

KNOWLEDGE OF MOTION. 463 



when these take place in a direction across the field of vision, in which 

 case, the image of the moving object has a motion across the retina, 

 and hence the motion is perceptible. Movements in the direction of 

 the line of vision, i. e., from, or towards, the centre of the eye, are not 

 immediately perceived, the image, as a whole, having no motion across 

 the retina, although its dimensions undergo increase or diminution, 

 according as it is approaching or receding. The apparent movement 

 of an object, from one point in the field of vision to another, depends 

 either on the motion of the image upon the retina, or on the motion of 

 the latter in regard to the image. The extent of this motion, is 

 measured by the angle formed in the eye, by two lines drawn through 

 the eyeball, the one from the point whence the object moves, the other 

 from the point where it is arrested ; hence the movement is termed the 

 angular motion of the object. This motion may be too slow to be 

 easily observed ; and, on the other hand, unless the perception of a 

 moving body be sufficiently prolonged in time,, it appears to stand still; 

 as when a rapidly moving body is instantaneously illuminated by an 

 electric flash; or as when the capillary circulation in the web of the 

 frog's foot, is momentarily looked at under the microscope. 



We judge of the real motion of objects from point to point in space, 

 when these are within the range of the binocular parallax, by a stere- 

 oscopic use of both eyes ; in estimating the actual direction and extent 

 of the motion, we are assisted by those changes in the convergence and 

 adjustment of the eyes, which are essential to keep the moving object 

 in sight. Moreover, changes in size, distinctness of outline, and color, 

 and the passage of the moving body before or behind others, assist us 

 in the formation of a judgment. In the case of objects beyond the 

 range of the binocular parallax, these latter are the only data on which 

 we base our conclusions. 



It has been remarked, that when a given object approaches, or re- 

 cedes from, us, the estimate formed by the mind of its real size, does 

 not become confused or altered. It must, however, be added, that this 

 is only true of objects moving at a moderate rate; for certainly, as 

 any one may find by experience, a locomotive engine, advancing rap- 

 idly towards the eye, on a straight piece of railroad, appears to swell 

 out, or, as it were, to grow. The movement is here too rapid for the 

 eye or mind not to be deceived. The explanation offered of the accu- 

 racy of our perceptions under ordinary circumstances, is, that, whilst 

 the increase of size in the retinal image of an approaching object, sug- 

 gests an increase in its dimensions, yet the convergence of the two 

 eyes necessary for the distinct vision of the object, is accompanied by 

 a reduction of its apparent magnitude. The two effects, therefore, 

 counteract each other. In the case of a receding object, the opposite 

 conditions occur. 



The movements of the eye itself, are accompanied by movements of 

 the retina, as it were, behind, or through, the images of the various 

 objects in the visual field; if the eyeballs be rapidly moved, these 

 objects appear to acquire a general motion. There is, however, no 

 apparent motion of the objects in regard to each other, for the relative 

 position of their images remains unaltered. When we are carried 



