Ecr.XL. j. OCULAR SPECTRA. 697 



1. Place a piece of red filk, about an inch in di- 

 ameter, as in plate i, at Seel. III. i, on a fheet of 

 white paper, in a ftrong light ; look fteadily upon 

 it from about the dilhinceof half a yard f}r a mi. 

 nute; then doling your eyelids cover them with 

 your hands, and a green fpeitrum will be feen in 

 your eyes, refembiing in form the piece of red filk : 

 after foine time, this fpeclrum will difappear and 

 fhortly re-appear ^ and this alternately three or four 

 times, if the experiment is well made, till at length 

 it vanifhes entirely. 



2. Place on a Iheet of white paper a circular piece 

 of blue filk, about four inches in diameter, in the 

 funmine ; cover the center of this with a circular 

 piece of yellow filk, about three inches in diameter j 

 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 centre of this 

 with a circle of indigo, about half an inch in dia- 

 meter ; make a fmall fpeck with ink in the very 

 centre of the whole, as in plate 3, at Seel. III. 3. 

 6. ; look iteadily for a minute on this central fpot, 

 and then doling your eyes, and applying your hand 

 at about an inch diftance before them, fo as to 

 prevent too much or too little light from patting 

 through the eyelids, you will fee the moft beauti- 

 ful circles of colours that imagination can conceive, 

 which are molt reieinbled by the colours occationed 

 by pouring a drop or two of oil on a Hill lake in a 

 bright day ; but thefe circular irifes of colours are 

 not only different from the colours of the filks above 

 mentioned, but are at the fame time perpetually 

 changing as long as they exift. 



3. When any one in the dark preflfes either cor- 

 ner of his eye with his ringer, 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 



