SECT. III. 4. i. THE RETINA. 13 



inches in diameter ; and the centre of the yellow filk with a 

 circle of pink (ilk, about two inches in diameter ; and the cen- 

 tre of the pink (ilk with a circle of green (ilk, about one inch in 

 diameter ; and the centre of this with a circle of indigo, about 

 half an inch in diameter ; make a fmall fpeck with ink in the 

 very centre of the whole, as in Plate III. look fteadily for a min- 

 ute on this central fpot, and then clofing your eyes, and applying 

 your hand at about an inch diftance before them, fo as to pre- 

 vent too much or too little light from palling through the eye- 

 lids, and you will fee the moft beautiful circles of colours that 

 imagination can conceive; which are moft refembled by the 

 colours occafioned by pouring a drop or two of oil on a ftill 

 hke in a bright day. But thefe circular irifes of colours are 

 not only different from the colours of the filks above mention- 

 ed, but are at the fame time perpetually changing as long as 

 they exift. 



From all thefe experiments it appears, that thefe fpeftra in 

 the eye are not owing to the mechanical impulfe of light im- 

 prefled on the retina ; nor to its chemical combination with 

 that organ ; nor to the abforption and emiflion of light, as is 

 fu ; ,pofed, perhaps erroneoufly, to take place in calcined (hells 

 and other phofphorefcent bodies, after having been expofed to 

 the light : for in all thefe cafes the fpeclra in the eye (hould ei- 

 ther remain of the fame colour, or gradually decay, when the 

 object: is withdrawn ; and neither their evanefcence during the 

 pretence of the object, as in the fecond experiment, 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 ex- 

 periment, could exift. 



IV. The fubfequent articles (hew, that thefe animal motions 

 or configurations of our organs of fenfe conltitute our ideas. 



1. If any one in the dark prefTes the ball of his eye, by ap- 

 plying his finger to the external corner of it, a luminous appear- 

 ance is obferved ; and by a fmart ftroke on the eye great flaflies 

 of fire are perceived. (Newton's Optics.) So when the arteries, 

 that are near the auditory nerve, make ftronger 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 necefTary to con- 

 ftitute the perception or idea of light and found. 



2. During the time of fleep, or in delirium, the ideas of im- 

 agination are miftaken for the perceptions of external objects ; 

 whence it appears, that thefe ideas of imagination are no other 



than 



