22 MOTIONS OF THE RETINA. SECT. III. 3. 



5. Place a piece of red filk about an inch in dia- 

 meter on a fheet of white paper in a ftrong light, as 

 in Plate I. ; look fteadily upon it from the diftance 

 of about half a yard for a minute ; then doling 

 your eye-lids, cover them with your hands and hand- 

 kerchief, and a green fpeclrum will be feen in your 

 eyes refembling in form the piece of red filk. After 

 fome feconds of time the fpeftrum will difappear, 

 and in a few more feconds will reappear ; and thus 

 alternately three or four times, if the experiment 

 be well made, till at length it vanifhes entirely. 



6. Place a circular piece of white paper, about 

 four inches in diameter, in the funfhine, cover the 

 center of this with a circular piece of black filk, 

 about three inches in diameter ; and the center of 

 the black filk with a circle of pink filk, about two 

 inches in diameter ; and the center of the pink filk 

 with a circle of yellow filk, about one inch in dia- 

 meter ; and the center of this with a circle of blue 

 fiik, about half an inch in diameter ; make a fmall 

 fpot with ink in the center of the blue filk, as in 

 Piate 111. look fteadily for a minute on this central 

 fpot, and then clofing your eyes, and applying your 

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

 prevent too much or too little light from paffing 

 through the eye-lids, and you will fee the moft beau- 

 tiful circles of colours that imagination can con* 

 ceive ; which are moft refembled by the colours oc- 

 cafioned by pouring a drop or two of oil on a ftili 

 lake in a bright day. But thefe circular irifes of co- 

 lours are not only different from the colours of the 

 filks above mentioned, but are at the fame time per* 

 petually changing as long as they exift* 



From all thefe experiments it appears, that thefe 

 fpectra in the eye are not owing to the mechanical 

 impulfe of light impreffed on the retina ; nor to its 

 chemical combination with that organ ; nor to the 

 abforption and emiffion of light j as is fuppofed 



perhaps 



