We are very fortunate to have artist Anthony McCall speaking with us this week. Here is a description in his own words of one of his most celebrated works. 
Line Describing a Cone is what I term a solid light film. It deals with the projected light beam itself, rather than treating the light beam as a mere carrier of coded information that is decoded when it strikes a flat surface.
The viewer watches the film by standing with his or her back towards what would normally be the screen, and looking along the beam towards the projector itself. The film begins as a coherent pencil of light, like a laser beam, and develops through 30 minutes into a complete, hollow cone.

Line Describing a Cone deals with one of the irreducible, necessary conditions of film: projected light. It deals with this phenomenon directly, independently of any other consideration. It is the first film to exist in real, three-dimensional space.
This film exists only in the present: the moment of projection. It refers to nothing beyond this real time. It contains no illusion. It is a primary experience, not secondary: that is, the space is real, not referential; the time is real, not referential.
No longer is one viewing position as good as any other. For this film, every viewing position presents a different aspect. The viewer therefore has a participatory role in apprehending the event: he or she can, indeed needs, to move around relative to the slowly emerging light form.
Continues in class...