ANALYSIS OF FIELD OF VISION. 517 



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. 186, we may in succession FlG - 186 - 

 have a vivid perception of the whole, or of dis- 

 tinct parts only ; of the six triangles near the 

 outer circle, of the hexagon in the middle, or 

 of the three large triangles. The more nume- 

 rous and varied the parts of which a figure is 

 composed, the more scope does it afford lor the 

 play of the attention. Hence it is that architectural orna- 

 ments have an enlivening effect on the sense of vision, since 

 they afford constantly fresh subject for the action of the mind. 



The duration of the sensation produced by a luminous 

 impression on the retina is always greater than that of the 

 impression which produces it. However brief the luminous 

 impression, the effect on the retina always lasts for about one- 

 eighth of a second. Thus, supposing an object in motion, say 

 a horse, to be revealed on a dark night by a flash of lightning. 

 The object would be seen apparently for an eighth of a second, 

 but it would not appear in motion ; because, although the 

 image remained on the retina for this time, it was really re- 

 vealed for such an extremely short period (the duration of a 

 flash of lightning being almost instantaneous), that no appre- 

 ciable movement on the part of the object could have taken 

 place in the period during which it was revealed to the retina 

 of the observer. And the same fact is proved in a reverse 

 w r ay. The spokes of a rapidly revolving wheel are not seen 

 as distinct objects, because at every point of the field of vision 

 over which the revolving spokes pass, a given impression has 

 not faded before another comes to replace it. Thus every part 

 of the interior of the wheel appears occupied. 



The duration of the after sensation or spectrum, produced by 

 an object, is greater in a direct ratio with the duration of the 

 impression which caused it. Hence the image of a bright 

 object, as of the panes of a window through which the light 

 is shining, may be perceived in the retina for a considerable 

 period, if we have previously kept our eye fixed for some time 

 on it. 



The color of the spectrum varies with that of the object 

 which produced it. The spectra left by the images of white or 



44 



