266 Work for the Telescope, 



the two points where it crosses his orbit, has no reference to it 

 whatever. But it is so placed as to make a considerable angle 

 (26° 49' 17") with the orbit of Saturn, and while it is carried 

 round the Sun with the planet, it remains always parallel to 

 itself, since a fresh direction could only be the result of a fresh 

 force impressed upon it from without ; and no such force exists. 

 Hence, in the course of one revolution of Saturn, or 29 \ of our 

 years, it turns first one and then the other side to the Sun ; 

 and in passing from the one to the other position, presents to 

 him for a short time its edge, in which situation, from its ex- 

 treme thinness, estimated by the American astronomer Bond 

 at less than 40 miles, it will not reflect light enough to be 

 visible at the distance of the Earth, without the greatest diffi- 

 culty. But this is not the sole condition of the phenomenon, 

 which depends not only on the position of the Sun, but of the 

 Earth also : as the Earth is but seldom exactly in the line be- 

 tween the Sun and Saturn, it may so happen that the edge of 

 the ring, when not presented to the Sun, may be turned to- 

 wards ourselves, and then again its thinness will withdraw it 

 from sight. And there is a third case of partial disappearance 

 when the Sun and Earth are on opposite sides of the ring, and 

 we look on the darkened side. This conjuncture is soon about 

 to take place. May 18, the Sun passes from the south side, 

 which we have been looking upon since February 1, to the 

 opposite one out of sight from the Earth, and consequently its 

 aspect will be entirely reversed from light to dark, with a 

 singular corresponding reversal of visibility : the portion cross- 

 ing the ball, hitherto undistinguishable on a bright background, 

 will become strongly visible as a dark line of fully 2" in breadth, 

 and the extremities, previously so distinct against the dark sky, 

 will disappear. After this epoch (May 18) the Earth's orbital 

 motion carrying our eye towards the plane of the ring, the 

 breadth of the dark streak will diminish ; but not in a regular 

 progression, for the Sun, gradually rising to a height of 1° 

 20' above the ring, will cast a shadow upon the ball, which, 

 increasing as the ring decreases, will keep in some degree 

 the apparent breadth of the band. A few telescopes will be 

 able, during this period, to distinguish the ring from its shadow, 

 by bringing out a most delicate line of light between them ; but 

 they will be few indeed. It was seen by Dawes in 1848 with 

 a 6y inch object-glass ; but his eye is an extraordinary one. 

 Schmidt also seems to have seen it in that year ; and Jacob 

 perceived it with an object-glass of 9 inches as recently as 

 last December. This interesting series of phases will terminate 

 on August 12, by the final disappearance of the ring as we pass 

 to its enlightened north side, which will continue uninterrup- 

 tedly in sight for nearly fifteen years ; and fifteen years are 



