444 OCULAR SPECTRA. Sect. XL. i. i, 



who has termed them accidental colours, as if fubjedted to no 

 eftablifhed laws, Ac. Par. 1743. M. p. 215. 



I muft here apprize the reader, that it is very difficult for dif- 

 ferent people to give the fame names to various (hades of colours ; 

 whence, in the following pages, fomething muft be allowed, if 

 on repeating the experiments the colours here mentioned fhould 

 not accurately correfpond with his own names of them. 



I. Affivity of the Retina in Vifion. 



From the fubfequent experiments it appears, that the retina 

 is in an atlive not in a paflive ftate during the exiftence of theie 

 ocular fpe£tra ; and it is thence to be concluded, that all vifion 

 is owing to the activity of this organ. 



1. Place a piece of red filk, about an inch in diameter, as in 

 plate 1, at Seel. III. 1, on a fheet of white paper, in a ftrong 

 light ; look fteadily upon it from about the diftance of half a 

 yard for a minute ; then clofmg your eyelids cover them with 

 your hands, and a green fpeclrum will be feen in your eyes, re- 

 iembling in form the piece of red filk : after fome time, thi3 

 ipeCtrum will difappear and (hortly reappear ; and this alter- 

 nately three or four times, if the experiment is well made, till at 

 length it vaniflies entirely. 



2. Place on a fheet of white paper a circular piece of blue 

 filk, about four inches in diameter, in the funffiine ; cover the 

 centre of this with a circular piece of yellow filk, about three 

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

 cle of pink filk, about two inches in diameter ; and the centre 

 of the pink filk with a circle of green filk, 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 3, at Se&. III. 3. 6. ; look 

 iteadily for a minute on this central fpot, and then ciofmg your 

 eves, 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 eyelids, you will fee the mod beautiful circles of 

 colours that imagination can conceive, which are mod refemblcd 

 bv the colours occasioned by pouring a drop or two of oil on a 

 ilill lake in a bright day ; but theie circular iriies 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. 



3. When any one !n the dark prefles either corner of his 

 eye with his finger, and turns his eye away from his finger, he 

 will fee a circle cf colours like thofe in a peacock's tail : and a 



fudden 



