1130 



VISION, 



Fig. 405.— Hering. 



The law was first formulated by Listing 1 on theoretical grounds, 

 and was shown experimentally by Helmholtz to be true for distant 

 vision. 



Helmholtz used the after-image method, in which the observer 

 should be seated opposite a screen at sufficient distance to render the 

 convergence inappreciable. The position of the head must be found 



from which the after-images of a 

 vertical and a horizontal strip do not 

 deviate from a vertical and a hori- 

 zontal line respectively on the screen 

 with vertical and lateral movements 

 of the eyes. When this position is 

 found, the line of fixation of each eye 

 is in the primary position, and the 

 correspondence of the after - images 

 with the lines shows that Listing's 

 law is true for simple vertical and 

 horizontal movements of the eyeball. 

 In order to show that the same is true 

 for oblique directions, the screen should 

 be capable of rotation in a vertical 

 plane round its centre, and the after- 

 images of oblique strips will be found not to deviate from lines on the 

 screen of corresponding obliquity. If, on the other hand, a fixed screen 

 is used, such as is shown in Fig. 405, the after-images of horizontal 

 and vertical strips will appear distorted, as shown in that figure. 

 This distortion is apparent, and depends on the fact that the lines of 

 the screen, which really cross at right angles, give 

 rise to retinal images in which the lines cross 

 obliquely. Nevertheless the objective lines are 

 seen at right angles, and the obliquity is ascribed 

 to the after-image, as is represented in Fig. 406. 

 Donders found that it made no difference 

 whether the visual axis was moved from the 

 primary to a secondary position in a straight 

 line or in any other way, and this fact was 

 referred to by Helmholtz as Donders' law. The 

 apparent distortion of the after-image is exactly 

 the same for each eye. 



Listing's law only applies to parallel or 

 approximately parallel positions of the visual 

 axes. With convergence there is evidence of a 

 swivel rotation of the eyeballs outwards, i.e. an 

 increase of the normal divergence upwards of the 

 median vertical sections of the two retinas. 



According to Donders, 2 the swivel rotation 

 of convergence may be shown by a simple experi- 

 ment, in which a horizontal line, doubled by con- 

 vergence for a nearer point, gives two images which form an angle open 

 downwards. The amount varies considerably in different individuals. 

 In seventeen cases Donders found it to vary from l° - 2 to 5 0- 5. 



1 See Ruete's "Lehrbuch d. Ophth.," 1857. 



- Jrch.f. d. ges. Physiol., Bonn, 1876, Bd. xiii. S. 418. 



Fig. 406.— Hering. 



