SECTION XXIII 

 THE SENSE OF SIGHT 



CHAPTER LXVII 

 PHYSIOLOGICAL OPTICS 



The Nature, Cause and Velocity of Light. — The study of the 

 phenomena connected with hght, and their application, is called optics. 

 Physiological optics is that subdivision of optics which deals with 

 these phenomena as appHed in a practical way to our visual mechanism. 

 In accordance with Aristotle, the universe consists of four mundane 

 elements, earth, fire, water, air and a fifth submundane, or ether. 

 This name was applied to this element on account of its ethereal cir- 

 cular movement and not on account of its "fire.'' At the present 

 time, of course, we are concerned solely with those ethereal impacts 

 which give rise to illumination, and particularly with those which 

 affect the retinae of our eyes, because, as commonly understood, light 

 is that form of energy which by its action upon this receptor, evokes 

 the phenomenon of vision. In this group must be placed the ethereal 

 vibrations forming the spectral colors, namely, vibrations possessing 

 a rate per second of 482,000,000,000,000 for red light and of 707,000,- 

 000,000,000 for violet light. 



The different sources of light may be divided into natural and artificial. The 

 most important among the former is the sun. Then follow the fixed stars, nebulae, 

 comets, meteors, lightning, auroras and lights modified by reflection and refraction, 

 such as that of the rainbow, clouds, and phosphorescent and fluorescent bodies. 

 Among the artificial sources might be mentioned the combustions of gas, oil, wood, 

 coal, etc., and the illumination produced by the electric current and mechanical 

 impacts. But, since we are dealing in the latter case with certain forms of stored 

 energy, all these sources of light must have had originally an exherent cause, 

 presumably the sun. 



Regarding the cause of light two theories have been propounded, namely, 

 the emission or corpuscular theory, generally accredited to Newton, and the undula- 

 tory theory of Huyghens and Euler. The first assumes that the different luminous 

 bodies actually discharge certain particles or molecules in straight lines. Conse- 

 quently, luminous vibrations are really transverse in their direction, while those of 

 sound are longitudinal. This assumption would lead us to infer that the retina 

 is stimulated by actual molecules of matter. The second theory holds that all 

 space is filled with an attenuated medium, called luminiferous ether, which is set 

 into rapid vibration. This conception would imply that the retina is stimulated 

 by the vibration of the molecules of the ether, in analogy with the excitation of 

 the organ of Corti by vibrations or waves occurring in the ordinary atmosphere. 



794 



