504 THE SPECIAL SENSES. 



retina to distinguish them apart. Consequently we can no longer 

 appreciate the form of the object. 



For the same reason, when we mix together minute grains of a 

 different hue, we produce an intermediate color. If yellow and 

 blue be mingled in this way, we no longer perceive the separate 

 blue and yellow grains, but only a uniform tinge of green; and 

 white and black granules, mixed together, produce, at a short dis- 

 tance, the appearance of a continuous shade of gray. 



Impressions, once produced upon the retina, remain for a short time 

 afterward. Usually these impressions are so evanescent after the* 

 removal of their immediate cause, and are so soon followed by 

 others which are more vivid, that we do not notice their existence. 

 They may very readily be demonstrated, however, by swinging 

 rapidly in a circle before the eyes, in a dark room, a stick lighted 

 at one end. As soon as the motion has attained a certain degree of 

 velocity, the impression produced on the retina, when the lighted 

 end of the stick arrives at any particular spot, remains until it has 

 completed its revolution and has again reached the same point ; 

 so that the effect thus produced upon the eye is that of a continu- 

 ous circle of light. The same fact has been illustrated by the 

 optical contrivance, known as the Thaumatrope, in which successive 

 pictures of similar figures in different positions are made to revolve 

 rapidly before the eye, and thus to produce the apparent effect of a 

 single figure in rapid motion ; since the eye fails to perceive the 

 intervals between the different pictures. 



The sense of vision, therefore, through the impressions of light, 

 gives us ideas of form, size, color, position, distance, and movement. 

 But these ideas may also be excited by impressions derived from 

 an internal source, as well as those produced by rays coming from 

 without. And it is one of the most striking peculiarities of the 

 sense of sight that these ideal or internal impressions which are 

 excited in it by various causes, are much more vivid and powerful 

 than those of any other of the senses. Thus, in a dream, we often see 

 external objects, with all their visible peculiarities of light, color 

 form, fec., nearly or quite as distinctly as when we are awake ; but 

 the imaginary impressions of sound, in this condition, are always 

 comparatively faint, and those of taste, smell, and touch, almost 

 entirely imperceptible. Even in a reverie, in the waking condi- 

 tion, when the absorption of the mind in its own thoughts is com- 

 plete, and we are withdrawn altogether from outward influences, 

 we see objects which have no present existence as if they were 



