THE SENSE ORGANS 479 



of measuring sensations two methods of investigation have been developed 

 which to a great extent avoid it, namely the method of threshold values and 

 the method of comparisons. 



In the method of threshold values gradually increasing stimuli are applied 

 to the sense organ under investigation, until a sensation is just perceived. 

 Thus increasing weights are applied to the skin until their pressure is felt, or 

 a light of increasing intensity is presented to the eye until its presence is seen. 

 Such threshold values give definite information concerning the sense organ 

 to which they refer, and allow us to compare the same effects in one and the 

 same sense organ under different conditions, and also in different individuals. 



In the method of comparisons two separate stimuli are applied, and 

 one or both are varied until the sensations caused by them are equal. Thus 

 a ray of yellow light may be compared with a ray mixed from red and green 

 light, and the intensities of the two adjusted until the sensations caused by 

 both are similar. Both these methods give concordant results if care be 

 taken to make the conditions standard, and both are for this reason largely 

 used for studying the sense organs. 



A careful study of these organs is necessary and important because it is 

 only through their agency that we derive information about ourselves, one 

 another, and the world in which we live. The limitations of our sense organs 

 therefore restrict our knowledge of the many transformations of energy that 

 are going on around us, except in so far as we are able to devise means of 

 extending them artificially. Thus our knowledge of the existence of ultra- 

 violet light began only with the discovery of photography. Wireless waves 

 circulated through the ether around unknown to us before the invention of 

 the coherer. 



CLASSIFICATION OF SENSE ORGANS. Sense organs differ in their 

 anatomical position and structure and also according to the nature of the 

 stimulus to which they react and the kind of sensation which they cause. 

 Any of these differences might be used as a basis of classification, but 

 the latter is found to be the best. 



Structure. Kind of Sensation. 



/ Touch ^ 



Skin r Feeling 



( Told / 





Cold 

 ( Light 



iss. 



V 



Distance / 



{Tone \ 



Harmony I Hearing 



Position J 



r Acid \ 



Tongue ....... J Sweet '. Taste 



( Bitter J 

 Nose ........ Smell 



Stimuli adequate for one sense organ are found to be inadequate for 



