ocular Spectra of Light and Colours • 3 ji 
and laftly pink, whilft the fpectrum of the field varied into red 
and green. 
Thefe fucceffions of different coloured fpedlra were not 
exactly the fame in the different experiments, though obferved, 
as near as could he, with the fame quantity of light, and 
other fimilar circumftances ; owing, I fuppofe, to trying too 
many experiments at a time ; fo that the eye was not quite 
free from the fpeftra of the colours which were previoufly 
attended to. 
The alternate exertions of the retina in the preceding fedtion 
refembled the ofcitation or pandiculation of the mufcles, as 
they were performed in diredlions contrary to each other, and 
were the confequence of fatigue rather than of pain. And 
in this they differ from the fucceffive diffimilar exertions of 
the retina, mentioned in this fedtion, which refemble in mi- 
niature the more violent agitations of the limbs in convulfive 
difeafes, as epilepfy, chorea S. Viti, and opifthotonos ; all 
which difeafes are perhaps, at firft, the confequence of pain, 
and have their periods afterwards eftablifhed by habit. 
vni. The retina , after having been excited into affiion by a Jlitnulus 
fomewhat greater than the laf mentioned , falls into a fixed 
fpafmodic a8tfon, which continues for fome days . 
1. After having looked long at the meridian fun, in making 
fome of the preceding experiments, till the difcs faded into a 
pale blue, I frequently obferved a bright blue fpedlrum of the 
fun on other objedls all the next and the fucceeding day, which 
conftantly occurred when I attended to it, and frequently when 
I did not previoufly attend to it. When I doled and covered 
X x 2 my 
