520 PSYCUOLOOY. 



difficult. Such fusions of many sensations into what, to conscious 

 perception, seems a simple whole, abound in all our senses. 



"Physiological optics affords other interesting examples. The per- 

 ception of the botiily form of a near object comes about through the 

 combination of two diverse pictures which the eyes severally receive 

 from it, and whose diversity is due to the different position of each eye, 

 altering the perspective view of what is before it. Before the invention 

 of the stereoscope this explanation could only be assumed hypothetically; 

 but it can now be proved at any moment by the use of the instrument. 

 Into the stereoscope we insert two flat drawings, representing the two 

 perspective views of the two eyes, in such a manner that each eye sees 

 its own view in the proper place ; and we obtain, in consequence, the 

 percejition of a single extended solid, as complete and vivid as if we 

 had the real object before us. 



" Now we can, it is true, by shutting one eye after the other and at- 

 tending to the point, recognize the difference in the pictures — at least 

 when it is not too small. But, for the stereoscopic perception of solidity, 

 pictures suffice whose difference is so extraordinarily slight as hardly 

 to be recognized by the most careful comparison ; and it is certain that, 

 in our ordinary careless observing of bodily objects, we never dream 

 that the perception is due to two perspective views fused into one, be- 

 cause it is an entirely different kind of perception from that of either 

 flat perspective view by itself. It is certain, therefore, that two different 

 sensations of our two eyes fuse into a third perception entirely different 

 from either. Just as partial tones fuse into the perception of a certain 

 instrument's voice ; and just as we learn to separate the partial tones 

 of a vibrating string by pinching a nodal point and letting them sound 

 in isolation ; so we learn to separate the images on the two eyes by 

 opening and closing them alternately. 



"There are other much more complex instances of the way in which 

 many sensations may combine to serve as the basis of a quite simple 

 perception. AVhen, for example we perceive an object in a certain 

 direction, we must somehow be impressed by the fact that certain of 

 our optic nerve-fibres, and no others, are impressed by its light. Fur- 

 thermore, we must rightly judge the position of our eyes in our head, 

 and of our head upon our body, by means of feelings in our eye-muscles 

 and our neck-muscles respectively. If any of these processes is dis- 

 turbed we get a false perception of the object's position. The nerve- 

 fibres can be changed by a prism before the eye; or the eyeball's position 

 changed by pressing the organ towards one side; and such experiments 

 show that, for the simple seeing of the position of an object, sensations 

 of these two sorts must concur. But it would be quite impossible to 

 gather this directly from the sensible impression which the oVjject 

 makes. Even when we have made experiments and convinced ourselves 

 in every possible manner that such must be the fact, it still remains 

 hidden from our immediate introspective observation. 



"These examples" [of 'synthetic perception,' perception in which 



