$2 6 Dr. Darwin's Experiments on the 
from the next clafs of ocular fpe&ra, which are the confe- 
quence of fatigue. Thefe diredl ipedtra are beft obferved in 
fuch circumftances that no light, but what comes from the 
abjedt, can fall upon the eye ; as in looking through a tube, 
of half a yard long, and an inch wide, at a yellow paper on 
the fide of a room, the direct fpedtrum was eafily produced on 
clofing the eye without taking it from the tube : but if the 
lateral light is admitted through the eye-lids, or by throwing 
the fpedlrum on white paper, it becomes a reverfe fpedlrum, 
as will be explained below. 
The other fenfes alfo retain for a time the impreffions that 
have been made upon them, or the a&ions they have been ex- 
cited into. So if a hard body is preffed upon the palm of the 
hand, as is pradtifed in tricks of legerdemain, it is not eafy to 
diftinguifh for a few feconds whether it remains or is removed ; 
and taftes continue long to exift vividly in the mouth, as the 
fmoke of tobacco, or the tafte of gentian, after the fapid 
material is withdrawn. 
v. A quantity of Jlimulus fomewhat greater than the lajl men - 
tioned excites the retinu into fpafmodic adiion y which ceafes and 
recurs alternately . 
i. On looking for a time on the fetting fun, fo as not 
greatly to fatigue the fight, a yellow fpedtrum is feen when 
the eyes are clofed and covered, which continues for a time, 
and then difappears, and recurs repeatedly before it intirely 
vaniflies. This yellow fpedtrum of the fun when the eye- 
lids are opened becomes blue ; and if it is made to fall on the 
green grafs, er on other coloured objedts, it varies its own 
colour 
