J24 PRODUCTION OF IDEAS. SECT. XIV. 4. 



which by acquired aflbciations introduce the tan- 

 gible ideas of bodies. Hence it is, that this fenfe 

 is fo readily deceived by the art of the painter to 

 our amufement and inftruclion. The reader will 

 find much very curious knowledge on this fubject 

 in Bifhop Berkley's EfTay on Vifion, a work of great 

 ingenuity. 



The immediate object however of the fenfe of 

 vifion is light ; this fluid, though its velocity is fa 

 great, appears to have no perceptible meehankal 

 impulfe, as was mentioned in the third Section, 

 but teems to ftimulate the retina into animal mo- 

 tion by its tranfmiffion through this part of the 

 fenferium : for though the eyes of cats or other 

 animals appear luminous in obfcure places ; yet it 

 is probable, that none of the light, which falls on 

 the retina, is reflected from it, but adheres to or 

 enters into combination with the choroide coat be- 

 hind it. 



The combination of the particles of light with 

 opake 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 largeft burning-glands, which in a mo- 

 ment vitrifies a particle of opaque matter. 



IV. Oftfa Organ of Hearing. 



IT is generally believed, that the tympanum of 

 the ear vibrates mechanically, when expofed to 

 audible founds, like the firings of one mufical in- 

 ftrument, when the fame notes are ftruck upon 

 another. Nor is this opinion improbable, as the 

 mufcles and cartilages of the larynx are employed 

 in producing variety of tones by mechanical vibra- 

 tion : 



