APPARENT DURATION OF SENSATIONS. 339 



tive sensations are smell, taste, and the thermal sense ; for these are 

 referred entirely to the nose, mouth, and heated or chilled surface of 

 the skin, and not to the outer world. The reference of these three 

 degrees of objective sensations, to their respective external causes, is 

 connected with different mental processes ; but, in all cases, an infer- 

 ence is drawn, that they depend on properties of external objects or 

 forces. Thus an agreeable odor is a property of the rose, and a green 

 color of chlorophyll. At first, however, in the infant, sensations are 

 not associated with ideas, or related, through perception, with external 

 objects; but the mind, by some innate process, soon compares sensa- 

 tions, caused by such objects, with those which arise in the interior, 

 and learns to discriminate them from the latter, and so to distinguish 

 the objective "self," from the outer world. The recognition of the 

 subjective "ego," is a still higher mental act. 



Though sensations are, in the normal condition, inseparably linked 

 with consciousness, they may be unattended to, or cease to be regarded 

 by the attentive mind, i. e., by the mind acting in the state called at- 

 tention; the sensations are then so transitory, that they have been 

 named, though, it is here submitted, erroneously, unconscious sensa- 

 tions, a concatenation of terms, apparently illogical, and contrary to 

 fact. In the most perfect sensation, the whole attention must be con- 

 centrated upon the sensory impression. 



The velocity of sensory impressions, in their paths from the periph- 

 eral organs to the sensorial nervous centres, has been already men- 

 tioned (page 223). This velocity is measurable, but so rapid, that, 

 within the limits of the body, the moment of sensation practically 

 coincides with the moment of impression. The intensity of a sensa- 

 tion, is proportional to that of its exciting cause, the state of the sen- 

 sorial apparatus being equal ; but we are unable to perceive any 

 measurably corresponding strength, in the sensations produced ; as, 

 for example, in degrees of flavor, or shades of brightness. The im- 

 pressions made upon the senses of touch and sight, however, yield us 

 definite measurements of space and time. The relative acuteness of 

 sensation, is determined by the ability to distinguish the feeblest 

 stimuli, or stimuli differing but very slightly in intensity. Weak 

 stimuli require a longer period of action than strong ones. 



The apparent duration of sensations, varies in the different senses, 

 and is also influenced by the state of the nerves concerned; thus, the 

 duration of impressions photographed on the retina, is sufficiently long, 

 to enable a luminous circle to be produced by whirling a point of light 

 through the air, and doubtless explains the streaming lights seen in 

 artificial fireworks, and the track of light which a meteor, or falling 

 star, leaves behind it; but the duration of light on the retina, is lim- 

 ited, otherwise one set of images would habitually blend with preced- 

 ing or succeeding ones, as the eye shifted its direction. Again, the 

 duration of auditory impressions is undoubted, but not practically suf- 

 ficient to interfere with the function of hearing. The impressions of 

 smell, and especially those of taste, are more durable, probably, how- 

 ever, because the odorous and sapid particles, which excite them, are 

 not removed, but continue to act on the sentient extremities of the 



