On the nature of things (books 4 - 6)   

From things and gliding pass away....

For ever every outside streams away
From off all objects, since discharge they may;
And when this outside reaches other things,
As chiefly glass, it passes through; but where
It reaches the rough rocks or stuff of wood,
There 'tis so rent that it cannot give back
An image. But when gleaming objects dense,
As chiefly mirrors, have been set before it,
Nothing of this sort happens. For it can't
Go, as through glass, nor yet be rent- its safety,
By virtue of that smoothness, being sure.
'Tis therefore that from them the images
Stream back to us; and howso suddenly
Thou place, at any instant, anything
Before a mirror, there an image shows;
Proving that ever from a body's surface
Flow off thin textures and thin shapes of things.
Thus many images in little time
Are gendered; so their origin is named
Rightly a speedy. And even as the sun
Must send below, in little time, to earth
So many beams to keep all things so full
Of light incessant; thus, on grounds the same,
From things there must be borne, in many modes,
To every quarter round, upon the moment,
The many images of things; because
Unto whatever face of things we turn
The mirror, things of form and hue the same
Respond. Besides, though but a moment since
Serenest was the weather of the sky,
So fiercely sudden is it foully thick
That ye might think that round about all murk
Had parted forth from Acheron and filled
The mighty vaults of sky- so grievously,
As gathers thus the storm-clouds' gruesome night,
Do faces of black horror hang on high-
Of which how small a part an image is
There's none to tell or reckon out in words.

Now come; with what swift motion they are borne,
These images, and what the speed assigned
To them across the breezes swimming on-
So that o'er lengths of space a little hour
Alone is wasted, toward whatever region
Each with its divers impulse tends- I'll tell
In verses sweeter than they many are;
Even as the swan's slight note is better far
Than that dispersed clamour of the cranes
Among the southwind's aery clouds. And first,
One oft may see that objects which are light
And made of tiny bodies are the swift;
In which class is the sun's light and his heat,
Since made from small primordial elements
Which, as it were, are forward knocked along
And through the interspaces of the air
To pass delay not, urged by blows behind;
For light by light is instantly supplied
And gleam by following gleam is spurred and driven.
Thus likewise must the images have power
Through unimaginable space to speed
Within a point of time,- first, since a cause
Exceeding small there is, which at their back