The different terms applied to the various morbid affections of vision
have been frequently misconceived, and consequently have occasioned much
confusion in their application. Those vitiated conditions which are
usually noticed may be classed as follows:
I. Night sight.
II. Day sight.
III. Long sight.
IV. Short sight.
V. Skew sight.
VI. False sight.
_Night sight_, specifically called _Lucifuga_, was also termed
_Nyctalopia_, from [Greek: nyx], _night_, and [Greek: ops], _eye_; it was
also known as the _Noctem amans_. This affection was thus named in
consequence of the person labouring under it being only able to see at
night, or in a deep shade; hence the first name: while nyctalopia has been
used by most modern writers in the opposite sense of _night-sight ache_,
agreeably, according to Mason Good's observations, to the technical or
implied meaning of _opia_, in which case it always applies to a diseased
vision; whence nyctalopia has been made to import day sight, instead of
night sight.
This disease appears to be dependent upon a peculiar irritability of the
retina, produced by two different causes,--a sudden exposure to a stronger
light than the eye has been accustomed to bear, or a deficiency of the
black pigment which lines the choroid tunic. If the iris be weak and
torpid, it is enlarged; if strong and contracted, diminished. Thus, those
who from peculiar circumstances reside in dark caverns and subterraneous
abodes, or who have long been confined in obscure dungeons, labour under
the first of these causes; instances of which were observed in two of the
captives liberated from the Bastille in 1789.
Ramazzini informs us that this affection is commonly observed among the
Italian peasants, amongst whom he was not able to trace any other
peculiarity than an enlargement of the pupil. This state of the vision,
however, has been attributed to the peculiar brightness of the Italian
sky, its clear atmosphere, and the relaxing warmth of the temperature.
The Italian peasants are therefore constantly exposed to all those causes
that tend to debilitate the iris, while they irritate the retina. We thus
find these causes acting with renewed power at the season when the disease
usually makes its attack,--the vernal equinox, when an increased flood of
solar rays breaks on them. Such is the dimness that this brightness
produces, that the peasantry frequently lose their way in the fields in
the glare of day; but on the approach of night they can see distinctly.
Hence are they obliged to remain for some weeks in the shade to recover
their sight.
A deficiency of the black pigment of the eye is occasionally found in
persons of a very fair complexion and light hair. This affection is
therefore common in the Albinoes. This circumstance arises from the
whiteness of the eyelashes and hair, whereby the retina is deprived of the
natural shade that softens the light in its descent. This debilitated race
generally inhabit warm and damp regions; they are seldom long-lived, and
frequently low-spirited and morose. The iris is of a pink colour, and this
circumstance, added to the constant winking that the weakness of the organ
occasions, gives them a distressing appearance. In horses, this want of
the dark pigment constitutes what is called the _wall eye_.
Acuteness in night vision is natural to most, if not to all, animals that
prowl in the dark. In the feline genus we observe that the iris can be
contracted much closer than in mankind, when exposed to a vivid glare; but
they also expand to a much greater degree when obscurity sets in. Owls,
bats, and many insects, possess a similar faculty.
_Day sight_, the nyctalopia of some authors, is said to be endemic in some
countries,--Poland, the West Indies, Brazil, and various intertropical
regions. This affection arises from causes totally different from the
former one. Here the eye is habitually exposed to too great a flood of
light, whence the retina becomes torpid. It has been said to be endemic in
some districts of France, particularly in the neighbourhood of Roche
Guyon, on the banks of the Seine; but here the soil is of a dazzling
white: and as it makes its attacks in the spring, and continues for three
months, it is supposed to arise from the keenness of the reflected light,
after the dreary winter months.
This disease has also been commonly observed in Russia, especially in the
summer, when the eye is exposed, with scarcely any intermission, to the
constant action of light, as the sun dips but little below the horizon,
and there is scarcely any interval of darkness. Hens are subject to this
affection, and cannot see to pick up their food in the dusk of the
evening. The complaint is, from this circumstance, called _hen blindness_.
