88 PSTCHOLOGT. 



ness violates all our habits of association. For tlie same 

 reason it is Tery easy to make an intaglio cast of a face, or 

 the painted inside of a pasteboard mask, look convex, in- 

 stead of concave as they are. 



Our sense of the position of things with respect to our 

 eye consists in suggestions of how we must move our hand 

 to touch them. Certain places of the image on the retina, 

 certain actively-produced positions of the eyeballs, are 

 normally linked with the sense of every determinate posi- 

 tion which an outer thing may come to occupy. Hence we 

 perceive the usual position, even if the optical sensation be 

 artificially brought from a different part of space. Prisms 

 warp the light-rays in this way, and throw upon the retina 

 the image of an object situated, say, at spot a of space in the 

 same manner in which (without the prismsj an object situ- 

 ated at spot h would cast its image Accordingly we feel 

 for the object at h instead of a. If the prism be before one 

 eye only we see the object at h with that eye, and in its 

 right position a with the other — in other words, we see it 

 double. If both eyes be armed with prisms with their angle 

 towards the right, we pass our hand to the right of all objects 

 when we try rapidly to touch them. And this illusory 

 sense of their position lasts until a new association is fixed, 

 when on removing the prisms a contrary illusion at first 

 occurs. Passive or unintentional changes in the position 

 of the eyeballs seem to be no more kept account of by the 

 mind than prisms are ; so we spontaneously make no allow- 

 ance for them in our perception of distance and movements. 

 Press one of the eyeballs into a strained position with the 

 finger, and objects move and are translocated accordingly, 

 just as when prisms are used. 



Curious illusions of movement in objects occur whenever 

 the eyeballs move without our intending it. We shall learn 

 in the following chapter that the original visual feeling of 

 movement is produced by any image passing over the retina. 

 Originally, however, this sensation is definitely referred 

 neither to the object nor to the eyes. Such definite refer- 

 ence grows up later, and obeys certain simple laws. We 

 believe objects to move : 1) whenever we get the retinal 

 movement-feeling, but think our eyes are still ; and 2) when- 



