Newton, and the perception of visible objects has been ably investigated by
the ingenious Dr. Berkeley and M. Malebranche; but these minute phenomena
of vision have yet been thought reducible to no theory, though many
philosophers have employed a considerable degree of attention upon them:
among these are Dr. Jurin, at the end of Dr. Smith's Optics; M. Æpinus, in
the Nov. Com. Petropol. V. 10.; M. Beguelin, in the Berlin Memoires, V. II.
1771; M. d'Arcy, in the Histoire de l'Acad. des Scienc. 1765; M. de la
Hire; and, lastly, the celebrated M. de Buffon, in the Memoires de l'Acad.
des Scien. who has termed them accidental colours, as if subjected to no
established laws, Ac. Par. 1743. M. p. 215.
I must here apprize the reader, that it is very difficult for different
people to give the same names to various shades of colours; whence, in the
following pages, something must be allowed, if on repeating the experiments
the colours here mentioned should not accurately correspond with his own
names of them.
I. _Activity of the Retina in Vision._
From the subsequent experiments it appears, that the retina is in an active
not in a passive state during the existence of these ocular spectra; and it
is thence to be concluded, that all vision is owing to the activity of this
organ.
1. Place a piece of red silk, about an inch in diameter, as in plate 1, at
Sect. III. 1., on a sheet of white paper, in a strong light; look steadily
upon it from about the distance of half a yard for a minute; then closing
your eyelids cover them with your hands, and a green spectrum will be seen
in your eyes, resembling in form the piece of red silk: after some time,
this spectrum will disappear and shortly reappear; and this alternately
three or four times, if the experiment is well made, till at length it
vanishes entirely.
2. Place on a sheet of white paper a circular piece of blue silk, about
four inches in diameter, in the sunshine; cover the center of this with a
circular piece of yellow silk, about three inches in diameter; and the
center of the yellow silk with a circle of pink silk, about two inches in
diameter; and the center of the pink silk with a circle of green silk,
about one inch in diameter; and the centre of this with a circle of indigo,
about half an inch in diameter; make a small speck with ink in the very
center of the whole, as in plate 3, at Sect. III. 3. 6.; look steadily for
a minute on this central spot, and then closing your eyes, and applying
your hand at about an inch distance before them, so as to prevent too much
or too little light from passing through the eyelids, you will see the most
beautiful circles of colours that imagination can conceive, which are most
resembled by the colours occasioned by pouring a drop or two of oil on a
still lake in a bright day; but these circular irises of colours are not
only different from the colours of the silks above mentioned, but are at
the same time perpetually changing as long as they exist.
3. When any one in the dark presses either corner of his eye with his
finger, and turns his eye away from his finger, he will see a circle of
colours like those in a peacock's tail: and a sudden flash of light is
excited in the eye by a stroke on it. (Newton's Opt. Q. 16.)
4. When any one turns round rapidly on one foot, till he becomes dizzy, and
falls upon the ground, the spectra of the ambient objects continue to
present themselves in rotation, or appear to librate, and he seems to
behold them for some time still in motion.
From all these experiments it appears, that the spectra in the eye are not
owing to the mechanical impulse of light impressed on the retina, nor to
its chemical combination with that organ, nor to the absorption and
emission of light, as is observed in many bodies; for in all these cases
the spectra must either remain uniformly, or gradually diminish; and
neither their alternate pretence and evanescence as in the first
experiment, nor the perpetual changes of their colours as in the second,
nor the flash of light or colours in the pressed eye as in the third, nor
the rotation or libration of the spectra as in the fourth, could exist.
It is not absurd to conceive, that the retina may be stimulated into
motion, as well as the red and white muscles which form our limbs and
vessels; since it consists of fibres, like those, intermixed with its
medullary substance. To evince this structure, the retina of an ox's eye
was suspended in a glass of warm water, and forcibly torn in a few places;
the edges of these parts appeared jagged and hairy, and did not contract,
and become smooth like simple mucus, when it is distended till it breaks;
which shews that it consists of fibres; and that its fibrous construction
became still more distinct to the sight, by adding some caustic alkali to
the water, as the adhering mucus was first eroded, and the hair-like fibres
remained floating in the vessel. Nor does the degree of transparency of the
retina invalidate the evidence of its fibrous structure, since Leeuwenhoek
has shewn that the crystalline humour itself consists of fibres. (Arcana
Naturæ, V. 1. p. 70.)
