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instead of a darkish, uniformly diffused coating, a pretty extensive spot 

 of various colours, and in some, most beautiful and brilliant. It is not 

 easy to say what is the use of this coloured spot, known by the name of 

 tafietum. 



The rays of light, reflected by this opaque substance, must, in passing 

 through the eye, cross those which are entering at the same time ; they 

 must, consequently, prevent distinct vision, or at least impair the impres- 

 sion, in a manner which it is impossible to determine. It has been said, 

 that the lower animals, provided with less perfect and often less numerous 

 senses than those of man, must have different ideas of the universe ; is it 

 not, likewise, probable, that in consequence of the indistinct vision oc- 

 casioned by the reflection from the tapetum, they may entertain erroneous 

 and exaggerated notions of the power of man ? And. notwithstanding the 

 power granted to man by the Creator over the lower animals, as we are 

 told in the book of Genesis, is it probable that those which Nature has 

 gifted with prodigious strength, or with offensive weapons, would obey 

 the lord of the creation, if they saw him in his feeble and destitute con- 

 dition, in a word, such as he is ? 



The heads of insects with numerous eyes, are joined to their body, and 

 move along with it : their existence is, besides, so frail, that it was re- 

 quisite that Nature should furnish them abundantly with the means of 

 seeing those objects which may be injurious to them. We shall not en- 

 ter, any farther, into these remarks relative to the differences in the organ 

 of sight, in the various kinds of animals. More ample details on this 

 subject belong, in an especial manner, to Comparative Anatomy. 



CXXI. Of the Organ of Hearing. Of Sound. Sound is not, like light, 

 a body having a distinct existence: we give the name of sound to a sen- 

 sation which we experience, whenever the vibrations of an elastic body 

 strike our ears. All bodies are capable of producing it, provided their 

 molecules are susceptible of a certain degree of re-action and resistance. 

 When a sonorous body is struck, its integrant particles experience a sud- 

 den concussion, are displaced and oscillate with more or less rapidity. 

 This tremulous motion is communicated to the bodies applied to its 

 surface ; if we lay our hand on a bell that has been struck by its clapper, 

 we feel a certain degree of this trembling. The air, which envelopes the 

 sonorous body, receives and transmits its vibrations with the more effect 

 from being more elastic. Hence, it is observed that, caeteris paribus, the 

 voice is heard at a greater distance in winter, when the air is dry and 

 condensed by the cold. 



The sonorous rays are merely series of particles of air along which the 

 vibration is transmitted, from the sonorous body, to the ear which per- 

 ceives the noise occasioned by its percussion. These molecules partici- 

 pate in the vibrations which are communicated to them ; they change 

 their form and situation, in proportion as they are nearer to the body that 

 is struck, and vice versa; for, sound becomes weaker, in proportion to 

 the increase of distance. But this oscillatory motion of the aerial mole- 

 cules, should be well distinguished from that by which the atmosphere, 

 agitated by the winds, is transported and changes its situation.- 

 And in the same manner as the balance of a pendulum moves inces- 

 santly within the same limits, so, this oscillatory motion affects the 

 molecules of the air within the space which they occupy, so that they 

 move to and fro during the presence or the absence of the vibration. 

 The atmospherical air, when set in motion, in a considerable mass at a 



