L. Trouvelot — Physical Ohsei-vations of Sah 



When looking at the rinos, attention is at once attracted 



king at t 



conspicuous dark line, apparently concentric with the outer 

 margin of the rings, and boldly surrounding the planet, and 

 adorning it by its sharp contrast. This dark line is known as 

 "the principal division of the rings," and is shown at a, fig. 1. 

 Owing to the effect of perspective, it always appears widest at 

 the two extremities of its major axis, on that portion called "the 

 ansae," as there only, it is seen without foreshortening. I have 

 carefully compared the intensity of this dark line with the sky 

 outside of the rings, and inside of the ansse ; and I have always 

 found it to be slightly lighter. All my observations also agree 

 in showing this line as appearing a little narrower on the side 

 farther from the observer, at c, fig. 1, than it appears on the 

 opposite side, at d. This phenomenon could readily be ex- 

 plained by supposing that the outside margin of the ring C is 

 on a plane higher than the ring B, and may, consequently, con- 

 ceal a narrow portion of the dark line. The assumption of such 

 an hypothesis seems to be fully supported by the observations, 

 as will be shown hereafter. It is furthermore to be remarked, 

 that the outside margin of the ring C has always appeared to 

 me to be more sharply defined on that part of the ellipse farther 

 from the observer than on the side nearest The case is the 

 same for the outer border of the ring A, which appears sharper 

 on its northern than on its southern side. In both cases, the 

 northern portion of the ellipse is limited by the matter comb- 

 ing the surface of the rings on their flat and illuminated side ; 

 while for the southern portion it is seen a little edgeways, ana 

 this may account for the vagueness of its outlines on tQ^s side. 

 Soon after the beginning of my observations, in October, 1«V A 

 my attention was called to a singular appearance not heretotore 

 noticed, as far as I am aware. Two small, dark, angular forms, 



