ATTENTION. 441 



adaptations. And we remark that our feeling of the strain of our 

 inward attentiveness increases with every increase in the strength of 

 the impressions on whose perception we are intent." * 



The natural way of conceiving all this is under the sym- 

 bolic form of a brain-cell played upon from two directions. 

 Whilst the object excites it from without, other brain-cells, 

 or jDerhaps spiritual forces, arouse it from within. The latter 

 influence is the 'adaptation of the attention.' The plenary 

 energy of the hrain-cell demands the co-operation of both fac- 

 tors : not when merely jDresent, but when both present and 

 attended to, is the object fully perceived. 



A few additional experiences will now be perfectly clear. 

 Helmholtz, for instance, adds this observation to the pas- 

 sage we quoted a while ago concerning the stereoscopic 

 pictures lit by the electric spark. 



" These experiments," he says, " are interesting as regards tlie part 

 which attention plays in the matter of double images. . . . For in 

 pictures so simple that it is relatively difficult forme to see them double, 

 I can succeed in seeing them double, even when the illumination is only 

 instantaneous, the moment I strive to imagine in a lively way how 

 they ought then to look. The influence of attention is here pure ; for 

 all eye movements are shutout."! 



In another place % the same writer says : 



" When I have before my eyes a pair of stereoscopic drawings which 

 are hard to combine, it is difficult to bring the lines and points that 

 correspond, to cover each other, and with every little motion of the eyes 

 they glide apart. But if I chance to gain a lively mental image {An- 

 schauimgshild) of the represented solid form (a thing that often occurs 

 by lucky chance), I then move my two eyes with perfect certainty over 

 the figure without the picture separating again." 



Again, writing of retinal rivalry, Helmholtz says : 



" It is not a trial of strength between two sensations, but depends 

 on our fixing or failing to fix the attention. Indeed, there is scarcely 

 any phenomenon so well fitted for the study of the causes which are 

 capable of determining the attention. It is not enough to form the 

 conscious intention of seeing first with one eye and then with the other ; 

 we must form as clear a notion as possible of what we expect to see. 

 Tlien it will actually appear. " § 



* Physiol. Psych., ii. 209. 



t Physiol. Optik, 741. % P. 728. 



§ Popular Scientific Lectures, Eng. Tmns., p. 295. 



