334 THE SPECIAL SENSES. 



to that described for stimulation of nerves in general (p. 104). Not 

 only is there a "light response" each time that the retina is stimu- 

 lated by light, but there is a similar electrical change, a "dark 

 response," when the light is suddenly withdrawn. This last 

 interesting fact would seem to indicate a stimulation process of 

 some kind in the retina due to darkness — that is, withdrawal 

 of the objective stimulus. Einthoven and Jolly* have applied 

 the sensitive string galvanometer to the study of this phenome- 

 non. They find that the electrical response of the illuminated 

 eye, when photographed, presents a curve of much complexity, 

 and they conclude that its complexity is due to the fact that 

 several different processes occur together in the stimulated 

 retina. They offer some evidence to indicate that three different 

 processes depending on the reaction of three different substances 

 may be distinguished. These substances react with different 

 velocities and with different changes in electric potential to 

 flashes of light and "flashes of darkness." What physiological 

 effects may be connected with these three processes cannot 

 yet be stated. The electrical reaction is a very sensitive one, 

 lights so weak as to be near the threshold for the human eye 

 give a distinct electrical change in the frog's retina, and an eye 

 that has been kept in the dark for some time (dark-adapted eye) 

 shows an increased sensitiveness. It is very interesting, also, 

 to find that the frog's retina responds to a range of light vibrations 

 that corresponds with the limits of the visible spectrum as seen 

 by the human eye. If the electrical response is a true indication 

 of functional activity, it would appear that the frog's vision has 

 about the same extent as our o^m as regards the ether waves 

 of different periods of vibration. 



The Visual Purple — Rhodopsin. — ^The change that takes place 

 in the rods and cones whereby the vibratory energy of the ether 

 waves is converted into nerve impulses is unknown. It has been 

 assumed by some observers that the light waves act mechanically, 

 the wave movements setting into vibration portions of the external 

 segments of the rods or cones, and that this mechanical movement 

 forms the direct excitant of the nerve impulses, f The general 

 view, however, is that the process is photochemical, ^that is, the 

 impact of the ether waves sets up chemical changes in the rods or 

 cones which in turn give rise to nerve impulses that are transmitted 

 to the brain. We have an analogy for this action in the known 

 change produced by light upon sensitized photographic films. In 

 the retina itself some basis for such a view is found in the existence 

 of a red pigment which is bleached by light. This interesting dis- 



* Einthoven and Jolly, "Quarterly Journal of Experimental Physiology,' 

 1, 37.3, 1908. 



t Zenker, " Archiv f. mik. Anatomie," 3, 248, 1867. 



