238 Notes and Memoranda. 



NOTES AND MEMORANDA. 



Note on Saturn's Rings, by Me. Proctor. — We hare received the fol- 

 lowing from Mr. Richard Proctor : — " It is certainly not absolutely impossible 

 that in some parts of the universe vapours may have a density greater than that 

 of water — that is to say, some hundreds of times greater than that of the densest 

 terrestrial vapours. By admitting the possible existence of such vapours near 

 Saturn, we widen the field of speculation as to the nature of Saturn's rings ; and 

 if we further assume that the rings are not under the dominion of gravity, there 

 is a still 'wider field for the admission of dissimilar theories.' The only positive 

 (but far from conclusive) evidence I know of against the former view, is the cir- 

 cumstance (proved by spectrum analysis) that Saturn's globe and rings are sur- 

 rounded by an atmosphere whose constitution is very similar to that of our own 

 atmosphere.* That terrestrial cloud reflects and intercepts light hardly proves 

 that vaporous rings would do so ; since cloud is not vapour, but formed bythe 

 condensation of aqueous vapour into minute, opaque, and (for the most part) 

 hollow spherules of water. However, if we concede the possible existence of 

 vapours of the density mentioned above, we may readily concede to them the 

 power of reflecting or intercepting light. Your remarks on the dark ring remind 

 me of a circumstance I had omitted to consider. Even if the reflective powers of 

 a satellite and its primary were equal, the former would appear as a dusky spot 

 in transiting the central parts of the latter's disc. For, by two well-known 

 optical properties, the apparent illumination of any point of the disc of a satellite 

 is equal to the real illumination, which varies as the cosine of the angle of inci- 

 dence of the illuminating rays. Hence it may readily be shown that the mean 

 illumination of the disc * the illumination of the centre of the disc (* I the content 

 of a sphere '. the content of its circumscribing cylinder)! '.'. 2 '. 3. I may notice 

 that, if the illumination of the dark ring could be determined, it would be very easy 

 (assuming the truth of the satellite theory) to determine approximately the illu- 

 mination of the part which crosses the ball. For instance, call the illumination of 

 the central parts of the disc of Saturn, or of a satellite, 1 ; assume the illumina- 

 tion of the dark ring to be -|, and, for simplicity, that there is no overlapping of 

 the discs of satellites : required to compare the illumination of three small equal 

 spaces — of the dark ring, of the dusky band across the ball (central part), and of 

 the central part of Saturn's disc. Call these spaces respectively A, B, and C. 

 The illumination of A, ^, would be increased in the proportion of 3 to 2 if the 

 whole disc of each satellite were as bright as the central part of the disc, or would 

 be T 3 (j. Thus the discs of satellites cover j%ths of space A ; and therefore of space 

 B ; and, further, ^ths of the background of space B are in shadow, so that the 

 mean illumination of the spotted background is ^. Since i 7 5 ths of this back- 

 ground, so illuminated, are visible, the light received from this part of space B is 

 j^fo ; and since the discs of satellites occupy the remaining 'ggtas of space B, and 

 give the mean illumination -f, the light received from this part of space B is x %, 

 or ,$gj ; hence the total amount of light from space B is ^fa. Thus the illumi- 

 nation of A I the illumination of B * the"illumination of C '.'. |' '. $&*'" 1, or 

 '. '. 20 * 69 '. 100. By assuming different values for the illumination of the dark 

 ring these proportions can be varied. It may be noticed that, if we assume the 

 illumination of A to be ■£, we obtain for the illumination of B its least possible 

 value — viz., -|." 



84th Planet. — Dr. R. Luther, of Bilk, discovered a new planet on the 25th 

 of August. Its mean position for 1865 is given by him in Astro nomische Nach- 

 richten, R.A. 324° 29' 34"'4. S.D. 14° 17' 49"'4. It is the size of a 10th mag- 

 nitude star, and has been named Clio. 



A Fungus in Ivory and Bone. — Professor Wedl found, on examining some 

 sections of teeth that had been macerated for a few days in water, that they were 



* Philosophical Transactions, 1S61, p. -123. 



t The parenthesis is introduced as suggesting a simple geometrical proof of the proportion 

 in question. 



