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.
Now thou wilt say the sunbeam shows itself
More dimly there than in the other parts,
By being there reflected farther back.
From this reply experiment will free thee
If e’er thou try it, which is wont to be
The fountain to the rivers of your arts.
Three mirrors shalt thou take, and two remove
Alike from thee, the other more remote
Between the former two shall meet thine eyes.
Turned towards these, cause that behind thy back
Be placed a light, illuming the three mirrors
And coming back to thee by all reflected.