Hence it appears, that as the muscles have larger fibres intermixed with a
smaller quantity of nervous medulla, the organ of vision has a greater
quantity of nervous medulla intermixed with smaller fibres; and it is
probable that the locomotive muscles, as well as the vascular ones, of
microscopic animals have much greater tenuity than these of the retina.
And besides the similar laws, which will be shewn in this paper to govern
alike the actions of the retina and of the muscles, there are many other
analogies which exist between them. They are both originally excited into
action by irritations, both are nearly in the same quantity of time, are
alike strengthened or fatigued by exertion, are alike painful if excited
into action when they are in an inflamed state, are alike liable to
paralysis, and to the torpor of old age.
II. OF SPECTRA FROM DEFECT OF SENSIBILITY.
_The retina is not so easily excited into action by less irritation
after having been lately subjected to greater._
1. When any one passes from the bright daylight into a darkened room, the
irises of his eyes expand themselves to their utmost extent in a few
seconds of time; but it is very long before the optic nerve, after having
been stimulated by the greater light of the day, becomes sensible of the
less degree of it in the room; and, if the room is not too obscure, the
irises will again contract themselves in some degree, as the sensibility of
the retina returns.
2. Place about half an inch square of white paper on a black hat, and
looking steadily on the center of it for a minute, remove your eyes to a
sheet of white paper; and after a second or two a dark square will be seen
on the white paper, which will continue some time. A similar dark square
will be seen in the closed eye, if light be admitted through the eyelids.
So after looking at any luminous object 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; hence, when the eyes
are turned on other less luminous parts of the sky, a dark spot is seen
resembling the shape of the sun, or other luminous object which we last
beheld. This is the source of one kind of the dark-coloured _muscæ
volitantes_. If this dark spot lies above the center of the eye, we turn
our eyes that way, expecting to bring it into the center of the eye, that
we may view it more distinctly; and in this case the dark spectrum seems to
move upwards. If the dark spectrum is found beneath the centre of the eye,
we pursue it from the same motive, and it seems to move downwards. This has
given rise to various conjectures of something floating in the aqueous
humours of the eyes; but whoever, in attending to these spots, keeps his
eyes unmoved by looking steadily at the corner of a cloud, at the same time
that he observes the dark spectra, will be thoroughly convinced, that they
have no motion but what is given to them by the movement of our eyes in
pursuit of them. Sometimes the form of the spectrum, when it has been
received from a circular luminous body, will become oblong; and sometimes
it will be divided into two circular spectra, which is not owing to our
changing the angle made by the two optic axises, according to the distance
of the clouds or other bodies to which the spectrum is supposed to be
contiguous, but to other causes mentioned in No. X. 3. of this section. The
apparent size of it will also be variable according to its supposed
distance.
As these spectra are more easily observable when our eyes are a little
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 have thence fallen into the hands of ignorant oculists;
but I believe they never are a prelude to any other disease of the eye, and
that it is from habit alone, and our want of attention to them, that we do
not see them on all objects every hour of our lives. But as the nerves of
very weak people lose their sensibility, in the same manner as their
muscles lose their activity, by a small time of exertion, it frequently
happens, that sick people in the extreme debility of fevers are perpetually
employed in picking something from the bed-clothes, occasioned by their
mistaking the appearance of these _muscæ volitantes_ in their eyes.
Benvenuto Celini, an Italian artist, a man of strong abilities, relates,
that having passed the whole night on a distant mountain with some
companions and a conjurer, and performed many ceremonies to raise the
devil, on their return in the morning to Rome, and looking up when the sun
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. (Life of Ben. Celini.)
