MO VEMENTS OF THE E YES. 1129 



than its corresponding distance on the temporal half. There will be a 

 certain distance, at which the point of intersection of the indirect lines of 

 vision will lie in the same vertical plane as the fixation point (Fig. 403) ; 

 nearer than this they will lie nearer (Fig. 404), beyond this they will be 

 farther (Fig. 402) than the fixation point. The horopter, as experiment- 

 ally determined, is more extended than the theoretical or mathematical 

 horopter, which Hering speaks of as the skeleton of the real horopter. 



MOVEMENTS OF THE EYES. 



Methods of measurement. The centre round which the eyeball 

 rotates has already been dealt with (p. 1029). The plane passing through 

 the two lines of fixation is known as the plane of fixation. In investigat- 

 ing the movements of the eyes, it is important to exclude movements 

 of the head, and this is done most satisfactorily by means of a fixed board 

 with a cast into which the teeth of the observer fit accurately. The head 

 may then be fixed in any position, and the amount of ocular move- 

 ment measured by means of movable fixation points on graduated scales. 



The movement of the eyeballs which presents the greatest difficulty 

 is the wheel or swivel rotation round the optic axis. Numerous methods 

 have been devised for observing it and for measuring its amount. It 

 may be observed directly by watching a marking of the iris, a 

 conjunctival vessel, or a mark placed on the conjunctiva (J. Miiller, 

 1826). It has been doubted whether the conjunctiva follows the 

 movements of the eyeball with sufficient exactness for this purpose, but 

 this seems to be the case. Another method is to make use either of 

 regular or irregular astigmatism. In the latter the movement of the 

 star-like figure may be observed by comparing it with a stationary 

 thread. In regular astigmatism the amount of rotation may be 

 measured by finding the cylindrical glass which corrects the defect 

 in different positions ; for satisfactory results, probably a degree of 

 astigmatism of over 1 D is necessary. Another method which has 

 been employed satisfactorily for exact measurements is the degree of 

 rotation of the blind spot. The method, due to Euete, which has been 

 most widely used, is the observation of the apparent rotation of an after- 

 image. A linear after-image will show the changed direction of any 

 retinal meridian on which a linear image has been formed, and this 

 method is well adapted for quantitative results. 



Another method by means of which any difference in the swivel 

 rotation of the two eyes may be detected, is the observation of half- 

 images of a doubled vertical line. This method is very delicate, and gives 

 more exact results than the after-image method, but the error due to 

 retinal incongruence (p. 1125) must be taken into account. 



Listing's law. When the head is fixed, there is, according to 

 Listing, only one position of the lines of fixation from which the 

 eyeball is able to move upwards and downwards and from side to side 

 without any swivel rotation. This position is known as the primary 

 position of the line of fixation. Listing's law states that when the 

 eyeball moves from the primary position to any secondary position, 

 the axis round which the movement of the eyeball takes place is one 

 at right angles to a plane passing through the line of fixation in both 

 its primary and its secondary positions. In other words, the axis 

 round which oblique movements take place lies in the same plane as the 

 axes round which simple horizontal and vertical movements take place. 



