Chemistry and Physics. 433 



point. There is much praise of the application of ozone to the 

 purification of water supplies as practiced in Europe, and this 

 application is strongly urged for this country, where at present it 

 appears to be comparatively neglected. The view that minute 

 quantities of ozone in the air make it agreeable and healthful for 

 inhalation is advocated, and the use of the substance for the 

 purification of air is recommended. h. l. w. 



5. A Theory of Color Vision. — At the present time there is 

 no generally accepted theory of color vision, and there is consid- 

 erable divergence of opinion as to the tests which should be 

 applied for color-blindness. Consequently the new point of view 

 proposed by R. A. Houstoun may be appropriately outlined in 

 this place. The author first shows that the Young-Helmholtz 

 theory, which accounts very well for the phenomena of color 

 mixing, involves a certain amount of arbitrariness and does not 

 demonstrate the existence of three primary color sensations. The 

 paper is divided into two parts : the first, which deals with the 

 retinal process, applies principles already more or less familiar ; 

 but the second part, which relates to the cerebral process, uses an 

 idea quite original in its application to color vision. 



(a) The first step in the argument consists in assuming that 

 there exist in the eye a very great number of vibrators, with a 

 free period in the green, and that these execute forced vibrations 

 under the influence of light waves. The amplitude of the forced 

 vibrations is a maximum when the free period of the vibrators 

 coincides with the period of the incident light. When formu- 

 lated mathematically this hypothesis leads to a visibility curve 

 which is similar to the one obtained experimentally by H. E. 

 Ives. These graphs suggest probability curves, for each has a 

 single maximum ordinate (in the green) and bends down toward 

 the axis of wave-lengths on both sides (red and violet). To con- 

 form to Fechner's law the additional assumption is made that, 

 when E" 1 acquires a small increment, the increase in the energy 

 absorbed is proportional to d(E' i )/E 2 , where E cosut represents 

 the force per unit mass exerted on a typical vibrator by the light 

 wave. This assumption implies that some of the vibrators cease 

 to act when E" 1 is increased. " When the energy of a vibrator 

 reaches a critical value, the force attaching the vibrator to its 

 center snaps, the latter then ceases to absorb light energy, and a 

 chemical change takes place. This critical value is not the same 

 for all the vibrators, but varies from vibrator to vibrator. We 

 may identify the chemical change with the bleaching of the 

 visual purple, but this identification is not necessary to the expla- 

 nation." When E is kept constant the same identical vibrators 

 do not remain in action all the time for there are two processes 

 going on in opposite directions which balance one another, visual 

 purple being bleached and constantly restored. When E increases, 

 the point of equilibrium is shifted. Owing to the bleaching and 

 restoration of the visual purple the vibrators must be assumed to 

 be in a perpetual state of agitation. Their free vibrations are 

 continually being renewed. 



