186 ON THE SENSES 



to pass obliquely downwards to the other side, but the slowest with least, the 

 swiftest with most obliquity. It results that the undulations differing in their 

 velocity strilse, one after another, on the intercepting screen. The different 

 colors of different portions of the stripe proceeds from this : that the Avaves 

 reflected from the screen to our eye operate differently on the nervous apparatus 

 of the organ according to the velocity of their vibrations, so that each generates 

 for itself a definite modification of the nervous current, and every such modifi- 

 cation a definite quality of sensation ; that is, of color. The longest waves 

 with ihe least vibratory velocity, which reach the eye from the upper end of the 

 stripe, and which hence, in the image on the retina, form, through the inversion 

 of the image, the lower end of the stripe, so operate on the nerve-exciting appa- 

 ratus that the sensation of a red color is the result ; the shortest and swiftest 

 waves from the other end of the stripe (which stripe is called the solar spec- 

 trum) produce in the same way the sensation of a violet color ; the waves of 

 intermediate length and velocity give rise in succession to the above-named in- 

 termediate sensations of color. And the same is the case not merely with the 

 waves directly disengaged from the sun's light, but with those issuing from 

 every visible object. An object appears red when the waves proceeding from 

 it are so constituted that each particle of the ether lying in its course executes 

 only 439 billions of vibrations in a second of time, while each wave is the 

 seventy -thousandth part of a millimetre iu length ; if the wave be only the 

 sixty-thousandth part of a millimetre long, while, on the other hand, its parti- 

 cles vibrate 697 billion times in a second, the object from which it pro- 

 ceeds will appear of a violet color. The observed object is hence neither red 

 nor violet ; it only throws the particles of the ether lying between itself and 

 our eye into vibrations of definite velocity, according to which they excite in 

 our mind, through the medium of the retina and optic nerve, the sensation of 

 red or violet. In like manner we erroneously seek the sound we hear in the 

 vibrating string, while the string simply vibrates ; the sound first exists in our 

 sensation, occasioned by the waves of a definite frequency of vibration proceed- 

 ing from the string. It is, in fact, the general error as regards all perceptions 

 of the senses that we ascribe the qiiality of the sensation to the exciting exter- 

 nal cause ; a necessary error, however, through which our sensations become 

 first available for the practical uses of life, as we have repeatedly shown on 

 previous occasions. To return to our theme : Each solar ray, which the prism 

 decomposed into seven undulatory elements distinguishable by their color, com- 

 prises other waves besides the colored ones. In the white solar light there are 

 ether waves of yet slower vibration than those which produce the sensation of 

 red, and such as are of yet greater vibratory velocity than those which yield a 

 violet color. In the spectrum, therefore, formed by the prism, there are waves 

 (the longest) which strike beyond the red extremity, as well as others (the 

 most frequent) which strike beyond the violet extremity ; but these we do not 

 see, although constituted precisely like the rays of color, except as regards the 

 difference of length. In other words, only those waves of the ether, the 

 number of Avhose vibrations falls within certain limits, (434-697 billions,) 

 are capable of so affecting the apparatus of the retina as to produce exci- 

 tation of its nerves, and therefore a sensation ; while, on the other hand, 

 as well those whose number of vibrations is too small as those whose number 

 is too great, are not qualified to excite a sensation. Again a complete analogy 

 with the nerves of hearing ! If the vibrations of a string are calculated to ex- 

 cite the auditory nerves so as to produce sound, there are equally limits to the 

 number of vibrations which must not be transcended ; the string must not os- 

 cillate from side to side less than sixteen times, nor more than 64,000 times in 

 a second. If it vibrate less often or more frequently, it sounds no longer. But 

 how, will the reader ask, can it be known that beyond the visible there are 

 present invisible waves of light 'I How can light be recognized which is not 



