310 THE SPECIAL SENSES. 



sponse," when the light is suddenly withdrawn. This last interest- 

 ing fact would seem to indicate a stimulation process of some kind 

 in the retina due to darkness, that is, withdrawal of the objective 

 stimulus. The 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 sensitive- 

 ness. 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 own 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.* 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- 

 covery was made by Boll,| and the facts were afterward carefully 

 investigated by Kuhne.t The red pigment, known usually as 

 visual purple or rhodopsin, is found only in the external segments 

 of the rods; the cones do not contain it. In the fovea, therefore, 

 which has only cones, the pigment is entirely absent. The existence 

 of the visual purple may be demonstrated very easily. A frog is 

 kept for some time in the dark; it is then killed and an eye removed 

 and bisected equatorially. If the vitreous is removed from the pos- 

 terior half the retina may be detached by means of a pair of forceps. 

 When the operation is performed in red or yellow light, as in photo- 

 graphic work, the detached retina on examination by daylight is 

 found to be a deep-red color; but after a short exposure it fades 

 rapidly, finally becoming colorless. If the frogs before operation 

 were exposed to strong daylight, the retina is found to be 



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



fBoll, "Archiv f. Physiologie," 1877, 4. 



j Kiihne, "Untersuch. a. d. physiol. Inst. d. Univ. Heidelberg," vol. i, 

 1878. Also "The Photochemistry of the Retina, etc.," translated by Foster, 

 London, 1878. 



