highest magni- 

 fying powers that the conditions of at- 

 mosphere would admit. Since 1905, 

 when the 18-inch Clark glass was first 

 mounted at Amherst, the ring has 

 been too much foreshortened and the 

 inner regions of the ansae too restricted 

 in area, until the apparition of 191 1, 

 during which every opportunity of ex- 

 ceptional definition has been embraced. 

 The objective has been fitted with an 

 exterior iris diaphragm, conveniently 

 operated from the eye-end; and the 

 absolute necessity of such an appliance 



in all telescopic work requiring fine 

 definition has been proved beyond a 

 doubt. 



The weather conditions of the pecu- 

 liar autumn of 191 1 gave many oppor- 

 tunities when resolution of the Satur- 

 nian ring near its extremities was sus- 

 pected ; but not until the perfectly 

 quiescent nights of October 28 and 29 

 was there a near approach to that 

 serenity and entire atmospheric calm 

 which I had before experienced but 

 twice: on the summit of Fuji-san in 

 1887, an< l m tne desert of Tarapaca in 

 northern Chile twenty years later. The 

 power on this occasion was pushed 

 nearer to the limit than I had ever 

 found it possible to do before at Am- 

 herst. The sky, too, was absolutely 

 clear of haze, so that a power of 950 

 gave only very slightly scattered il- 

 lumination in the field. In moments of 

 best definition a power of 1400 was 

 found to perform satisfactorily with an 

 aperture of 16 inches. 



Near the extremities of the inner 

 bright ring there was a lenticular 

 shading, as drawn by Proctor and less 

 pronouncedly by Barnard; and it was 

 in this especial region that, in moments 

 of the best vision, a certain sparkling 

 fiocculence was more or less steadily 

 glimpsed ; scintillant much as fine 

 snowflakes sun-illumined at the close 

 of a storm. There was no longer in 

 the writer's mind any doubt that the 

 separate component satellites of the 

 ring had been seen, at least in that part 

 of the inner Huygenian ring which is 

 adjacent to the extremities of its major 

 axis. 



A Latin dispatch was therefore for- 

 warded to Sir David Gill, as follows: 

 Saturni anulorum clarorum exteriorum- 

 que axium maiorum prope extrema, me 

 adiuvantibus validissimis tclescopiis, 

 quondam Hocculentiam scintillantem ob- 

 servavi, quam ocularum dissipationem 

 anuli esse inter ■pretatus sum. 



By a like fatality that rendered 

 Schiaparelli's canali into canals, ocu- 

 larum dissipatio became, not optical 

 resolution, its true English equivalent, 

 but dissipation, — a simple translitera- 

 tion which implied a breaking up or 

 dissolution of the ring: an idea wholly 



