PERCEPTION. 519 



add to the former the ideas of partial, or secon- 

 dary qualities, such as temperature, the peculiar 

 actions which produce taste and smell, the sounds 

 conveyed from certain bodies, and lastly their 

 visible appearances. 



The picture formed on the retina by the re- 

 fracting power of the humours of the eye, is the 

 source of all the perceptions which belong to the 

 sense of vision ; but the visible appearances 

 which these pictures immediately suggest, when 

 taken by themselves, could have given us no 

 notion of the situation, distances, or magnitudes 

 of the objects they represent ; and it is altogether 

 from the experience acquired by the exercise of 

 other senses that we learn the relation which 

 these appearances have with those objects. In 

 process of time the former become the signs and 

 symbols of the latter ; while abstractedly, and 

 without such reference, they have no meaning. 

 The knowledge of these relations is acquired by 

 a process exactly analogous to that by which we 

 learn a new language. On hearing a certain 

 sound in constant conjunction with a certain idea, 

 the two become inseparably associated together 

 in our minds ; so that on hearing the name, the 

 corresponding idea immediately presents itself. 

 In like manner, the visible appearance of an 

 object is the sign, which instantly impresses us 

 with ideas of the presence, distance, situation, 

 form, and dimensions of the body that gave rise 



