It would be most effective if visual observation was impossible.

From the data and visual observations they were able to identify all the flights of the birds.

This was more than a fact of visual observation.

It was the last planetary moon to be discovered directly by visual observation.

In addition the forest shielded both sides from visual observation.

These are one man's visual observations presented as invitations to a dialogue.

The traditional way to identify foam index values is by visual observation.

All visual observations showed point images, with no sign of a dust coma.

We are going to be relying almost entirely on visual observations here.

Once inside, the only way they could be detected would be by visual observation.