3 i 8 Dr, Darwin’s Experiments on the 
the fhape of the fun, or other luminous objeft which we laft 
beheld. This Is the fource of one kind of the dark- coloured 
tnufcte volit antes. If this dark fpot lies above the center of the 
eye, we turn our eyes that way, expe&ing to bring it into the 
center of the eye, that we may View it more diftindtly ; and in 
this cafe the dark fpe&rum feems to move upwards. If the 
dark fpe&rum is found beneath the center of the eye, we pur- 
fue it from the fame motive, and it feems to move downwards. 
This has given rife to various conje&ures of fomething floating 
in the aqueous humours of the eyes ; but whoever, in attend? 
ing to thefe fpots, keeps his eyes unmoved by looking fteadily 
at the corner of a cloud, at the fame time that he obferves the 
dark fpe£tra, will be thoroughly convinced, that they have no 
motion but what is given to them by the movement of our 
eyes in purfuit of them. Sometimes the form of the fpeftrum, 
when it has been received from a circular luminous body, will 
become oblong ; and fometimes it will be divided into two cir- 
cular fpeftra, which is owing to our changing the angle made 
by the two optic axifes, according to the diftance of the clouds 
or other bodies to which the fpe&rum is fuppofed to be conti- 
guous. The apparent lize of it will alfo be variable according 
to its fuppofed diftance ; but when fuch a fpe&rum is received 
with only one eye, the other being covered, its form and num- 
ber are invariable. 
As thefe fpe&ra are more eafily obfervable when our eyes are 
a little weakened by fatigue, it has frequently happened, that 
people of delicate conftitutions have been much alarmed at 
them, fearing a beginning decay of their fight, and have 
thence fallen into the hands of ignorant oculifts ; but I believe 
they never are a prelude to any other difeafe of the eye, and 
that it Is from habit alone, and our want of attention to them, 
7 that 
