ocular Spetdra of Light and Colours* g\ j- 
mufcles, there are many other analogies which exift between 
them. They are both originally excited into a&ion by irrita* 
tions,. both a£t nearly in the fame quantity of time, are alike 
ftrengthened or fatigued by exertion, are alike painful if ex- 
cited into adtion when they are in an inflamed Hate, are alike* 
liable to paralyfis, and to the torpor of old age. 
IT. OF. SPECTRA FROM DEFECT OF SENSIBILITY* 
51 be retina is not fo eafily excited into action by lefs irritatiom 
after having been lately fubjehled to greater. 
1. When any one paffes from the bright daylight into a ’ 
darkened room*. the irifes of his eyes expand themfelves to their 
ntmoft extent in a few feconds of time; but it is very long^ 
before the optic nerve, after having, been ftimulated by the, 
greater light of the day, becomes fenfible of the lefs degree of 
it in the room ; and,, if the room is not too obfcure, the irifes. 
will again contrail themfelves in fome degree, as the' fen ft-- 
bility of the retina returns. 
2, Place about half an inch fquare of white paper on a black 
hat, and looking fteadily on the center of it for a minute, re- 
move your eyes to - a fheet of white paper ; and after a fecond 
or two a dark fquare will be feen on the white paper, which , 
will continue fome time. A- fimilar dark fquare will be feen in 
the clofed eye, if light be admitted through the eye- lids. 
So after looking at any luminous objedl of a fmali. fize, as 
at the fun, for a (hort time, fo as not much to fatigue the 
eyes, this part of the retina becomes lefs fenfible to fmaller 
quantities of light; hence, when the eyes are turned on other 
lefs luminous parts of the Ikv, a dark fpot is feen re.fembling 
i the . 
