RET 
■Some time. A similar dark square will be 
seen in the closed eye, if light be admitted 
through, the eye-lids. So after looking at 
.any luminous body of a small size, as at the 
Sun, for a short time, so as not much to 
fatigue the eyes, this part of the retina 
becomes less sensible to smaller quantities 
of light : henee when the eyes are turned 
upon other less luminous parts of the sky, a 
dark spot is seen resembling the shape of 
the luminous body. To the same cause 
Dr. R. Darwin ascribes those dark co- 
loured floating spots which are easily per- 
ceptible when the eyes are a little weak- 
ened by fatigue, and during illnesses which 
are attended with great debility. He says, 
that as these speetra are most easily dis- 
cernible when our eyes are weakened by 
fatigue, it has frequently happened that 
people of delicate constitutions have been 
much alarmed at them, fearing a beginning 
decay of their sight, and thence have fallen 
into the hands of ignorant oculists. They 
are not, however, he observes, the pre- 
ludes to any disease, and it is only from our 
habitual inattention to them that we do not 
see them on all objects every hour of ouf 
lives. As the nerves of very weak people, 
he continues, lose their sensibility by a 
small duration of exertion, it frequently 
happens that sick people, in the extreme 
debility of fevers, are perpetually em- 
ployed in picking something olF from the 
bed clothes, owing to their mistaking the 
cause of these dark spots. An Italian ar- 
tist, a man of strong abilities, relates, that 
having passed the whole night on a distant 
mountain, with some companions and a 
conjuror, and performed many ceremonies 
to raise the devil, on their return in the 
morning to Rome, looking up when the 
sim began to rise, they saw numerous devils 
run on the tops of the houses as they passed 
along. So much were the spectra of their 
weakened eyes magnified by fear, and 
made subservient to the purposes of fraud 
or superstition. 
Again, make with ink, on white paper, a 
very blaek spot about half an inch in dia- 
meter, with a tail about an inch in length, 
so as to represent a tadpole. Look steadily 
at this spot for about a minute, and on 
moving the eye a little, the figure of the 
tadpole will be seen on the white part of 
the paper, whieh figure will appear whiter 
or more luminous than the other part of the 
paper. This Dr. R. Darwin brings as one 
proof, that when the retina has been sub- 
jected to a less excitement, it is more easily 
RET 
brought into action by being subjected to 
a greater. A surface appears black in con- 
sequence of its absorbing all the rays of 
light ; that part of the retina, therefore, 
which is unemployed while looking at the 
spot, is afterwards more sensible of the light 
from the white paper, than those parts 
which had previously been exposed to it. 
On. closing the eyes after viewing the black 
spot on the white paper, a red spot is seen 
of the form of the black spot ; for that part 
of the retina on which the figure of the 
black Spot was formed, being more sensible 
to the light than the other parts, is capable 
of being brought into action by the red rays 
which penetrate the eye-lids. Upon the 
same principle Dr. R. Darwin accounts for 
the following fact. A writer in the Berlin 
Memoires observes, that when he held a 
book, so that the sun shone upon his half 
closed eye-lids, the black letters which he 
had long inspected, became red. There is 
a similar story told by Voltaire of a Duke 
of Tuscany, who was playing at dice with a 
general of a foreign army, and believing 
that he saw red spots on the dice, por- 
tended dreadful events, and retired in con- 
fusion. The observer, after looking for a 
minute on the black spots of a die, in a 
bright day, and carelessly closing his eyes, 
would see red spots corresponding to the 
black spots on the die, and if they were 
intense from the fatigue or weakness of the 
optic organ, those appearances would con- 
tinue, and on looking at the die, would be 
supposed to be upon it, just as before stated, 
persons in a very weak state often see black 
spots which they refer to tlie bed clothes. 
RETICULA, or Reticule, in astronomy, 
a contrivance for the exact measuring the 
quantity of eclipses. The reticule is a lit- 
tle framfe, consisting of thirteen fine silken 
threads, equidistant from each other, and 
parallel, placed in the focus of object- 
glasses of telescopes ; that is, in the place 
where the image of tlie luminary is painted 
in its full extent j of consequence, there- 
fore, the diameter of the sun or moon is 
hereby seen divided into twelve equal parts 
or digits ; so that to find the quantity of 
the eclipse, there is nothing to do but to 
number the luminous and tlie dark parts. 
As a square reticule is only proper for the 
diameter, not for the circumference, of the 
luminary, it is sometimes made circular by 
drawing six concentric equi-distant circles. 
This represents the phases of the eclipse 
perfectly. 
RETINA, in anatomy, the expansion of 
