ABSTRACT
The reflectance of a reflecting surface is intrinsic to the substance, i.e., the kind of substance it is. Reflectance is a diagnostic ratio, a fraction. The relative reflectances of all the surfaces in the layout are also invariant. The natural spotting of a surface characterizes the substance that underlies it, as does its texture. But that fact is complicated by the presence in the human habitat of artificially spotted flat surfaces like drawing-pads and canvases, walls, screens, and writing paper. The original unspotted surface can often be recognized. Substances change and surfaces undergo periodic changes of illumination but neither can be seen without a specifying change of some sort in the ambient optic array at a point of observation. Animals perceive surfaces and their properties, since animal behavior must be controlled by what the surfaces and their substances afford.
