being taken as a
centre, then the existing Universe of stars is the result of
_irradiation_ from that centre.

Now, the laws of irradiation are _known_. They are part and parcel of
the _sphere_. They belong to the class of _indisputable geometrical
properties_. We say of them, "they are true--they are evident." To demand
_why_ they are true, would be to demand why the axioms are true upon
which their demonstration is based. _Nothing_ is demonstrable, strictly
speaking; but _if_ anything _be_, then the properties--the laws in
question are demonstrated.

But these laws--what do they declare? Irradiation--how--by what steps does
it proceed outwardly from a centre?

From a _luminous_ centre, _Light_ issues by irradiation; and the
quantities of light received upon any given plane, supposed to be
shifting its position so as to be now nearer the centre and now farther
from it, will be diminished in the same proportion as the squares of the
distances of the plane from the luminous body, are increased; and will
be increased in the same proportion as these squares are diminished.

The expression of the law may be thus generalized:--the number of
light-particles (or, if the phrase be preferred, the number of
light-impressions) received upon the shifting plane, will be _inversely_
proportional with the squares of the distances of the plane.
Generalizing yet again, we may say that the diffusion--the scattering--the
irradiation, in a word--is _directly_ proportional with the squares of
the distances.

[Illustration]

For example: at the distance B, from the luminous centre A, a certain
number of particles are so diffused as to occupy the surface B. Then at
double the distance--that is to say at C--they will be so much farther
diffused as to occupy four such surfaces:--at treble the distance, or at
D, they will be so much farther separated as to occupy nine such
surfaces:--while, at quadruple the distance, or at E, they will have
become so scattered as to spread themselves over sixteen such
surfaces--and so on forever.

In saying, generally, that the irradiation proceeds in direct proportion
with the squares of the distances, we use the term irradiation to
express _the degree of the diffusion_ as we proceed outwardly from the
centre. Conversing the idea, and employing the word "concentralization"
to express _the degree of the drawing together_ as we come back toward
the centre from an outward position, we may say that concen