444 OCULAR SECTRA. SECT. XL. i. i. 



who has termed them accidental colours, as if fubjefted 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 name to various (hades of colours ; 

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

 on repeating the experiments the colours here mentioned (hould 

 not accurately correfpond with his own names of them. 



I. Aftivity of the Retina in Vtfton. 



From the fubfequent experiments it appears, that the retina 

 is in an aftive not in a paflive ftate during the exiftence of thefc 

 occular fpedra ; and it is thence to be concluded, that all vifion 

 is owing to the aftivity of this organ. 



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

 plate i, at Se6r. III i, 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 clofing your eyelids cover them wiih 

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

 fembling in form the piece of red filk : after fome time, this 

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

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

 length it vanifhes entirely. 



2. Place on a (heet of white paper a circular piece of blue 

 filk, about four inches in diameter, in the funfhine , cover the 

 centre of this with a circular piece of yellow (ilk, 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 (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 3, at Seft III. 3.6.; 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 eyelids, 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 

 dill lake in a bright day ; but theie circular irifes of colours arc 

 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 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 



