SPATIAL PERCEPTION. 1147 



sensibility of the retina to movement, usually receive a more complicated 

 psychological explanation. Helmholtz x explained the well-known rail- 

 way carriage and waterfall illusions, by supposing that an observer 

 becomes accustomed to regard the voluntary impulses set up when 

 looking at a moving object, as those adapted for the fixation of a resting 

 object. When the eyes are turned to a resting object, the motor 

 impulses continue, and bring about movements of the eyes ; and since 

 the observer supposes that his eyes are stationary, objects appear to him 

 to move in the opposite direction to the previously observed objective 

 movement. Helmholtz believed that this illusion did not occur with 

 accurate fixation. 



This explanation cannot apply to the modification of the illusions 

 observed with a spiral disc (Fig. 417). When rotated this gives the 

 appearance of circular bands 

 moving inwards and outwards, 

 according to the direction of 

 rotation. When the rotation is 

 stopped, the rings appear to move 

 in the opposite direction, and if 

 any object is observed it will 

 appear to swell or diminish in size. 

 If a grey screen is observed, or if 

 the eyes are closed, the visual 

 field shows a peculiar streaming 

 appearance inwards or outwards, 

 according to the direction of 

 rotation. 



As a modification of the ex- 

 periment, Dvorak 2 laid on a large 

 disc with a spiral a smaller con- 

 centric disc with the spiral in 

 the opposite direction, and on 



this a smaller disc with the spiral in the same direction as the first. 

 The after-image showed, with apparent movement, the opposite directions 

 in the different parts. The appearances occurred with accurate fixation, 

 guaranteed by obtaining a well-defined after-image of a white patch in 

 the centre. According to Helmholtz's explanation, this would involve 

 different ideas of movements simultaneously present for different parts 

 of the retina. 



There seems no doubt that these illusions depend on an after-image 

 change of some kind taking place in the retinal (cerebro-retinal) apparatus, 

 and are independent of movements of the eyes. Dvorak has investigated 

 other conditions to find if change of stimulation can produce an after- 

 image, and has found that if the illumination of a room is alternately 

 raised and lowered, that the alternation appears to continue for a time 

 after the illumination is made constant ; during the objective alternation 

 he noticed contraction and dilatation of the pupil, which did not continue 

 during the subjective alternation. These subjective alternations may, 

 however, have been of the same nature as the periodical variations of the 

 after-image. 



When a figure made up of narrow concentric black and white rings 



1 "Handbuch d. physiol. Optik," 2te Aufl., S. 746. 



2 Sitzungsl. d. Jc. AJcad. d. Wissensch., Wien, 1870, Bd. Ixi. Abth. 2, S. 257. 



