UAS Visual Line Of Sight
§ 107.31 Visual line of sight aircraft operation.
With vision that is unaided by any device other than corrective lenses, the operator or visual observer must be able to see the unmanned aircraft throughout the entire flight in order to:
(a) Know the unmanned aircraft’s location;
(b) Determine the unmanned aircraft’s attitude, altitude, and direction;
(c) Observe the airspace for other air traffic or hazards; and
(d) Determine that the unmanned aircraft does not endanger the life or property of another.
§ 107.33 Visual observer.
If a visual observer is used during the aircraft operation, all of the following requirements must be met:
(a) The operator and the visual observer must maintain effective communication with each other at all times.
(b) The operator must ensure that the visual observer is able to see the unmanned aircraft in the manner specified in §§ 107.31 and 107.37.
(c) At all times during flight, the small unmanned aircraft must remain close enough to the operator for the operator to be capable of seeing the aircraft with vision unaided by any device other than corrective lenses.
(d) The operator and the visual observer must coordinate to do the following:
(1) Scan the airspace where the small unmanned aircraft is operating for any potential collision hazard; and
(2) Maintain awareness of the position of the small unmanned aircraft through direct visual observation.
You can not use a telescope or binoculars. You can not rely on first person viewing (FPV), or looking through the camera lens as a way to have line-of-sight viewing. FPV is legal but it doesn't satisfy the visual requirements. Your observor can provide the line-of-sight and the operator can use FPV, but someone has to always have unaided eyes on the UAS.
Note that the UAS has to alwasy be near enough to the operator that s/he can see it as prescribed. This means you can NOT daisy-chain the line-of-sight requirement by using one or more observers. The reason for this is obvious. The operator must see-and-avoid other aircraft. It is not plausible to assume the operator can do that by relaying through observers.