U PRODUCTION G;:ct. XIV. j. 



The immediate object however of tlie fenfe of vifion is fight ; 



is fiuid, though its velocity is io great, appears to have no per- 

 ceptible mechanical impnlfe, as was mentioned in the third 

 Section, but feems to ftimuiate the retina into animal motion 

 by its tranfmiflion through this part of the fenforium : for 

 though the eyes of cats or other animals appear luminous in ob- 

 fcure places ; yet it is probable, that none of the light, which 

 falls on the retina, is reflected fr f >m it, but adheres to or enters 

 into combination with the choroide coat behind it. 



The combination of the particles of light with opaque bodies, 

 and therefore with the choroide coat of the eye, is evinced from 

 the heat which is given out, as in other chemical combinations. 

 For the fun-beams communicate no heat in their paflage through 

 tranfparent bodies, with which they do not combine, as the air 

 continues cool even in the focus of the largefl burning-glafies, 

 which in a moment vitrifies a particle of opaque matter. 



IV. Of the Organ of Hearing. 



It is generally believed, that the tympanum of the ear vi- 

 brates mechanically, when expofed to audible founds, like the 

 firings of one mutical inftrument, when the fame notes are 

 ftruck upon another. Nor is this opinion improbable, as the 

 mufcles and cartilages of the larinx are employed in producing 

 variety of tones by mechanical vibration : fo the mufcles and 

 bones of the ear feem adapted to increase or diminifh the ten- 

 lion of the tympanum for the purpofes of iimilar mechanical 

 vibration:. 



But it appears from di flection, that the tympanum is not the 

 immediate organ of hearing, but that, like the humours and cor- 

 nea of the eye, it is only of ufe to prepare the object for the 

 immediate organ. For the portio mollis of the auditory nerve 

 is not fpread upon the tympanum, but upon the vefcibulum, and 

 cochlea, and iemi-circular canals of the ear ; while between the 

 tympanum and the cxpanfion of the auditory nerve the cavity is 

 faid by Dr. Cotunnus and Dr. Meehel to be filled with water ; 

 as they had frequently obferved by freezing the heads of *\z?A 

 animals before they difTecled them ; and water being a more 

 denfe fluid than air is much better adapted to the propagation 

 of vibrations. We may add, that even the external opening of 

 the ear is not abfclutely neceflatfy for the perception of found . 

 for forne people, who from thefe defects would have been com- 

 pletely deaf, I ave diftuiguifhed acme or grave founds by the tre- 

 mouTS of a flick held between their teerh propagated along the 

 borfes of the head, (Haller Phyf T V. p. 295). 



lie nee 



