874 TKANSACTIONS OF SECTION I. '; 



The other, a ph^-siological "explanation, is that furnished by Hering. He 

 regards the surrounding field as aS'ecting the condition of the central one by 

 producing in it a change of the opposite type to that going on in the general 

 Held — spatial induction. 



If the reacting lield be stimulated originally by the colour which would 

 normally be induced in it by the surrounding or inducing field, then simultaneous 

 contrast is no longer seen until a certain time has elapsed to jillow the area 

 again to resume its normal condition. Thus even brief exposure of the eye to red 

 spectral light will prevent the appearance of the red tint in the central field which 

 is surrounded by a larger green field. One may observe the gradual increase in 

 the induced effect as fatigue passes off", but it does not reach its normal degree 

 until some time after the usual signs of fatigue, as shown by the negative after- 

 image, have disappeared. 



It is interesting, in the light of Ilering's views on the black-white substance, 

 to note the effects of a previous fixation of a central lampblack disc in the green- 

 coloured field. If one examines a patch of black in a spectral green field for a 

 few minutes and then removes the black disc and allows white light to pass 

 through in its place, the induced pink simultaneous contrast-effect is not seen at 

 once, but prior to its appearance one has the sensation of a central white field 

 which slowly, beginning from the periphery, shows the gradual pink colouration 

 of the field. 



That is to saj', during the period when the excitability of the reacting 

 field has been raised by the previous examination of a black field, the inducing 

 colour does not bring out its normal effect. 



This phenomenon is more readily explained on the Ilering than on the 

 Ilelmholtz hypothesis. 



(iii) Some Aspects of Mental Fatigue. By H. Sackville Lawson. 



This paper was concerned with the measurement of mental fatigue by means of 

 the sesthesiometer, which is in reality a modified compass, and has been used in 

 connection with physiological experiments on skin-sensitiveness. Intellectual 

 effort causes an accumulation of waste and poisonous material in the brain-cells. 

 As a result of this there is found a diminished skin-sensitiveness in all parts of 

 the body. Hence the measurement in millimetres of the loss in skin-sensitiveness 

 would appear to be a guide to the mental energy expended ; and the principal of 

 the ajsthesio meter is to record mental fatigue in terms of this nature. 



In conducting these experiments it has been found that fatigue symptoms may 

 or may not have a physiological counterpart. The individual experimented on 

 may feel tired and yet be objectively fresh, or, on the other hand, he may feel tired 

 and at the same time find justification for this feeling on physiological grounds. 



There is yet another kind of fatigue met with in the victims of that 'roten sin' 

 known to Chaucer as accidie, which maketh a man 'bevy, thoughtful, and wrawe' 

 and 'full of slouthe, wanhope, and sompnolence.' Many an accidiose patient feels 

 tired and betrays all the outward casual symptoms of fatigue. And yet a careful 

 reading with the resthesiometer confirms nothing of these symptoms; indicates, ou 

 the contrary, that now, as of old, the best remedy for such 'nerves' is hard work. 



The following are some of the kinds of information to be obtained by the 

 systematic use of the sesthesiometer : — 



1. How mnny units of skin-sensitiveness a student may lose without injury. 



2. The relative fatigue induced by a study of the various subjects in the school 

 curriculum. 



3. Information as to the most correct method of arranging the subjects in the 

 time-table. 



The physiological effect on the student of different methods of teaching may 

 also be shown. 



Lastly, the differences let ween physiological and psychical fatigue are rendered 

 evident, 



