ocular Speffira of Light and Colours . 325 
and on clofing your eyes the colours of the fpeftrum will appear 
fimilar to the two areas, as in fig. g . ; but if the eye is kept 
too long upon them, the colours of the fpe&rum will be the 
reverfe of thofe upon the paper, that is, the internal circle 
will become blue, and the external area yellow ; hence forne 
attention is required in making this experiment. 
3. Place the bright flame of a fpermaceti candle before a 
black obje£t in the night ; look fteadily at it for a fhort time, 
till it is oblerved to become fomewhat paler ; and on clofing 
the eyes, and covering them carefully, but not fo as to comprefs 
them, the image of the blazing candle will continue diftin&ly 
to be vifible. 
4. Look fteadily, for a fhort time, at a window in a dark 
day, as in Exp. 2. S. 111. and then clofing your eyes, and 
covering them with your hands, an exa£i delineation of the 
window remains for fome time vifible in the eye. This expe- 
riment requires a little practice to make it fucceed well ; fince, 
if the eyes are fatigued by looking too long on the window, or 
the day be too bright, the luminous parts of the window will 
appear dark in the fpeftrum, and the dark parts of the frame- 
work will appear luminous, as in Exp. 2. S, in. And it is 
even difficult for many, who firft try this experiment, to per- 
ceive the fpeftrum at all ; for any hurry of inind, or even too 
great attention to the fpe£trum itfelf, will difappoint them, till 
they have had a little experience in attending to fuch fmali 
fen fatten s. 
The fpe£tra defcrlbed in this ie£lion, termed direct ocular 
fpeftra, are produced without much fatigue of the eye ; the 
irritation of the luminous objedl being foon withdrawn, or its 
quantity of light being not fo great as to produce any degree of, 
imeafinefs la the organ of vifion ; which diftinguiiheS' them 
from., 
