Sect. III. m. THE RETINA. $ 



Hence, when we fay animal motion is excited by irritation, wa 

 do not mean that the motion bears any proportion to the me- 

 chanical impulfe of the ftlmulus ; nor that it is affected by the 

 general gravitation of the two bodies ; nor by their chemical 

 properties 5 but folely that certain animal fibres are excited in- 

 to action by fomething external to the moving organ. 



In this fenfe the ftimulus of the blood produces the contrac- 

 tions of the heart ; and the fubftances we take into our ftomach 

 and bowels ftimulate them to perform their neceffary functions. 

 The rays of light excite the retina into animal motion by their 

 ftimulus ; at the fame time that thofe rays of light tfiernfelves 

 are phyfically converged to a focus by the inactive humours of 

 the eye. The vibrations of the air ftimulate the auditory nerve 

 into animal action ; while it is probable that the tympanum o£ 

 the ear at the fame time undergoes a mechanical vibration. 



To render this circumftance more eafy to be comprehended, 

 motion may be defined to be a variation of figure ; for the whole 

 univerfe may be confidered as one thing pofiefling a certain fig- 

 ure ; the motions of any of its parts are a variation of this fig- 

 ure of the whole : this definition of motion will be further ex- 

 plained in Section XIV. 2. 2. on the production of ideas. 



Now the motions of an organ of fenfe are a fucceffion of con- 

 figurations of that organ ; thefe configurations fucceed each 

 other quicker or flower ; and whatever configuration of this or- 

 gan of fenfe, that is, whatever portion of the motion of it is, or 

 has ufually been, attended to, conftitutes an idea. Hence the 

 configuration is not to be confidered as an effect of the motion 

 of the organ, but rather as a part or temporary termination of it j 

 and that, whether a paufe fucceeds it, or a new configuration 

 immediately takes place. Thus when a fucceffion of moving 

 objects are prefented to our view, the ideas of trumpets, horns, 

 lords and ladies, trains and canopies, are configurations, that is, 

 parts or links of the fucceflive motions of the organ of vifion. 



Thefe motions or configurations of the organs of fenfe differ 

 from the fenforial motions to be defcribed hereafter, as they ap- 

 pear to be fimply contractions of the fibrous extremities of thofe 

 organs, and in that refpect exactly referable the motions or con- 

 tractions of the larger mufcles, as appears from the following 

 experiment. Place a circular piece of red filk about an inch in 

 diameter on a fheet of white paper in a ftrong light, as in Plate 

 I.-— look for a minute on this area, or till the eye becomes fome- 

 what fatigued, and then, gently clofing your eyes, and fnading 

 them with your hand, a circular green area of the fame appar- 

 ent diameter becomes vifible in the clofed eye. This green 

 area is the colour reverfe to the red area, which had b^tn pre- 

 vol. I. C viouilv 



