Most of the light came from the compluvium and the open peristylium.

Atrium A large hall, lit by an opening in the centre of the roof called a compluvium.

This went though into the atrium with compluvium and impluvium.

Directly below the compluvium was the impluvium.

Compluvium, The roof over atrium which was purposely slanted to drain rain water into the impluvium pool.

It generally contained an opening in the roof (compluvium) above a pool (impluvium) originally intended as a reservoir for domestic use.

In the centre was a square roof opening called the compluvium in which rainwater could come, draining inwards from the slanted tiled roof.

Thence he passed into a huge atrium whose impluvium pool beneath the compluvium in the roof shimmered mirrorlike from an overhead sun.

As the water evaporates, the surrounding air is cooled and becomes heavier and flows into the living spaces and is replaced by air drawn through the compluvium.

The rain from the roof was collected in gutters around the compluvium, and discharged from thence into a tank or open basin in the floor called the impluvium.

Designed to carry away the rainwater coming through the compluvium of the roof, it is usually made of marble and placed about 30 cm below the floor of the atrium.

The house was planned to be inwardly orientated towards open colonnaded courtyards where rainwater was collected from the sloping roofs of the compluvium (open central square) to the impluvium (a receptacle below).

In the displuviatum the roofs, instead of sloping down towards the compluvium, sloped outwards, the gutters being on the outer walls; there was still an opening in the roof, and an impluvium to catch the rain falling through.

The combination of the compluvium and impluvium formed an ingenious, effective and attractive manner of collecting, filtering and cooling rainwater and making it available for household use as well as providing cooling of the living spaces.

Had the weather been less unnaturally cold, they might have remained there, since it was well past dinnertime, but the open rectangle of the compluvium in the roof was acting like a vortex, and the pool below it was a twinkling crust of rapidly melting snowflakes.

The Tuscanicum responds to the greater number apparently of those at Pompeii, in which the timbers of the roof are framed together, so as to leave an open space in the center, known as the compluvium; it was through this opening that all the light was received, not only in the hall itself, but in the rooms around.