CHAP, in.] SIGHT. 947 



away a green negative image is produced. If now the vision be 

 directed from the fixed point either up or down along the verti- 

 cal 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 nega- 

 tive image coincides in turn with each of the crosses of the 

 pattern on the wall, the horizontal limb coinciding with a horizon- 

 tal line arid the vertical limb with a vertical line. This shews 

 that during the up and down and during the side to side move- 

 ment, during the rotation of the eyeball round its horizontal or 

 round its vertical axis, no swivel rotation has taken place, for 

 otherwise the negative image would have been turned round, 

 and its cross would make an angle with the image of the cross 

 on the wall. If the pattern on the wall be changed so that the 

 lines while still at right angles to each other are oblique, not 

 vertical and horizontal (this is most conveniently done by using 

 not a wall but a large board and turning the board round), and 

 the observation be repeated except that the eye is turned not 

 vertically or horizontally but obliquely so as to follow the lines 

 of the pattern, it will still be found that the cross of the nega- 

 tive image coincides with the cross of the pattern, and that 

 whatever be the angle round which the board lias been turned. 

 This shews that Listing's law holds good not only for up and 

 down and side to side movements but also for oblique move- 

 ments, for movements of rotation round an axis which whatever 

 its obliquity lies in a plane at right angles to the visual axis. 



In the ordinary movements of the eye then, a swivel rota- 

 tion round the visual axis does not take place ; and this limi- 

 tation, since it holds good for the two eyes used together, as 

 well as for one eye used by itself, serves to secure single vision 

 with two eyes inasmuch as it avoids changes which might cause 

 the images of external objects to fall on the parts of the two 

 retinas which were not " corresponding parts." In certain move- 

 ments of the eyes, however, a certain amount of swivel rotation 

 does take place. This is .especially seen in somewhat unusual 

 movements. For instance when the head is turned down to the 

 shoulder, or again when in directing vision to any object, tlu- la-ad 

 is moved from side to side, the eyes do not move with the head ; 

 they appear to remain stationary, very much as the needle of 

 a snip's compass remains stationary when the head of the ship 

 is turned. The change in the position of the visual axes to 

 which the movement of the head would naturally give rise U 

 met by compensating movements of the eyeballs ; were it not 

 so, steadiness of vision would be impossible ; and these compen- 

 sating movements are found, on careful examination, to include 

 a certain amount of swivel rotation round the visual axes. In 

 certain other more usual movements some amount of such a 

 swivel rotation is also present; and indeed, though so long as 

 the visual axes remain parallel, movement in any direction may 



