APPENDIX 



43 



environmental conditions, a camouflage 

 for all of these conditions is difficult if not 

 impossible to achieve. 



Another practical illustration of the 

 importance of brightness contrast is shown 

 in Fig. 36, which scarcely needs any inter- 

 pretation. These data were obtained by 

 subjects who looked through aircraft glass, 

 clean plexiglas, and dirty plexiglas. This 

 figure shoAvs that dust and grease on wind- 

 shields act as an effective screen between 

 the pilot and the outside w^orld. Particles 

 of dirt and grease scatter light haphazardly 

 into the bundle of light rays which form an 

 image of the object on the retina. This 

 decreases the contrast and destroys the 

 sharpness of the image. Diminished con- 

 trast also results from scratching or fogging 

 of the transparent material, because each 

 scratch or water droplet is a source of 

 scattered light. 



Summary 



In this chapter we have tried to introduce 

 some basic concepts, units of measurement, 

 and functions in the field of visual psycho- 

 physiology. Although this is a long chapter, 

 it is short in relation to the amount of 

 research it has attempted to summarize. 

 It has obviously been necessary to select 

 from among the possible topics we might 

 have covered. This is by no means the 

 complete story of how we see, but it should 

 provide some understanding of the kinds of 

 data which can be put to use in applied 

 problems. 



Our second aim in this chapter has been 

 to point out research needs in this field. 

 We saw that much visual research needs 

 reinvestigation because (a) it has not been 

 sufficiently thorough to be of use to the 

 applied visual scientist, and (b) the sub- 

 jects have usually been so small in num- 

 ber — often only one — that the applied vis- 

 ual scientist cannot be sure whether the 

 functions are representative of the average 

 eye. The thorough reinvestigation of many 

 of these functions with many subjects 



constitutes an enormous undertaking, but 

 the practical benefits to be derived from the 

 data are great. 



Appendix 



Measurement and Nomenclature in 

 Visual Science 



The Concepts of Light and Color 



One difficulty with the study of light and color 

 is that it does not fall exclusively within any of the 

 major domains of science. It is now generally 

 understood even by lay people that we see things 

 either because they emit radiant energy or because 

 radiant energy is reflected from them. In order 

 for any object to be seen, radiant energy must 

 come from the object and get into the eye. Ra- 

 diant energy is similar to any other form of energy 

 and its measui-ement is strictly a physical problem. 

 But in order to be "seen," radiant energy must be 

 converted into nervous energy by means of 

 photochemical transformations in the eye and this 

 nervous energy must then be transferred to the 

 brain via the optic nerve. Both of these processes 

 fall within the purview of the physiologist. 

 Finally, the nervous energj- in the brain must 

 somehow be transformed into the inner experi- 

 ences, sensations and perceptions, which consti- 

 tute the "stuff" of seeing. The study of this 

 process falls into the province of the psychologist. 

 It is difficult, of course, to study this chain of 

 events in its entirety, and one source of difficulty 

 in the past has been that each group of scientists 

 has had its own terminology and frame of reference 

 for studying the same phenomena. A synthesis of 

 these points of view is essential to the proper 

 understanding of visual data. 



Light and Color. When an observer's eye is 

 stimulated by radiant energy and he reports that 

 he sees light, the "light" he is referring to is not 

 the same as radiant energy. It is radiant energy 

 which has somehow been transformed into a part 

 of a person's experience. Two definitions of 

 light which have been adopted as standard are the 

 following: 



For purposes of illuminating engineering, 

 light is radiant energy . . . evaluated according to 

 its capacity to produce visual sensation. (100) 

 Light is that aspect of radiant energj' of 

 which a human observer is aware through the 

 visual sensations which arise from the stimula- 

 tion of the retina of the eye. (18) 

 Color is almost synonymous with light and has 

 been defined as follows: 



Color consists of the characteristics of 



