The wind was stronger here, pouring up the wide valley from some invisible source below.

Sometimes music of a heavenly sweetness played, coming from an invisible source, but never for more than a short interval.

The sound of plaintive cries on all sides, their source invisible in the dark.

The doctor guessed that the molecules of the air itself were being activated by some invisible source to create light.

For a steady, invisible source of heat, consider radiant floor systems.

Only the space in the middle of the couches remained clear, lit from some invisible source.

Suddenly the flame receded, sinking back into its invisible source.

I stopped, experimented, moved in a direction away from the invisible source.

These are also used for photo detectors in the form of invisible light source.

Buffy runs off to follow an invisible source of laughter.