I02 2 THE SENSES 



a good light, to be larger than an exactly equal black circle on a white 

 ground. The explanation is as follows: Owing to the aberration of the 

 refractive media of the eye (p. 987), all the rays proceeding from the 

 luminous object are not brought accurately to a focus on the retina, 

 and the image is surrounded by diffusion circles (p. 988) which encroach 

 upon the unilluminated boundary. Physically these represent a 

 weaker illumination than that of the image proper, and therefore the 

 latter ought to stand out in its real size as a brightei area surrounded by 

 weaker haloes. That this is not the case, and that the image is pro- 

 jected in its full brightness for a certain distance over its dark boundary, 

 is due to the fact that the eye does not recognize very small differences 

 of brightness. When the accommodation is not perfect, the diffusion 

 circles are, of course, much wider, and irradiation is better marked when 

 the object is a little out of focus. 



The Movements of the Eyes. That the eyes may be efficient 

 instruments of vision, it is necessary that they should have the 

 power of moving independently of the head. An eye which could 

 not move, though certainly better than an eye which could not see, 



would* yet be as imperfect after 

 its kind as a ship which could 

 run before the wind, but could 

 not tack. The mere fact that 

 the angle between the visual 

 axes must be adapted to the 

 distance of the object looked at 

 Fig 432 renders this obvious; and the 



beauty of the intrinsic mechan- 

 ism of the eyeball has its fitting complement in the precision, 

 delicacy, and range of movement conferred upon it by its extrinsic 

 muscles. 



Not only are movements of convergence and divergence of the 

 eyeballs necessary in accommodating for objects at different dis- 

 tances, but without compensatory movements of the eyes it would 

 be impossible to avoid diplopiawith every movement of the head; 

 for the images of an object fixed in one position of the head would 

 not continue to fall on corresponding points of the retinae in another 

 position. 



All the complicated movements of the eyeball may be looked 

 upon as rotations round axes passing through a single point, which 

 to a near approximation always remains fixed, and is situated about 

 177 mm. behind the centre of the eye. 



The position which the eyeballs take up when the gaze is directed to 

 the horizon, or to any distant point at the level of the eyes, is called 

 the primary position. Here the visual axes are parallel, and the plane 

 passing through them horizontal. While the head remains fixed in this 

 position, the eyeballs can rotate up or down around a horizontal axis, 

 or from side to side around a vertical axis; or upwards and inwards, 

 downwards and O'.twards, downwards and inwards, and upwards and 

 outwards around oblique axes, which always lie in the same plane as 

 the vertical and horizontal axes of rotation i.e., in the vertical plane 



