454 SPECIAL PHYSIOLOGY. 



from the axis of vision, are seen only in their general outline, while 

 smaller and darker objects may be invisible. The actual images on 

 the retina are, however, equally clear at all parts of that membrane, 

 and hence the diminution in the distinctness of vision, must proceed 

 from deficient receptivity of its lateral parts. This is observable in 

 regard to color as well as form. It is said that the retinal sensibility 

 diminishes more quickly in the upward and downward directions than 

 laterally. The existence of a limited area in the retina, specially set 

 apart for distinct vision, enables us to concentrate our attention upon 

 special objects in the visual field, undisturbed by the simultaneous 

 images of surrounding objects. Moreover, by this arrangement alone, 

 we are able to direct the optic axes of the eyes in exact and known 

 directions, so as to gain a knowledge of the direction or position of 

 visible objects, and, further, to adjust the axes of the two eyes, so that 

 they shall meet in any given object, a condition essential, as will be 

 presently explained, for the occurrence of single vision with two eyes. 



The adjustment of the optic axes upon any object, is accomplished 

 by the movements of the body and head, and especially by those of 

 the eyeballs themselves, which are very rapid, singularly free, and 

 perfectly under the control of the will, the globe of the eye turning, 

 like a sphere, upon its poles or axes, as it rests, in its capsule, on its 

 smooth cushion of fat. By means of the complex movements of the 

 two eyeballs, already described (p. 422), the field of vision of each eye, 

 and that of the two combined, is perfectly under our command, so 

 that the optic axes can be made to converge, with the most extreme 

 accuracy, upon the smallest object which we can see distinctly. This 

 is effected voluntarily, not, however, as a simple direct act of the will 

 upon the ocular movements, but only indirectly, by the mind seeking, 

 through the eyes, the desired attainment of the distinct vision of any 

 given object. The mind, however, is cognizant, through the muscular 

 sensations, that the desired act is performed, and that the position of 

 the eye is duly adjusted. The combination of harmonious movements 

 of the two eyes, is beyond our direct control, and is perhaps provided 

 for by a commissural, or other, disposition of the governing nerve- 

 fibres of both eyes, in the oculo-motor nervous centres. As already 

 mentioned, the direction of objects is referred to their correspondence 

 with, or deviation from, the position of the optic axes ; and every 

 retinal impression is referred to its proper line of direction in the outer 

 world. The position of these axes is known to us very accurately, by 

 means of impressions, conveyed through the muscular sense, of the 

 condition of the several muscles of the eyeball. The notion of the 

 direction of objects, is therefore not a simple sensation, but the result 

 of a judgment, formed by the mind, from certain impressions conveyed 

 to it. 



The apparent magnitude of an object, is determined by the size of 

 its retinal image, in other words, by the size of its visual angles, or 

 angles formed in the eye, by lines drawn from its extremities or mar- 

 gins through the optic centre of the eye. When the visual angle of 

 an object is known, the object is said to subtend such an angle. The 

 apparent magnitude of an object, is influenced by its distance from the 



