The Relationship Between Stimulation and Stimulus Information (Gibson, 1979, Chapter 4)

The first three chapters describe the world to be perceived, at the ecological scale. This chapter opens the section on how we visually perceive that world - via information. Remember, a key part of the ecological analysis is doing things in this order (see the Introduction). 

The work of the next few chapters is to lay out a theory of ecological optics. Lots of science studies light, but as with the physical world/environment distinction, Gibson will insist on distinguishing between the physics of light and the ecology of it; only the latter will be relevant to a theory of perception. 

Sabrina also blogged this chapter here. 

The Meaningful Environment (Gibson, 1979, Chapter 3)

In Chapter 1, Gibson identified the animal and it's environment as the two mutually defining parts of an ecological system. In Chapter 2, he developed a vocabulary for describing an environment (as opposed to the physical world). Now, in Chapter 3, he will use that vocabulary to identify that the environment of an animal is meaningful, literally full of meaning, and this will mean that meaning is there to be discovered, rather than constructed (as in every theory of perception so far). 

Sabrina also blogged this chapter here.