422 



psYCHOLoar. 



nomenon called by that name is this, that if we look with 

 each eye upou a differeut picture (as iu the annexed stereo- 

 scopic slide), sometimes one j)icture, sometimes the other, 



Fig. 36. 



or parts of both, will come to consciousness, but hardly 

 ever both combined. Helmholtz now says : 



" I find that I am able to attend voluntarily, now to one and now 

 to the other system of lines ; and that then this system remains visi- 

 ble alone for a certain time, whilst the other completely vanishes. 

 This happens, for example, whenever I try to count the lines first of 

 one and then of the other system. . . . But it is extremely hard to 

 chain the attention down to one of the systems for long, unless we 

 associate with our looking some distinct purpose which keeps the ac- 

 tivity of the attention perpetually renewed. Such a one is counting the 

 lines, comparing their intervals, or the like. An equilibrium of the 

 attention, persistent for any length of time, is under no circumstances 

 attainable. The natural tendency of attention when left to itself is to 

 wander to ever new things ; and so soon as the interest of its object is 

 over, so soon as nothing new is to be noticed there, it passes, in spite of 

 our will, to something else. If we wish to keep it upon one and the same 

 object, we must seek constantly to find out something new about the 

 latter, especially if other powerful impressions are attracting us away." 



And again criticising an author who had treated of at- 

 tention as an activity absolutely subject to the conscious 

 will, Helmholtz writes : 



" This is only restrictedly true. We move our eyes by our will ; but 

 one without training cannot so easily execute the intention of making 

 them converge. At any moment, however, he can execute that of 

 looking at a near object, in which act convergence is involved. Now 



