here above appears,
Is caus'd, I deem, by bodies dense or rare."

She then resum'd: "Thou certainly wilt see
In falsehood thy belief o'erwhelm'd, if well
Thou listen to the arguments, which I
Shall bring to face it. The eighth sphere displays
Numberless lights, the which in kind and size
May be remark'd of different aspects;
If rare or dense of that were cause alone,
One single virtue then would be in all,
Alike distributed, or more, or less.
Different virtues needs must be the fruits
Of formal principles, and these, save one,
Will by thy reasoning be destroy'd. Beside,
If rarity were of that dusk the cause,
Which thou inquirest, either in some part
That planet must throughout be void, nor fed
With its own matter; or, as bodies share
Their fat and leanness, in like manner this
Must in its volume change the leaves. The first,
If it were true, had through the sun's eclipse
Been manifested, by transparency
Of light, as through aught rare beside effus'd.
But this is not. Therefore remains to see
The other cause: and if the other fall,
Erroneous so must prove what seem'd to thee.
If not from side to side this rarity
Pass through, there needs must be a limit, whence
Its contrary no further lets it pass.
And hence the beam, that from without proceeds,
Must be pour'd back, as colour comes, through glass
Reflected, which behind it lead conceals.
Now wilt thou say, that there of murkier hue
Than in the other part the ray is shown,
By being thence refracted farther back.
From this perplexity will free thee soon
Experience, if thereof thou trial make,
The fountain whence your arts derive their streame.
Three mirrors shalt thou take, and two remove
From thee alike, and more remote the third.
Betwixt the former pair, shall meet thine eyes;
Then turn'd toward them, cause behind thy back
A light to stand, that on the three shall shine,
And thus reflected come to thee from all.
Though that beheld most distant do not stretch
A space so ample, yet in brightness thou
Will own it equaling the rest. But now,
As under snow the ground, if the warm ray
Smites it, remains dismantled of the hue
And cold, that cover'd it before, so thee,
Dismantled in thy mind, I will inform
With light so lively, that the tremulous beam
Shall quiver where it falls. Within the heaven,
Where peace divine inhabits, circles round
A body, in whose virtue dies the being
Of all that it contains. The following heaven,
That hath so many lights, t