THE PERCEPTION OF SPACE. 



231 



To make objects at different distances seem equidistant, 

 careful precautions must be taken to have them alike in 

 appearance, and to exclude all outward reasons for ascrib- 

 ing to the one a different location from that ascribed to the 

 other. Thus Donders tries to prove the law of projection 

 by taking two similar electric sparks, one behind the other 

 on a dark ground, one seen double ; or an iron rod placed 

 so near to the eyes that its double images seem as broad as 

 that of a fixated stove-pipe, the top and bottom of the objects 

 being cut off by screens, so as to prevent all suggestions 

 of perspective, etc. The three objects in each experiment 

 seem in the same plane.* 



Add to this the impossibility, recognized by all observ- 

 ers, of ever seeing double with iYiefovece, and the fact that 

 authorities as able as those quoted in the note on Wheat- 

 stone's observation deny that they can see double then with 

 identical points, and we are forced to conclude that the 

 projection-theory, like its predecessor, breaks doion. Neither 

 formulates exactly or exhaustively a law for all our perceptions. 



Ambiguity of Betinal Impressions. 



WJiat does each theory try to do ? To make of seen location 

 a fixed function of retinal impression. Other facts may be 



Fig. 57. 



brought forward to shoiv hoiv far from fixed are the perceptive 

 functions of retinal impressions. We alluded a while ago to 

 the extraordinary ambiguity of the retinal image as a re- 

 vealer of magnitude. Produce an after-image of the sun 

 and look at your finger-tip : it will be smaller than your 

 nail. Project it on the table, and it will be as big as a 

 strawberry; on the wall, as large as a plate; on yonder 

 mountain, bigger than a house. And yet it is an unchanged 



*Archiv f. Ophthal., Bd. xvii. Ablh. 2, pp. 44-6 (1871). 



