ocular Spedlra of Light and Colours ♦ 315 
three Inches In diameter ; and the center of the yellow filk with 
a circle of pink filk, about two inches in diameter ; and the 
center of the pink filk with a circle of green filk, about one 
inch in diameter ; and the center of this with a circle of in- 
digo, about half an inch in diameter ; make a fmall fpeck with 
ink in the very center of the whole, as in fig, 2. ; look deadily 
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 pafling 
through the eyelids, you will fee the mod beautiful circles of 
colours that imagination can conceive, which are mod refem- 
bled by the colours occafioned by pouring a drop or two of oil 
on a dill 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 perpetually changing as 
long as they exid. 
3. When any one in the dark prefles either corner of his 
eye with his finger, and turns his eye away from his finger, he 
will fee a circle of colours like thofe in a peacock’s tail : and a 
fudden flafh of light is excited in the eye by a droke on it, 
(Newton’s Opt. Qu. 16.) 
4. When any one turns round rapidly on one foot, till he 
becomes dizzy and falls upon the ground, the fpedlra of the 
ambient obje&s continue to prefent themfelves in rotation, or 
appear to librate, and he feems to behold them for fome time 
dill in motion. 
From all thefe experiments it appears, that the fpe£tra in 
the eye are not owing to the mechanical impulfe of light im- 
preffed on the retina, nor to its chemical combination with 
that organ, nor to the abforption and emiflion of light, as is 
obferved in many bodies : for in all thefe cafes the Ipedra mud 
T t 2 either 
