And she: “Right truly shalt thou see immersed
In error thy belief, if well thou hearest
The argument that I shall make against it.
Lights many the eighth sphere displays to you
Which in their quality and quantity
May noted be of aspects different.
If this were caused by rare and dense alone,
One only virtue would there be in all
Or more or less diffused, or equally.
Virtues diverse must be perforce the fruits
Of formal principles; and these, save one,
Of course would by thy reasoning be destroyed.
Besides, if rarity were of this dimness
The cause thou askest, either through and through
This planet thus attenuate were of matter,
Or else, as in a body is apportioned
The fat and lean, so in like manner this
Would in its volume interchange the leaves.
Were it the former, in the sun’s eclipse
It would be manifest by the shining through
Of light, as through aught tenuous interfused.
This is not so; hence we must scan the other,
And if it chance the other I demolish,
Then falsified will thy opinion be.
But if this rarity go not through and through,
There needs must be a limit, beyond which
Its contrary prevents the further passing,
And thence the foreign radiance is reflected,
Even as a colour cometh back from glass,
The which behind itself concealeth lead.