UNIVERSITY 



THE SENSES. 



seen. Moreover, if we want carefully to examine any object, we always 

 direct the eyes straight to it, so that its image shall fall on the yellow 

 spot where an image of a given area will cover a larger number of cones 

 than anywhere else in the retina. It has been found that the images of 

 two points must be at least 3/4 apart on the yellow spot in order to be 

 distinguished separately; if the images are nearer together, the points 

 appear as one. The diameter of each cone in this part of the retina is 

 about 3/j.. 



Estimation of Movement. We judge of the motion of an object, 

 partly from the motion of its image over the surface of the retina, and 

 partly from the motion of our eyes following it. If the image upon the 

 retina moves while our eyes and our body are at rest, we conclude that 

 the object is changing its relative position wit*h regard to ourselves. In 

 such a case the movement of the object may be apparent only, as when 

 we are standing upon a body which is in motion, such as a ship. If, on 

 the other hand, the image does not move with regard to the retina, but 

 remains fixed upon the same spot of that membrane, while our eyes fol- 

 low the moving body, we judge of the motion of the object by the sensa- 

 tion of the muscles in action to move the eye. If the image moves over 

 the surface of the retina while the muscles of the eye are acting at the 

 same time in a manner corresponding to this motion, as in reading, we 

 infer that the object is stationary, and we know that we are merely 

 altering the relations of our eyes to the object. Sometimes the object 

 appears to move when both object and eye are fixed, as in vertigo. 



The mind can, by the faculty of attention, concentrate its activity 

 more or less exclusively upon the sense of sight, hearing, and touch alter- 

 nately. When exclusively occupied with the action of one sense, it is 

 scarcely conscious of the sensations of the others. The mind, when deeply 

 immersed in contemplations of another nature, is indifferent to the ac- 

 tions of the sense of sight, as of every other sense. We often, when 

 deep in thought, have our eyes open and fixed, but see nothing, because 

 of the stimulus of ordinary light being unable to excite the brain to 

 perception, when otherwise engaged. The attention which is thus 

 necessary for vision, is necessary also to analyze what the field of vision 

 presents. The mind does not perceive all the objects presented by the 

 field of vision at the same time with equal acuteness, but directs itself 

 first to one and then to another. The sensation becomes more intense, 

 according as the particular object is at the time the principal object of 

 mental contemplation. Any compound mathematical figure produces a 

 different impression according as the attention is directed exclusively to 

 one or the other part of it. Thus in fig. 433 A, we may in succession 

 have a vivid perception of the whole, or of distinct parts only; of the 

 six triangles near the outer circle, of the hexagon in the middle, of the 



