614: HANDBOOK OF PHYSIOLOGY. 



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

 sation 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 al- 

 ternately. 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 actions 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 per- 

 ception, 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 contem- 

 plation. Any compound mathematical figure produces a different im- 

 pression according as the attention is directed exclusively to one or the 

 other part of it. Thus in Fig. 413, 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, or of the three large 

 triangles. The more numerous and varied the parts of which a figure 

 is composed, the more scope does it afford for the play of the attention. 

 Hence it is that architectural ornaments have an enlivening effect on 

 the sense of vision, since they afford constantly fresh subject for the 

 action of the mind. 



Color Sensations. If a ray of sunlight be allowed to pass through 

 a prism, it is decomposed by its passage into rays of different colors, 

 which are called the colors of the spectrum; they are red, orange, yellow, 

 green, blue, indigo, and violet. The red rays are the least turned out of 

 their course by the prism, and the violet the most, whilst the other colors 

 occupy in order places between these two extremes. The differences in 

 the color of the rays depend upon the number of vibrations producing 

 each, the red rays being the least rapid and the violet the most. In 

 addition to the colored rays of the spectrum, there are others which are 

 invisible, but which have definite properties, those to the left of the red, 

 and less refrangible, being the calorific rays which act upon the ther- 

 mometer, and those to the right of the violet which are called actinic or 



