2 LIGHT AND ITS PERCEPTION 



physical light) . The qualities of a light-sensation bear only a close, not 

 an absolute, relationship to the objective attributes of a physical light 

 which produces it. Thus, different colors may be seen under special cir- 

 cumstances when the corresponding different frequencies of light are 

 not being steadily presented to the eye at all, or the same color may 

 result from totally different mixtures of frequencies. Two lights with the 

 same energy-content may appear different in brightness while two others, 

 equally bright, may differ greatly in actual physical intensity. Color and 

 brightness are thus subjective correlates of the objective frequency and 

 intensity. The former can be perceived but not measured, while the latter 

 can be measured with inanimate instruments but cannot be perceived 

 with the eye. 



A sobering array of optical illusions may be seen by the reader in any 

 good reference work on psychology, and will serve to teach, still more 

 emphatically, the lesson that: "Our eyes do not see; but we see with our 

 eyes." Photoreception is one thing — it may be conscious, the reception 

 of the external stimulus of light upon the sill of the "window of the 

 soul" — or it may lead reflexly to quite unconscious activities such as the 

 change of the size of the pupil, the aiming of the eyes, the blinking of 

 the lids when the eye is about to be struck by something, and so on. 



Vision is something more. It is the complex and sometimes deceptive 

 product of the interaction of the simple information which travels along 

 the optic nerve and the manipulations, as yet unfathomable, which this 

 information undergoes in the brain before it is presented to the con- 

 sciousness for action or other disposal. 



A photoreceptor may be constituted by a single part of a one-celled 

 animal; by one of a number of similar, scattered, photosensory cells in 

 an invertebrate's skin; by a patch of cells closely aggregated into a plate, 

 or lining a pit; or by an ocellus or eye (Fig. 1). This last term is best 

 reserved for those photoreceptors in which there is a light-sensitive layer 

 of cells upon which accessory parts converge the light rays received from 

 environmental objects. An eye, then, ordinarily contains at least a photo- 

 sensory epithelium or retina, and a lens. An image may however be 

 formed upon the retina by a pinhole (as in the chambered nautilus) 

 instead of by a lens; or, the lens in a given type of eye may be employed 

 to concentrate the light in order that the eye may work in dimmer illum- 

 inations, instead of to form an image so that the mind may have a picture. 

 Finally, a number of 'concentrator' units may be congregated so that a 

 mosaic image can be built up in the consciousness itself, and it is upon 



