The Chapters 6, 7, 8, and 9 deal with the human observer. In Chapter 6, the physical and anatomical aspects of vision are
discussed, more in particular the structure of the nervous system. This chapter concentrates on the visual performance aspects,
and in Chapter 8 the visual perception. Colour vision is discussed in Chapter 9.
Chapter 7 deals with the different functions of the human visual system, concentrating on the image-forming aspects of the
activities of the visual sense. A central issue is the duplicity theorem, stating that the main optical elements are the cones
and the rods, each having their own characteristics. The cones are mainly active in daytime, allowing for photopic vision,
and the rods, being active in dim conditions, for scotopic vision. The spectral sensitivity curves for photopic, mesopic,
and scotopic vision are discussed in relation to human and visual performance. The primary visual functions are discussed,
viz. the adaptation, the contrast sensitivity, and the visual acuity.