CHAP, in.] SIGHT. 141 



two images of an external object ceasing to fall on corresponding 

 parts. But if the retinal circles were twirled round their 

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

 would rotate in a clock-hand fashion and if the movements 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. 



789. 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 move- 

 ments 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 without 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 secondary 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 ( 782) 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 horizontal 

 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 primary position stare 

 at the red cross until on turning the eye away a green negative 

 image is produced. If now the vision be directed from the fixed 

 point either up or down along the vertical line of the pattern 

 on the wall, or from side to side along the horizontal line, it will 

 be found that the cross of the negative image coincides in turn 



