MOVEMENTS OF THE EYEBALLS 587 



these movements of rotation ever occur under normal circumstances. This 

 may be tested in the following way : The gaze is first directed at a brilliant 

 line of light, e.g. a straight electric incandescent filament. The gaze is then 

 directed to a uniform white surface. On this white surface a negative after- 

 image of the vertical line is seen. It is found that in whatever direction the 

 eyes be turned, upwards, downwards, or obliquely to right or left, the after- 

 image always retains its vertical direction. If there were rotation of the 

 eyeballs on their antero- posterior diameters, the stimulated portions of the 

 retina, i.e. those which are the seat of the after-image, would lie obliquely, 

 and the apparent direction of the negative after-image would be also oblique. 

 We see therefore that, under normal circumstances, no rotation of the eyeball 

 on its antero- posterior diameter takes place. The actions of the different 

 muscles are always so co-ordinated that all movements of the eyeballs take 

 place round axes, which lie in a plane passing through the centre of rotation 

 of the eyeballs and at right angles to the visual axes. 



In man movements of the eyes are always bilateral and take place in such 

 a way that an image of one and the same object will fall on the central spot 

 of each retina. These movements are simple in character and are of only two 

 kinds, viz. : 



(1) Movements of both eyes with maintenance of parallelism of the visual 

 axes ; to this class belong the movements of conjugate deviation employed in 

 following the passage of an object across the field of vision from right to left, 

 or vice versa, as well as less extensive upward and downward movements 

 of both eyeballs. 



(2) The movement of convergence of the axes of bo.th eyes, which is 

 always associated with accommodation for near objects, and therefore with 

 contraction of the ciliary muscles and of the constrictors of the pupils. 



Other movements can be effected, but only with effort. Thus we 

 can converge the axes of the eyes so as to look at a near object lying to one 

 side of us. Such an action is, however, associated with considerable effort, 

 and in nearly all cases is replaced by movements of the head. Whenever 

 we wish to examine an object closely we turn the head so that the object lies 

 between the two eyes, and a simple movement of convergence serves to bring 

 the image of the object on to the two foveae centrales. 



BINOCULAR VISION CORRESPONDING POINTS 



When we fix our gaze on any object, although an image of the object is 

 formed on each retina, the object appears as single and not as double. On 

 the other hand, gentle pressure on one eyeball, so as to shift its gaze slightly 

 from that imposed on it by the co-ordinated action of the ocular muscles, at 

 once causes the object to appear double. This shows that the single appear- 

 ance of an object seen with the two eyes is due to the fact that the image 

 from this object must fall upon points in the retina, simultaneous stimulation 

 of which produces only a single sensation. These points are known as 

 ' corresponding points.' It is evident that to each point in one retina only 

 one point can exist in the other retina which corresponds to it, since for every 



