OTHER THEORIES 293 



by the pigment cells under the influence of a bright light would be 

 somewhat different in character from that which is formed in darkness." 

 This supposition affords the basis of an explanation of the photopic and 

 scotopic luminosity curves, of the Purkinje phenomenon, of erythropsia 

 or red- vision, and of green- vision. 



The disappearance of lights falling upon the fovea is attributed 

 to temporary absence of visual purple from the fovea. 



" It is very probable that light acting upon the visual purple is, 

 according to its wave-length, absorbed by particular atoms or molecules, 

 the amplitude of their vibrations being increased. These vibrations 

 may cause corresponding vibrations in certain discs of the outer segments 

 of the cones, which seem especially constructed to take up vibrations. 

 We know that when lisfht falls on the retina it causes an electric current. 

 We know how the telephone is able through electricity to convey waves 

 of sound, and something similar may be present in the eye, the apparatus 

 being especially constructed for vibrations of small wave-length. The 

 current of electricity set up by hght may cause the sensation of light, 

 and the vibrations of the atoms or molecules the sensation of colour. 



" In all vital processes there is a condition of katabolism or chemical 

 change in the protoplasm, and an anabolic or building-up process, in 

 which the protoplasm is restored to its normal state. We have there- 

 fore to consider two definite processes in the visual purple — namely, 

 a breaking down of the visual purple photochemically by light and its 

 restoration by the pigment cells and rods. Under ordinary conditions 

 of light, and during the whole of the daytime, the visual purple is con- 

 tinually being bleached and re-formed. It is obvious, therefore, that 

 when the eye has been kept in the dark and is then exposed to light, an 

 observation taken immediately will not be comparable with one taken 

 a few seconds afterwards, because in the first observation we have only 

 to consider the katabolic change ; whilst in the second observation the 

 anabolic change has to be considered as well, as the visual purple has 

 to be re-formed for subsequent seeing. There appears to be very little 

 evidence in ordinary circumstances of this anabohc process ; for instance, 

 if we fatigue the eye with sodium light in a dark room, and then immedi- 

 ately examine a spectrum, we find that though all the yellow has dis- 

 appeared there is no increase in the blue ; in fact, the blue seems rather 

 diminished than otherwise. Again, there is not the slightest diminution 

 in either the red or green, showing conclusively that yellow cannot be 

 a compound sensation made up by a combination of red and green^." 



^ Hunierian, Lectures, p. 21, 1911. 



