474 THE HUMAN BODY. 



creases proportionately to the logarithm of the strength of the 

 stimulus. Examples of this, which is known as " Weber's " 

 or "Fechner's psycho-physical " laiv will be hereafter 

 pointed out, and are readily observable in daily life; we 

 have, for example, a luminous sensation of certain intensity 

 when a lighted candle is brought into a darkroom; this sen- 

 sation is not doubled when a second candle is brought in; 

 and is hardly affected at all by a third. The law is only 

 true, however (and then but approximately), for sensations 

 of medium intensity; it is applicable, for example, to light 

 sensations of all degrees between those aroused by the light 

 of a candle and ordinary clear daylight: but it is not true 

 for luminosities so feeble as only to be seen at all with diffi- 

 culty, or so bright as to be dazzling. 



Besides their variations in intensity, dependent on varia- 

 tions in the strength of the stimulus, our sensations also 

 vary with the irritability of the sensory apparatus itself; 

 "which, is not constant from time to time or from person to 

 person. In the above statements the condition of the sense- 

 organ and its nervous connections is presumed to remain 

 the same throughout. 



Perceptions. In every sensation we have to carefully 

 distinguish between the pure sensation and certain judg- 

 ments founded upon it; weJiave to distinguish between what 

 we really feel and what we think we feel; and very often 

 firmly believe we do feel when we do not. 



The most important of these judgments is that which 

 leads us to ascribe certain sensations, those aroused through 

 organs of special sense, to external objects that outer 

 reference of our sensations which leads us to form ideas 

 oncerning the existence, form, position, and properties of 

 external things. Such representations as these, founded on 

 our senses, are called perceptions. Since these _p.1wq.yp imply 

 some ^^ntal_aj3tivity_in_MdItion to a mere feeling, their 

 full discussion belongs to the domain of Psychology. Phy- 

 siology, however, is concerned with them so far as it can 

 determine the conditions of stimulation and neurosis 

 under which a given mental representation concerning a 

 sensation is made. It is quite certain that we can feel 



