ASTRONOMY FOR 1889, 1890. 149 



invisible. While several observers confirmed L)r. Terby's discovery, 

 notiiing to correspond siitiflcieutly witU bis description could be made 

 out by others, though provided with luacli more powerful api)aratus. 

 Professor Hall lias expressed the opinion that it was an optical effect 

 of contrast. 



The very fine division of the outer ring detected with the 36-inch Lick 

 refractor early in 1888 was again seen in 1889 at a distance of about 

 one-sixth of the breadth of ring A from its outer edge. A dark shad- 

 ing extended inwards from the new division almost to the inner edge of 

 the ring. Professor Holden has noted also an extremely narrow brighter 

 polar cap about 5 seconds of arc wide, in a direction parallel to the 

 equator, and perpendicular to this about the width of the Cassini divi- 

 sion at the anste. 



An interesting monograph on Saturn, the result of fourteen years 

 work, is contributed by Prof. Asaph Hall as Appendix ii to the Wash- 

 ington Observations, 1885. The characteristic of this memoir is great 

 caution, and the three drawings of the planet, where a few scanty 

 markings represent all that Professor Hall can certainly see with a 

 fine telescope, should re-assure those who have been dissatisfied with 

 their modest instruments because thej^ could not therewith recognize 

 the elaborate detail described by more imaginative observers. To 

 quote the author's own words: ''The appearance of Saturn in our 

 2C-inch refractor undergoes great changes from night to night, and 

 sometimes even from hour to hour during the same night. Probably 

 these changes are due to variations in our own atmosphere and in the 

 action of the objective, and they do not therefore indicate real changes 

 in the planet. Whenever we have a steady and transparent atmos- 

 phere, the outlines of the planet, the faint belts and markings on the 

 ball, the shadow of the ball on the ring, the dusky ring, and the Cassini 

 division are clear and distinct, and the abnormal phenomena sometimes 

 seen are not visible. Without exception, my experience is that on 

 good nights the planet always has this natural appearance. But on 

 poor nights, when the image is blazing and unsteady, one can see and 

 imagine many strange things about this wonderful object." 



Professor Hall finds for the rotation period of the planet from obser- 

 vations of the white spot (1876, December 7 to 1877, January 2) 10^ 

 14°' 23«. 8 ± 2«. 3 mean time (see Astron. Nachr. No. 2146). Careful 

 discussions are also given of the position and dimensions of the ring. 



The notch in the outline of the shadow was never seen at Washing- 

 ton, either by Professor Hall or his assistant. " The curvature of the 

 outline of the shadow presented an anomaly in 1876 when the convexity 

 appeared to be turned towards the ball, contrary to what we should 

 expect from geometrical considerations. The notes show that some- 

 thing of this kind was seen after the re-ilppearance of the ring in 1878. 

 After the ring was well opened, the curvature of the outline always 

 appeared natural or turned away from the ball." {Observatory.) 



