254 
POPULAR SCIENCE REVIEW. 
iniioiis in appearance, consist of flights of minute bodies, each 
travelling on its own orbit around the planet.* 
But altliough to the mathematician capable of following 
Professor Maxwell through all the processes of a complicated 
proof, the demonstration of the satellite theory of the rings 
may seem complete, there can be no doubt that the more 
convincing evidence of observation is wanted to bring the fact 
home to the minds of the general student. Now we cannot 
hope that the most powerful telescopes which man can con- 
struct will suffice to reveal the separate bodies which form 
the ring. When the ring’s edge is turned towards ul it appears 
as an almost evanescent line of light, and doubtless if that line 
had not length as well as breadth, we could not detect au}^ 
trace of its existence. Yet there is every reason to believe that 
the apparent breadth of that fine line of light is many times 
larger than the apparent diameter of any single satellite be- 
longing to the rings. In this way, then, observation is not 
likely to help us. 
But there is a mode in which evidence might be gathered 
respecting the conformation of the rings, by any observer who 
had patience to conduct the requisite series of observations. 
If we consider the case of a series of flat rings (whose thick- 
ness may be neglected) formed as of old the rings of Saturn 
were supposed to be, we shall see that the apparent brilliancy 
of the rings ought to vary with the amount of opening. We do 
not refer to the total amount of light received from the ring, 
but to the apparent brilliancy of any point upon the system. 
When a plain surface is illuminated, the science of optics tells us 
that the illumination is proportional to the cosine of the angle 
of incidence. In fact, we know from experience that the higher 
^the sun is above our horizon the greater is the amount of light 
received on the earth’s surface around us. Precisely so would 
it be with the rings if they had plane surfaces. And further, it 
is a law of optics that the apparent brilliancy of any point of a 
luminous object is equal to the real brilliancy at that point, 
whatever may be the distance of the object, or the angle at 
which the line of light meets the surface (neglecting alwa}^s 
what does not here concern us — the influence of any absorptive 
medium which may be interposed between us and the object). 
Now, this being so, it is very evident that if the rings were 
• I know not w’ny this conchision sliould he commonly atlrihuted to my- 
self. Notliin^r, I think, can he clearer than the terms in which while 
dealing with the subject of the nature of the rings, in chapter v. of my 
treatise on S.ilnr.i, I have nssi^-ned to I'rof. Maxwell the full credit for a 
discover}' with wliich I have had absolutely nothing whatever to do, save 
t ) admire and describe it. 
