SECT. XIV. 3- PRODUCTION OF IDEAS. 123 



during our waking hours can thus acquire a know- 

 ledge of the external world. Which neverthelefs 

 we cannot do in our dreams, becaufe we have nei- 

 ther perceptions of external bodies, nor the power 

 of volition to enable us to compare them with the 

 ideas of imagination. 



III. OfVifion. 



OUR eyes obferve a difference of colour, or of 

 fhade, in the prominences and depreflions of ob- 

 jects, and that thole fhades uniformly vary, when 

 the fenfe of touch obferves any variation. Hence 

 when the retina becomes flimulated by colours or 

 fhades of light in a certain form, as in a circular 

 fpot ; we know by experience, that this is a fign, 

 that a tangible body is before us; and that its 

 figure is refembled by the miniature figure of the 

 part of the organ of vifion, that is thus ftimulat- 

 ed. 



Here whilft the ftimulated part of the retina re- 

 fembles exactly the vifible figure of the whole in 

 miniature, the various kinds of ftimuli from dif- 

 ferent colours mark the vifible figures of the mi- 

 nuter parts ; and by habit we inilantly recall the 

 tangible figures. 



Thus when a tree is the object of fight, a part 

 of the retina refembling a flat branching figure is 

 Simulated by various fhades of colours ; but it is 

 by fuggeftion, that the gibbofity of the tree, and 

 the mofs, that fringes its trunk, appear before us. 

 Thefe are ideas of fuggeftion, which we feel or 

 attend to, affociated with the motions of the reti- 

 na, or irritative ideas, which we do not attend 

 to. 



So that though our vifible ideas refemble in mi- 

 niature the outline of the figure of coloured bodies, 

 in other refpecb they ferve only as a language, 



K which 



