ON COLOUE-BLINDNESS. 



197 



organs of smell being insensible to the too rapid vibrations 

 of tho simpler molcoiilos. 



Now every possible tone between tho extreme limits of 

 audition can be beard by the ear. It cannot be said that all 

 tones arc derivable from a few fundamentals ; for although 

 fundamentals may yield overtones, yet these overtones are 

 perceived by ua as modifying tho quality of sound (Helm- 

 holz) much more frequently than as tho actual tone heard 

 as primary. 



So too evorv possible temperature between tho limits of 

 oar heat-perceptions is perceived by the skin ; and it is also 

 probable that every possible variety of smell is perceivable 

 as such by the olfactory nerves ; and that the smells which 

 we pei'oeive are not compounded from a few elementary 

 perceptions. We are acquainted with homologous series of 

 chemical compounds, in which tho smells progressively 

 change with progressive alteration in the molecular weights, 

 and yet no one smell can be declared a mixtui'c. 



Reasoning from tho analogy of the othcT' senses, therefore, 

 it may be concluded that the optic nerve is sensitive to all 

 possible vibration between certain limits, and not merely to 

 three or four kinds of vibrations, each kind having its own 

 special period. 



Another argument for this view may be drawn from prob- 

 ability. Suppose a young child to bo sensitive as regards 

 red to some absolutely definite number of vibrations, and to 

 none otlier so far as tlio colour red is ooncei-ned. Now, it is 

 unlikely that such a child's perception will remain sta- 

 tionary, for the nerve fibres will enlarge with the growth of 

 the body. It would therefore appear that adults sliould 

 have totally different colour-perceptions from children, and 

 I am not aware that this is the case ; I should certainly 

 deny it as regards myself. It might be well, however, to 



