The imaginary line which appears across the picture at eye level:

The imaginary line which appears across the picture at eye level is known as the "horizon line" in art and photography. Here are the steps to understand it:

1. The horizon line in art is a perspective tool that helps artists establish depth and spatial relationships in their artwork. It's an imaginary line that is drawn across the paper or canvas to represent the viewer's eye level and delineate where the sky meets the ground.

2. The position of the horizon line can greatly affect the mood of a painting. For example, a high horizon line can make the viewer feel as if they are looking down on a scene, while a low horizon line can make the viewer feel as if they are looking up at a scene or are part of the action.

3. The horizon line also helps artists determine the placement and scale of objects within the scene. Objects that are below the horizon line appear larger and closer to the viewer, while objects above the horizon line appear smaller and further away.

4. In photography, the horizon line is often used to help compose the photo. By aligning the horizon line with one of the lines in the rule of thirds, photographers can create a balanced and interesting composition.

5. The horizon line is not always visible, especially in scenes that are filled with objects, but it's always there and it's an important tool for artists and photographers.