Sect. XIV. 3, OF IDEAS. ' 87 



fweetnefs of fugar occurs to us, the ftimuli of furrounding ob- 

 jects, as the edge of the table, on which we prefs, or green 

 colour of the grafs, on which we tread, prevent the other ideas 

 of the hardnefs and whitenefs of the fugar from being excited, 

 by affociation. Or if they Ihould occur, we voluntarily com- 

 pare them with the irritative ideas of the table or grafs above 

 mentioned, and detect their fallacy. We can thus diftinguiih 

 the ideas caufed by the ftimuli of external objects from thofe, 

 which are introduced by aflbciation, fenfation, or volition ; and 

 during our waking hours can thus acquire a knowledge of the 

 external world. Which neverthelefs we cannot do in our 

 dreams, becaufe we have neither perceptions of external bodies, 

 nor the power of volition to enable us to compare them with 

 the ideas of imagination. 



III. Of Vifion. 



Our eyes obferve a difference of colour, or of made, in the 

 prominences and depreffions of objects, and that thofe (hades 

 uniformly vary, when the fenfe of touch obferves any variation. 

 Hence when the retina becomes ftimulated by colours or lhades 

 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 ftimulated. 



Here whilft the ftimulated part of the retina refembles exact- 

 ly the vifible figure of the whole in miniature, the various kinds 

 of ftimuli from different colours mark the vifible figures of the 

 minuter parts ; and by habit we inftantly recall the tangible 

 figures. 



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

 refembling a flat branching figure is ftimulated by various lhades 

 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, affo- 

 ciated with the motions of the retina, or irritative ideas, which 

 we do not attend to. 



So that though our vifible ideas refemble in miniature the 

 outline of the figure of coloured bodies, in other reipects they 

 ferve only as a language, which by acquired affociations intro- 

 duce the tangible ideas of bodies. Hence it is, that this fenfe 

 is fo readily deceived by the art of the painter to our amufe- 

 ment and inftruction. The reader will find much -very curious 

 knowledge on this fubjea in Bifhop Berkeley's May on Vifion, 

 a work of great ingenuity. 



The 



