18 COLOUR VISION 



the physiological path, so that the resultant nervous activities set up in 

 the higher centres of the brain are very different from those which 

 started in the peripheral mechanism. Try as we may to dissociate the 

 physiological from the attendant psychological phenomena we are 

 unable to do so, for the act of analysis itself is a psychological process. 

 We are practically limited to the consideration of the relationships 

 which can be made out with greater or less certainty to exist between 

 the stimuli and the resulting sensations. 



With regard to these relationships there are certain generalisations 

 which have been thought to hold good for all the so-called " senses." 

 One of these generalisations, enunciated in 1785 by Bonnet and 

 independently by Johannes Midler in 1826^, is called Miiller's Law of 

 the Specific Energies of the Senses. It is to be noted that the word 

 energy is not used here in its ordinary physical sense. Miiller stated 

 that we can experience no kinds of sensation through the inter- 

 mediation of external causes which we cannot also experience through 

 the sensation derived from the condition existing in the nervous 

 organs without external agency. These internal causes call up 

 different sensations in the different senses according to the nature of 

 each sense. Further, the same external stimuli arouse in the different 

 sense mechanisms different sensations according to the particular sense ; 

 and different external or internal stimuli, acting upon the same sense 

 mechanism, ahvays arouse the same sensation. The essential features 

 of Miiller's law are contained in the sentences in italics. He 

 amplified the law by saying that a sensation is not the transference of 

 a quality or condition of external bodies to the consciousness, but the 

 transference of a quality or condition of a sensory mechanism to the 

 consciousness, though that quality or condition is occasioned by external 

 stimuli. These qualities or " energies " of the senses are specific to 

 each sensory mechanism. Miiller left it open as fo the site of the 

 specific energies, whether in the brain or elsewhere in the nervous paths, 

 but he considered it certain that the central endings of the sensory 

 nerve-paths in the brain were capable of arousing the specific sensations 

 independently of the conducting paths. It is now generally held that 

 the seat of the specific sensations is in the brain. If this assumption 

 be true Miiller's law relieves us of the necessity of predicating the con- 

 duction of different kinds of impulses by the peripheral nerve-paths. 

 It is customary to divide the stimuli which can arouse a specific 



^ Zur vergleichendcn Physiologlt dcs Gesichtssinnes, Leipzig, 1826 ; Ha/idb. d. Physiol, 

 d. Menschen, Coblenz, 1840. 



