12 



KNOWLEDGE 



[Januaby 2, 1899. 



rings, or cross each other irregularly in their motions. 

 The frequent observation of sub-divisions, permanent or 

 otherwise, in the rings (Fig. 3) is thus only what should 

 naturally be expected ; but under such circumstances final 

 disintegration is inevitable. 



An interesting corollary of the discrete character of the 

 system is the intimate relation existing between the 

 grouping of the particles and the luminosity of the ring. 



There where the particles are grouped nearer together 

 (outer edge of inner bright ring), we have increased 

 luminosity ; there where they are more coarsely scattered 

 (outer ring), the brightness is diminished. Should the 

 particles be too widely strewn (dusky ring), then we have 

 the conditions necessary for the production of transparency. 

 There can be but little doubt that the outer ring is also 

 slightly transparent ; not, of course, to the extent of 

 showing us the planet's limb through it, but still suffi- 

 ciently transparent to render its shadow grey and not 

 quite black (Fig. 4). 



As its epithets of " crape ring" or " gauze veil " imply, 



Fig 4. — Duskiness of Outei- Ring's Shadow, suggesting the idea of 

 Transparency. 1894, July 4 (91 in. O.G-., power 220). 



the dusky ring of Saturn has been hitherto considered as 

 " one of the greatest marvels of our day."* But the 

 remarks we have made on the subject of the wonders 

 associated with the square- shouldered appearance of the 

 planet, have warned us to be sceptical of irrational inter- 

 pretations. Of explanations of the nature of the dusky 

 ring the writer knows none, excepting " Burton's ingenious 

 suggestion, that the separate particles, though brighter 

 than the sky, may be less luminous than the ball."t 

 But the very ingenuity of this assmnption casts a doubt 

 on its correctness. 



We can scarcely believe (a) that one part of a system of 

 discrete particles, having manifestly a common origin, is 

 white, another grey ; and (i) that the sombre meteors all 

 managed to group themselves near the planet to the abso- 

 lute exclusion of the white ones. The probabilities of 

 such a state of things being natural are but few. " The 

 moon," said Tyndall, " appears to us as if — 



" ' Clothed in white samite, mystic, wonderful ; ' 



but were it covered with the blackest velvet it would still 

 hang as a white orb in the heavens, shining upon our 

 ■world substantially as it does now." { 



Starting from the hypothesis that the luminous diffusive 

 power of the particles is everywhere the same, and seizing 

 all natural conditions likely to be found here, we might 

 perhaps scan the true nature of the " gauze " ring. We 

 saw that the relation between the agglomeration of the 



• Webb, "Celestial Objects": Saturn. 



t nid. t " On Light," 5th ed , p. 33. 



particles and the luminosity of the ring implies that the 

 bodies constituting the " dark " ring are more coarsely 

 scattered than those constituting the bright rings. In 

 this case the segments at the ansre would be, just what 

 they actually are in the telescope, parts of a nebular annulus ; 

 while, across the planet, the passage of the ring would be 

 invisible as far as t lie particles themselves are concerned. 



But, inasmuch as the stream casts its shadow on the 

 globe of Saturn, every individual meteor yielding its own 

 modicum of shade, we readily perceive that the trace 

 of the " dark " ring across the planet would be a dusky 

 shading, due solely to the shadow of the particles on 

 the ball, a shadow which would be visible through the 

 gaps separating that flight of meteors (Fig. 5). 



The correctness of this theory may be checked by its 

 consequences. Should the sun be higher above the 

 plane of the ring 

 than the earth, 

 the breadth of 

 the shadow 

 across the 

 planet would 

 shrink along the 

 minor axis ; 

 should the sun 

 be lower than 

 the earth, the 

 breadth would 

 be increased by 

 the additional 

 shadow of the 

 inner edge of 

 the bright ring. 

 In both cases, 

 however, the 

 outline of the 

 shadow cast on the globe would not be a rigorous continua- 

 tion of the nebular ansfe. But, as the heliocentric latitude 

 of Saturn cannot exceed 2' 30', and as the proximity of the 

 ring to the planet is very great, the slight differences of 

 outline we have mentioned would, under all ordinary 

 circumstances, be entirely masked by errors of vision. 



As a last consequence of the theory we are examining 

 it ought to be expected that the darkness of the dusky 

 belt across the globe would be, to a slight extent, a 

 function of the opening of the system. When the value 

 of the angle formed by the plane of the ring and the 

 visual ray is small, the particles of the " dark " 

 ring ought in an appreciable measure to hide their own 

 shadow, while their apparently closer grouping would in 

 this case slightly interfere with the usual transparency 

 of the ring, a conclusion which seems to be actually 

 corroborated by observation. 



Fig. 5. — Diagram illustrating the writer's 

 theory on the probable constitution of the 

 Crape Ring. (Dimensions of particles and their 

 shadows monstrously exaggerated.) 



Lord Iveagh has made two munificent offers to the 

 nation, the fulfilment of which will involve a cost to the 

 donor of a sum amounting to not less than half a million 

 of money. The first offer, which has already been accepted, 

 is that of a quarter of a million to be employed in the 

 endowment of scientific research. It is explained in 

 the following letter which was addressed to the Press on 

 December 22nd by Lord Lister, the Chairman of the 

 Council, and Sir Henry E. Roscoe, the Honorary Treasurer 

 of the Jenner Institute of Preventive Medicine : — " We ask 

 permission to announce in your columns a splendid offer in 

 aid of scientific research which has been placed in our 

 hands. British and Irish men of science have long 



