Todd — Optical Resolution of the Saturnian Ring. 153 



vorable conditions of our lower atmosphere are alone respon- 

 sible for failure hitherto to resolve the ring into its component 

 satellites. 



The writer has for many years observed Saturn at every 

 favorable opportunity, and with the highest magnifying pow- 

 ers that the conditions of atmosphere would admit. In 1905, 

 when the 18-inch Clark glass was first mounted at Amherst, 

 the ring was too much foreshortened and the inner regions of 

 the ansae too restricted in area. In 1907 when the glass was 

 taken to Chile for photographing Mars, the ring was passing 

 its period of edgewise visibility : the desert seeing, however, 

 was most of the time superb, and resolution of the ring would 

 have been relatively easy, from our station at Alianza in the 

 foothills of the Andes, had the presentation of the ring been 

 favorable. 



Since remounting the telescope at Amherst, every opportu- 

 nity of exceptional definition has been embraced. Further- 

 more, 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 

 pupil of the eye automatically opens and closes, in adaptation 

 to the strength of illumination of the object toward which it is 

 turned ; and the addition of a great objective to the optical 

 system requires further adaptation of the amount of light it 

 gathers to that particular magnifying power which the special 

 condition of the always turbulent air will allow. 



The weather conditions of the peculiar autumn of 1911 gave 

 many opportunities when resolution of the Saturnian ring near its 

 extremities was suspected ; 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, and 

 in the 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 Amherst. The 

 sky, too, was absolutely clear of haze, so that a power of 950 

 gave only very slightly scattered illumination in the field. In 

 moments of best definition a power of 1400 was found to per- 

 form satisfactorily with an aperture of 16 inches. 



Near the extremities of the inner bright ring there was a 

 lenticular shading, as drawn by Proctor (1837-88), 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 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 194. — February, 1912. 

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