946 LISTING'S LAW. [BOOK m. 



part of Fig. 158 could be rotated round the vertical or round the 

 horizontal axis or round any intermediate oblique axis without 

 the two images of an external object ceasing to fall on corre- 

 sponding parts. But if the retinal circles were twirled round 

 their respective visual axes, the lines of separation, v. m. and 

 h. m., would rotate in a clock-hand fashion, and if the move- 

 ments of the two eyes were unequal or in opposite directions, 

 a dislocation of corresponding parts would ensue, and vision 

 would become double. The limitation to the movements of 

 the eyeball so as to avoid a swivel rotation is in the interests 

 of binocular vision. 



591. Not only do we find ourselves thus limited in our 

 power when we attempt by a direct effort of our will to execute 

 particular movements of the eyeball, but a similar limitation 

 obtains in the natural movements of the eye in vision. The 

 various movements of the eyeballs which we carry out when 

 we are looking at things conform to a general law, which is 

 known as "Listing's Law," and which may be described as 

 follows. 



We stated a little while back that the " primary position " 

 of the eyeball is one in which the visual axis lies parallel to the 

 median plane and is directed to the distant horizon. When the 

 eyeball is changed from this primary position into any other 

 position, all of which may be called secondary positions, the 

 change is effected without any swivel rotation round the visual 

 axis itself; the visual axis may be directed up and down, or 

 from side to side or in any intermediate oblique manner with- 

 out any such swivel rotation taking place. In other words the 

 movements by which the eyeball is brought from the primary 

 position into any of the secondary positions are, in all cases, 

 movements of rotation round the horizontal axis, or round the 

 vertical axis, or round an axis, which though oblique, being 

 neither horizontal nor vertical, lies in the same plane that they 

 do; that is to say every movement from the primary to a secon- 

 dary position is a movement of rotation round an axis lying in 

 a plane which passing through the centre of rotation is vertical 

 to the visual axis. 



The experimental proof of " Listing's law " may be obtained 

 by the help of negative images ( 584) in the following manner. 

 Let the eye be directed to a grey wall or board which, otherwise 

 of uniform appearance, is marked by parallel vertical, and hori- 

 zontal lines, placed at some little distance from each other so as 

 to give a pattern of squares. At one of the intersections, which 

 is to be used as the fixed point of vision, place two narrow strips 

 of red paper in the form of a cross, one vertical coinciding with 

 the vertical line and the. other horizontal coinciding with the 

 horizontal line. Having brought the eye carefully into the pri- 

 mary position stare at the red cross until on turning the eye 



