THE SENSES BY WHICH WE OBSERVE. 319 



touch, taste, smell, hearing, and sight. It is a peculiarity 

 of the nerves of each particular sense, that an excitement 

 of them produces only the kind of cerebral impression 

 belonging to that sense. Thus excitement of the optic 

 nerve, whether produced by light, mechanical or chemical 

 stimulus, or by disease, produces only the impression of 

 light, while the sensation of sound only is produced by the 

 most diverse causes exciting the nerves of hearing. And 

 as the nerves of the senses collectively ramify through the 

 whole of our physical frame, and are all of them liable to 

 be irritated by functional change or disease, it is obvious 

 that when they are so affected we are subject to all kinds 

 of sensations and perceptions, and are apt to make corre- 

 sponding erroneous observations, unless we take proper 

 precautions, and carefully examine the circumstances. As 

 I have already indicated, in Chapter IX., some of the 

 chief sources of erroneous observation, and it is beyond 

 the scope of this book to enter into the details of correct 

 observation in particular sciences, I must refer the reader 

 to books on special sciences for information ; and as a 

 knowledge of the peculiarities, variations, and deceptions 

 of each of the senses would assist a young scientific inves- 

 tigator to avoid making false or defective observations, I 

 may recommend a perusal of a description of them con- 

 tained in Professor Bain's work on ' The Senses and the 

 Intellect.' 



The power of observation differs not only in kind, but 

 also in degree ; some of our acts of observation are so feeble 

 as only to feebly excite our consciousness, others excite it 

 powerfully. Attention may be considered a high degree 

 of volitional observation, and is a conscious mental effort 

 to observe a sensation, perception, or idea, already present 

 to the mind. The highest degree of observational action 

 is both conscious and volitional. The feebler degrees of 



