462 THE SENSORIAL FUNCMONS. 



add to the former the ideas of partial, or secondary 

 qualities, such as temperature, the peculiar actions 

 which produce taste and smell, the sounds con- 

 veyed from certain bodies, and lastly, their visible 

 appearances. 



The picture formed on the retina by the refract- 

 ing 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 ex- 

 perience 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 analagous 

 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 hear- 

 ing the name, the corresponding idea immediately 

 presents itself In like manner, the visible ap- 

 pearance 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 to it. This association is, in man at least, 

 not original, but acquired. The objects of sight 

 and touch, as Bishop Berkeley has justly observed, 

 constitute two worlds, which although they have 

 a very important correspondence and connexion, 



