What do the lines and dots in the focal plane of an optical instrument represent?

The lines and dots in the focal plane of an optical instrument represent a reticle. A reticle is a grid or pattern placed in the optical axis of a sighting device, like a rifle scope or surveying instrument, that helps the user align and take accurate measurements or shots.

In survey instruments, the reticle provides a reference point or scale that assists in determining angles and distances. It usually includes intersecting lines or points that aid in sighting precise targets. The reticle is essential for enhancing accuracy in measurement and can be designed in various patterns depending on the specific application, such as crosshairs for targeting or more complex designs for surveying.

Other options, like a lens, refer to components that focus light rather than provide a sight reference. A viewfinder serves as a tool to help align the instrument's optical axis with the subject but does not provide the measurement reference lines itself. Projection typically relates to the display of an image or information but does not directly correlate to the reference system used in the focal plane. Therefore, the presence of lines and dots specifically indicates the function of a reticle in these instruments.