ocular Speffira of Light and Colours . 339 
the fpe£lrum was thrown on white paper, with the eyes open, it 
became only a lighter green. 
Though a certain quantity of light facilitates the formation 
of the reverfe fpeftrum, a greater quantity prevents its forma- 
tion, as the more powerful ftimulus excites even the fatigued 
parts of the eye into a£lion; otherwife we fhould fee the 
fpedtrum of the laft viewed objefb as often as we turn our 
eyes. Hence the reverfe fpeftra are beft feen by gradually ap- 
proaching the hand near the clofed eyelids to a certain diftance 
only, which muft be varied with the brightnefs of the day, or 
the energy of the fpe&rum. Add to this, that all dark fpedrra, 
as black, blue, or green, if light be admitted through the eye- 
lids, after they have been fome time covered, give reddifh 
fpedtra, for the reafons given in fed. III. exp. 1. 
From thefe circumftances of the extraneous light coinciding 
with the fpontaneous efforts of the fatigued retina to produce a 
reverfe fpe&rum, as was obferved before, it is not eafy to gain 
a direft fpe&rum, except of objects brighter than the ambient 
light ; fuch as a candle in the night, the fetting fun, or view- 
ing a bright object through an opake tube ; and then the reverie 
fpedtrum is inftantaneoufly produced by the admiffion of fome 
external light ; and is as inftantly converted again to the dire£t 
fpe&rum by the exclufion of it. Thus, on looking at the fet- 
ting fun, on doling the eyes, and covering them, a yellow 
fpedru m is feen, which is the direct fpeftrum of the fetting 
fun ; but on opening the eyes on the Iky, the yellow* 
fpeArum is immediately changed into a blue one, which 
is tae reverfe fpeftrum of the yellow fun, or the direft fpec- 
trum of the blue Iky, or a combination of both. And this 
is again transformed into a yellow one on doling the eyes, 
and fo reciprocally, as quick as the motions of the opening 
and doling eyelids. Hence, when Mr. Melvill obferved 
Y y 2 the 