Dr. Heberden has communicated the following curious case of this species
of affection: "A man about forty years old had in the spring a tertian
fever, for which he took too small a quantity of bark, so that the returns
of it were weakened without being removed. Three days after his last fit,
being then employed on board a ship in the river, he observed at
sun-setting that all objects began to look blue, which blueness gradually
thickened into a cloud; and not long after he became so blind as hardly to
perceive the light of a candle. The next morning about sunrising his sight
was restored as perfectly as ever. When the next night came on, he lost
his sight again in the same manner, and this continued for twelve days and
nights. He then came ashore, where the disorder of his eyes gradually
abated, and in three days was entirely gone. A month after he went on
board another ship, and after three days' stay in it the night blindness
returned as before, and lasted all the time of his remaining in the ship,
which was nine nights. He then left the ship, and his blindness did not
return while he was upon land. Some little time afterwards he went into
another ship, in which he continued for ten days, during which time the
blindness returned only two nights, and never afterwards." It appears,
however, that this individual had previously laboured under an affection
produced by the use of lead, which had left him in a state of much nervous
debility. Notwithstanding this circumstance, this case clearly proves that
the affection is liable to be increased and brought on by local influence.
_Long sight._ In this species of vision the iris is habitually dilated,
and not easily stimulated into contraction. Several varieties of this
affection have been observed. Dr. Wells, in the Philosophical
Transactions, relates the case of a young person who, from a permanent
dilatation of the pupil, saw near objects with much difficulty and
confusion, but remote bodies with singular accuracy. The power of moving
the upper lid was completely lost. This dilatation of the pupil, which may
be artificially produced by the application of belladonna, can be remedied
by the use of convex glasses.
_Short sight._ In this case the iris is contracted, and the cornea, which
in long sight is too much flattened, is too convex or polarised; therefore
spectacles of an opposite character, and with concave glasses, become
necessary. Mice are said to be short-sighted; hence the affection has been
termed myopia or myopiasis, literally "mouse-sight."
_Skew sight_, or _sight askew_, is a condition of our vision only accurate
when the object is placed obliquely, in consequence of some partial
obfuscation of the cornea, frequently from slight scars, scarcely, if at
all, observable. In this lateral vision the axis of the eye affected
usually coincides with that of the sound eye. In squinting, on the
contrary, the two axes do not coincide.
In _false sight_, imaginary objects float before the sight; or, at other
times, objects assume imaginary forms and qualities. The latter species
has been divided in cases where the objects that are supposed to be seen
have no real existence, and in cases where actual objects have assumed
qualities that do not appertain to them. The first are termed ocular
phantasms or spectres; the latter, ocular transmutations or illusions.
These spectres sometimes form dark spots, called by physicians _muscae
volitantes_. In another species, a net-work seems to be spread before the
eyes; hence called _visus reticularis_. In a third form sparks
scintillate, and this appearance is experienced when the eye has been
struck. The eye is also troubled with an imaginary sense of dazzling,
constituting the _myrmaryge_ of the Greek writers; at other times, an
iridescent appearance, exhibiting the colours of the rainbow, is
experienced, although sometimes this impression is confined to a single
colour. Dr. Heberden relates the case of a lady of advanced age, lodging
on the eastern coast of Kent, in a house that looked immediately upon the
sea, and exposed to the glare of the morning sun. The curtains of her room
were white, a circumstance which added to the intensity of the light. When
she had been there about ten days, she observed one evening, at the time
of sunset, that first the fringes of the clouds appeared red, and soon
after the same colour was diffused over all the objects around her,
especially if they were white. This lasted the whole night, but in the
morning her sight was again perfect. This alternation of morbid and sound
sight prevailed the whole time the lady resided on the coast, which was
three weeks; and for nearly as long after she left it, at which time it
ceased suddenly of its own accord.
