XXXV.] THE SENSES. 09 



which are tasted being dissolved in the moisture of tlie 

 mouth. 



3. The nerves of smell are not nerves of any other sense. Smell. 

 Sensations of smell bear a greater resemblance to sensa- }\^ resem- 



. Dlance to 



tions of taste than do the sensations of any other two tasto. 

 senses to each other : this is, no doubt, a consequence of 

 the similarity in the physical conditions of their pro- 

 duction ; for sensations of smell are due to the vapours 

 which are smelled being dissolved in the moisture of the 

 nostrils. In other respects, also, the senses of taste and of 

 smell resemble each other. Smell, like taste, is not an 

 intellectual sense ; it gives no cognition of space, and its 

 impressions are scarcely at all capable of being reproduced 

 in memory. 



4. The nerves of sight are not nerves of any other sense. Sight, or 

 All the sensations of sight may with perfect accuracy be ^Y orr" 

 called colours ; for white may be regarded as a coloiir, 

 black is the absence of any colour, and lustre is only a 

 particular way in which light falls. According to the 

 theory now generally received, and I think on demon- 

 strative evidence, radiance consists of undulations in an 

 ethereal medium. The undulations are of various lengths, 



but, so far as we are able to ascertain, they do not differ 

 one from the other in any essential property, except that 

 those rays whereof the undulations are between certain Only some 

 ascertained limits of length have the power of excitins the '^"'""""^ 



° ^ ° unilula- 



sensation of light ; or rather (for the property is a purely tions pro- 

 physiological one), the nerves of the eye have the power of ^^p^j!g*^)!' 

 perceiving the sensation of light when they are acted on I'g^t, 

 by those rays ; and, what is most remarkable, rays which 

 appear to differ only in the length of their undulations 

 excite sensations of colour which differ not in intensity and these 

 but in kind — siich as red, green, and blue. This is an ^^'^'}^^ 



various 



ultimate fact of sensation, and its reason is consequently sensations 

 quite inscrutable.^ The subject of colours and their com- °^^"^""'- 

 binations being a very special one, I defer my further 

 consideration of it to Note B at the end of this chapter. 

 The sense of sight gives a cognition of space, though I 



1 See Note A at end of chapter. 



h2 



