In Book 4, Lucretius discusses the senses and concludes with a section on love and lust. He describes the way we see things as "images of things" which are "perpetually peeled off the surface of objects and flying about this way and that through the air." He also explains that these films can generate spontaneously in the air. He uses the example of clouds to illustrate this idea. They seem to generate by themselves and they are constantly shifting in form. This is his way of explaining the way we see things. The films can not pass through certain things such as mirrors and glass. The mirror image of the film is continually reproduced every time the mirror is moved. Likewise, every time the object is moved, the films will change. We get the idea that each film is like a single slide in a roll of film. Every instant and every movement requires a new film to be produced.
Lucretius combines this with a theory of the speed of films and the speed of light, which he says is instantaneous. He uses the mirror image to support this instantaneous speed. The mirror image is reflected immediately. Thus, light and/or the films must travel instantaneously as well. He explains that our senses are reliable in discerning the films but that we each might interpret them differently. For example, we can have a film of a man and a horse and, combining them, we can interpret a centaur.
In his final section on love, he warns that the torrents of falling in love are dangerous and destructive. He also warns against worshiping the loved one as an idol. He adds that the passion of love is something that should be practiced like a habit and learned over time. With this notion of practicing, some interpret that Lucretius is also endorsing promiscuity.