SECT. 111.4. MOTIONS OF THE RETINA. 2* 



perhaps crroncoufly, to take place in calcined fhells 

 and other phofphorefcent bodies, after having been 

 expofed to the light : for in all thefe cafes the fpec- 

 tra in the eye ftiould either remain of the fame co- 

 lour, or gradually decay, when the object is with- 

 drawn ; and neither their evanefcence during the 

 prefence of their object, as in the fecond experi- 

 ment, nor their change from dark to luminous, as 

 in the third experiment, nor their rotation, as in 

 the fourth experiment, nor the alternate prefence 

 and evanefcence of them, as in the fifth experiment, 

 nor the perpetual change of colours of them, as in 

 the laft experiment, could exift. 



IV. The fubfequent articles ihew, that thefe ani- 

 mal motions or configurations of our organs of 

 fenfe conftitute our ideas. 



1. If any one in the dark prefles the ball of his 

 eye, by applying his finger to the external corner 

 of it, a luminous appearance is obferved ; and by 

 a fmart ftroke on the eye great flafhes of fire are 

 perceived. (Newton's Optics.) So that when the 

 arteries, that are near the auditory nerve, make 

 flronger pulfations than ufual, as in fome fevers, 

 an undulating found is excited in the ears Hence 

 it is not the prefence of the light and found, but 

 the motions of the organ, that are immediately ne- 

 ceflary to conftitute the perception or idea of light 

 and found. 



2. During the time of fleep, or in delirium, the 

 ideas of imagination are midaken for the percepti- 

 ons of external objects ; whence it appears, that 

 thefe ideas of imagination are no other than a rei- 

 teration of thofe motions of the organs of fenfe, 

 which were originally excited by the ftimulus of 

 external objects : and in our waking hours the fim- 

 ple ideas, that we call up by recollection or by 

 imagination, as the colour of red, or the fmell of 



a rofe, 



