SECT. XXVII. FALLACY OF THE SENSES. 281 



body upon which it falls, resembles the sympathetic agitation of 

 a string when the sound of another string which is in unison with 

 it is transmitted through the air. Light, heat, sound, and the 

 waves of fluids are all subject to the same laws ; their undulatory 

 theories are perfectly similar : hence the interference of two 

 hot rays must produce cold, that is, they must extinguish one 

 another : darkness results from the interference of two undula- 

 tions of light, silence ensues from the interference of two undu- 

 lations of sound, and still water or no tide is the consequence 

 of the interference of two tides. The propagation of sound, 

 however, requires a much denser medium than that of light and 

 heat ; its intensity diminishes as the rarity of the air increases : 

 so that, at a very small height above the surface of the earth, 

 the noise of the tempest ceases, and the thunder is heard no 

 more in those boundless regions where the heavenly bodies 

 accomplish their periods in eternal and sublime silence. 



A consciousness of the fallacy of our senses is one of the most 

 important consequences of the study of nature. This study 

 teaches us that no object is seen by us in its true place, owing to 

 aberration ; that the colours of substances are solely the effects of 

 the action of matter upon light ; and that light itself as well as 

 heat and sound are not real beings, but mere motions communi- 

 cated to our perceptions by the nerves. The human frame may 

 therefore be regarded as an elastic system, the different parts of 

 which are capable of receiving the tremors of elastic media, and 

 of vibrating in unison with any number of superimposed undula- 

 tions, all of .which have their perfect and independent effect. 

 Here our knowledge ends : the mysterious influence of matter on 

 mind will in all probability be for ever hid from man. 



