CLASSIFICATION. 423 



The nature of a ray of homogeneous light is strictly 

 defined, either by its place in the spectrum or by the cor- 

 responding wave-length, but a ray of mixed light admits 

 of no simple classification ; any of the infinitely numerous 

 rays of the continuous spectrum may be present or absent, 

 or present in various intensities, so that we can only class 

 and define a mixed colour by defining the intensity and 

 wave-length of each ray of homogeneous light which is 

 present in it. Complete spectroscopic analysis and the 

 determination of the intensity of every part of the spec- 

 trum yielded by a mixed ray is requisite for its accurate 

 classification. Nearly the same may be said of complex 

 sounds. A simple sound undulation, if we could meet 

 with such a sound, would admit of precise and exhaustive 

 classification as regards pitch, the length of wave, or the 

 number of waves reaching the ear per second being a suf- 

 ficient criterion. But almost all ordinary sounds, even 

 those of musical instruments, consist of complex aggregates 

 of undulations of several different pitches, and in order to 

 classify the sound we should have to measure the inten- 

 sities of each of the constituent sounds, a work which has 

 been partially accomplished by Professor Helmholtz, as 

 regards the vowel sounds. The different tones of voice 

 distinctive of different individuals must also be due to the 

 intermixture of minute waves of various pitch, which are 

 at present quite beyond the range of experimental in- 

 vestigation. We cannot, then, at present, attempt to 

 classify the different kinds or timbres of sound. 



The difficulties of classification re even greater when a 

 varying phenomenon cannot be shown to be a mixture of 

 simpler phenomena. If we attempt, for instance, to 

 classify the tastes of natural and artificial substances, we 

 may rudely group them according as they are sweet, 

 bitter, saline, alkaline, acid, astringent, or fiery ; but it is 

 evident that these groups are bounded by no sharp lines 



