318 THE SPECIAL SENSES. 



acting. If, for instance, the retina is being stimulated by a light of 

 1 candle power an increase of illumination to 1.1 candle power may 

 make a perceptible difference in sensation. But if the retina is 

 being illuminated b^y a light of 10 candle power an increase to 10.1 

 candle power would probably make no perceptible difference. For 

 a certain range of stimulation, in fact, it has been stated that the 

 increase in stimulus must be a constant fractional part of the stimu- 

 lus already acting. That is, in the hypothetical case given, if, with 

 1 candle power, an increase to 1.1 candle power makes a just per- 

 ceptible difference in consciousness, then with 10 candle power an 

 increase of y q- of the acting stimulus, namely 1 candle power will 

 be necessary to cause a perceptible difference. The relation as 

 expressed in this form is known as Weber's law ; but it seems prob- 

 able that, while the general fact is true, this exact expression of it 

 holds only approximately for an intermediate range of stimulation. 

 In this matter of a threshold stimulus the sensitiveness of the 

 retina shows also certain interesting differences in the foveal as 

 compared with the peripheral field. The difference is especially 

 marked when the reaction of the retina in strong lights is compared 

 with its reaction in dim lights. 



The Light-adapted and the Dark-adapted Eye. The con- 

 dition of the retina changes when after exposure to light it is sub- 

 mitted to darkness, the change being most marked in the peripheral 

 field. The change is known as an adaptation, and in this respect 

 the retina differs from the sensitive photographic plate. When the 

 eye, for instance, is kept in the dark, the sensitiveness in the periph- 

 eral field increases during an hour or so, while that of the foveal field 

 is apparently unchanged. With such a dark-adapted eye, there- 

 fore, there will be a certain dim light which will be seen by the 

 peripheral parts of the retina, but perhaps will cause no reaction 

 upon the fovea. For such a degree of light, therefoi^, the fovea 

 would be blind. This general fact has, indeed, long been known. 

 Anyone may notice in late twilight, when the stars are beginning 

 to appear, that a very faint star may disappear when looked at, that 

 is, when its image is brought upon the fovea ; to see it one must direct 

 his eyes a little to the side, so as to bring its image into the periph- 

 eral field. This greater sensitiveness of the dark-adapted eye in 

 the peripheral field where the rods predominate over the cones seems 

 to be associated with the movement of the pigment in the pigment 

 epithelium (see above) and the resulting regeneration of the visual 

 purple in the external segments of the rods. The increase in the 

 visual purple in the dark may, indeed, account for the increased 

 sensitiveness to light in the rod-region and explain why a similar 

 increase fails to occur in the fovea, where only cones are present. 

 The curve given in Fig. 136 shows that in the dark-adapted eye 