There exists another variety of false sight, that Plenk has denominated
_metamorphopsia_, and in which objects appear changed in their natural
qualities, producing error of form, error of motion, error of number, and
error of colour. I had a patient in Lisbon who fancied that all the horses
he saw carried horns or extensive antlers. A young lady whom I attended
beheld every one of a gigantic height. Dr. Priestley has given a curious
case of error of colour in five brothers and two sisters, all adults. One
of the brothers could form no idea whatever of colours, though he judged
very accurately of the form and other qualities of objects; hence he
thought stockings were sufficiently distinguished by the name of
stockings, and could not conceive the necessity of calling them white or
black. He could perceive cherries on a tree; but only distinguished them,
even when red-ripe, from the surrounding leaves by their size and shape.
One of the brothers appeared to have a faint sense of a few colours, but
still a very imperfect notion; and, upon the whole, they did not seem to
possess any other distinguishing power than that of light and shade, into
which they resolved all the colours presented to them,--so that dove or
straw colour were regarded as white; and green, crimson, and purple, as
black or dark. On looking at a rainbow, one of them could distinguish that
it consisted of stripes, but nothing more. Dr. Nicholl relates the case of
a boy who confounded green with red; and called light red and pink, blue.
His maternal grandfather and one uncle had the same imperfection. The
latter was in the navy, and having a blue coat and waistcoat, purchased a
pair of red breeches to match. The same physician knew a gentleman who
could not distinguish green from red; a cucumber and a boiled lobster did
not offer the least difference in colour. His brother and his niece
laboured under a similar affection.
Some philosophers are of opinion, that in the power of conceiving colours
there is a striking difference in individuals, and are inclined to think
that in many instances the supposed defects of sight ought to be ascribed
to a defect in the power of conception, arising probably from some early
habit of inattention. This theory is scarcely tenable. The utmost
inattention and indifference regarding surrounding objects could never
lead to a delusive view of any colour; also, it is more than probable
that, in the case of a child in whom such a defective vision was observed,
his attention would be incessantly called on by those around him, to
correct, if possible, so strange a delusion. Moreover, this defect of
vision, as we have seen, appears in some instances to be hereditary; and
to prevail in families.
Phrenologists of course are of opinion that the judgment of colours
resides in a particular organ, remarkably full and prominent in painters
distinguished by the perfection of their colouring. According to Gall, a
local deficiency of brain is observable where the power of distinguishing
colours is wanting.
The sense of vision exhibits more variety in the different classes of
animals than any of the others. In man, and the greater number of
quadrupeds, this organ is guarded by an upper and a lower lid, both of
which in man are fringed with lashes. This is not the case in most
quadrupeds. In the elephant, opossum, seal, cats, other mammalia, birds,
and all fishes, we find a third eyelid, or nictitating membrane, as it is
called, arising from the internal angle of the eye, capable of covering
and protecting the eye from danger, either wholly or in part. In the dog
this membrane is narrow; in oxen and horses it extends half over the
eyeball. It is by means of this veil that eagles are capable of fixing
their eyes on the noon-day sun. The largest eyes in proportion to the size
of the animal are found in birds,--nearly the smallest in whales; but the
most diminutive are those of the shrew and mole, the latter's not
exceeding the size of a pin's head.
The situation of the organs of vision differs materially. In man and
monkeys they are placed directly under the forehead; in some fishes, such
as the turbot and flounder tribes, both eyes are placed in the same side
of the head. In the snail they are situated on the horns; and in the
spider, distributed over various points of the body, and in different
arrangements.
Eyes, however, are not indispensable to become sensible of the presence of
light. Several zoophytes, that do not possess the organs of vision, are
perfectly alive to its influence. A distinct organ is not always
indispensable for a distinct sense. It is probable that in those animals
that appear to be endowed with particular senses, without displaying
particular organs relating to them, the senses are diffused like that of
touch, over the whole surface. This subject has been admirably commented
on by Cuvier.