26 MOTIONS OF THE RETINA. SECT, III. j. 



ternal objects like our mufcles ; are affociated toge- 

 ther like our mufcular motions ; act in limilar time 

 with them ; are fatigued by continual exertion like 

 them; and that the organs of fenfe are fubject to 

 inflammation, numbnefs, palfy, convulfion, and 

 the defects of old age, in the fame manner as the 

 mufcular fibres. 



1. All our perceptions or ideas of external ob- 

 jects are univerfally allowed to have been originally 

 excited by the ftimulus of thofe external objects ; 

 and it will be (hewn in a fucceeding fectlon, that it 

 is probable that all our mufcular motions, as well 

 thofe that are ^become voluntary as thofe of the 

 heart and glandular fyftem, were originally in like 

 manner excited by the ftimulus of fomething exter- 

 nal to the organ of motion. 



2. Our ideas are alfo affociated together after 

 their production precifely in the fame manner as our 

 mufcular motions ; which will likewifebe fully ex- 

 plained in the fucceeding fection. 



3. The time taken up in performing an idea is 

 like wife much the fame as that taken up in perform- 

 ing a mufcular motion. A mufician can prefs the 



- keys of an harpfichord with his fingers in the or- 

 der of a tune he has been accuftomed to play, in as 

 little time as he can run over thofe notes in his 

 mind. So we many times in an hour cover our eye- 

 balls with our eye- lids without perceiving that we 

 are in the dark ; hence the perception or idea of 

 light is not changed for that of darknefs in fo fmali 

 a time as the twinkling of an eye; fo that in this 

 cafe the mufcular motion of the eye-lid is perform- 

 ed quicker' than the perception of light can be 

 changed for that of darknefs. So if a fire-ftick be 

 \vlnrfed round. in the dark, a luminous circle ap- 

 pears to the obferver ; if it be whirled fomewhaC 

 flower, this circle becomes interrupted in one part ; 

 and then the time taken up in fuch a revolution of 



the 