3. Place a square inch of white paper on a large piece of straw-coloured
silk; look steadily some time on the white paper, and then move the centre
of your eyes on the silk, and a spectrum of the form of the paper will
appear on the silk, of a deeper yellow than the other part of it: for the
central part of the retina, having been some time exposed to the stimulus
of a greater quantity of white light, is become less sensible to a smaller
quantity of it, and therefore sees only the yellow rays in that part of the
straw-coloured silk.
Facts similar to these are observable in other parts of our system: thus,
if one hand be made warm, and the other exposed to the cold, and then both
of them immersed in subtepid water, the water is perceived warm to one
hand, and cold to the other; and we are not able to hear weak sounds for
some time after we have been exposed to loud ones; and we feel a chilliness
on coming into an atmosphere of temperate warmth, after having been some
time confined in a very warm room: and hence the stomach, and other organs
of digestion, of those who have been habituated to the greater stimulus of
spirituous liquor, are not excited into their due action by the less
stimulus of common food alone; of which the immediate consequence is
indigestion and hypochondriacism.
III. OF SPECTRA FROM EXCESS OF SENSIBILITY.
_The retina is more easily excited into action by greater irritation
after having been lately subjected to less._
1. If the eyes are closed, and covered perfectly with a hat, for a minute
or two, in a bright day; on removing the hat a red or crimson light is seen
through the eyelids. In this experiment the retina, after being some time
kept in the dark, becomes so sensible to a small quantity of light, as to
perceive distinctly the greater quantity of red rays than of others which
pass through the eyelids. A similar coloured light is seen to pass through
the edges of the fingers, when the open hand is opposed to the flame of a
candle.
2. If you look for some minutes steadily on a window in the beginning of
the evening twilight, or in a dark day, and then move your eyes a little,
so that those parts of the retina, on which the dark frame-work of the
window was delineated, may now fall on the glass part of it, many luminous
lines, representing the frame-work, will appear to lie across the glass
panes: for those parts of the retina, which were before least stimulated by
the dark frame-work, are now more sensible to light than the other parts of
the retina which were exposed to the more luminous parts of the window,
3. Make with ink on white paper a very black spot, about half an inch in
diameter, with a tail about an inch in length, so as to represent a
tadpole, as in plate 2, at Sect. III. 3. 3.; look steadily for a minute on
this spot, and, on moving the eye a little, the figure of the tadpole will
be seen on the white part of the paper, which figure of the tadpole will
appear whiter or more luminous than the other parts of the white paper; for
the part of the retina on which the tadpole was delineated, is now more
sensible to light, than the other parts of it, which were exposed to the
white paper. This experiment is mentioned by Dr. Irwin, but is not by him
ascribed to the true cause, namely, the greater sensibility of that part of
the retina which has been exposed to the black spot, than of the other
parts which had received the white field of paper, which is put beyond a
doubt by the next experiment.
4. On closing the eyes after viewing the black spot on the white paper, as
in the foregoing experiment, a red spot is seen of the form of the black
spot: for that part of the retina, on which the black spot was delineated,
being now more sensible to light than the other parts of it, which were
exposed to the white paper, is capable of perceiving the red rays which
penetrate the eyelids. If this experiment be made by the light of a tallow
candle, the spot will be yellow instead of red; for tallow candles abound
much with yellow light, which passes in greater quantity and force through
the eyelids than blue tight; hence the difficulty of distinguishing blue
and green by this kind of candle light. The colour of the spectrum may
possibly vary in the daylight, according to the different colour of the
meridian or the morning or evening light.
M. Beguelin, in the Berlin Memoires, V. II. 1771, observes, that, when he
held a book so that the sun shone upon his half-closed eyelids, the black
letters, which he had long inspected, became red, which must have been thus
occasioned. Those parts of the retina which had received for some time the
black letters, were so much more sensible than those parts which had been
opposed to the white paper, that to the former the red light, which passed
through the eyelids, was perceptible. There is a similar story told, I
think, in de Voltaire's Historical Works, of a Duke of Tuscany, who was
playing at dice with the general of a foreign army, and, believing he saw
bloody spots upon the dice, portended dreadful events, and retired in
confusion. The observer, after looking for a minute on the black spots of a
die, and carelessly closing his eyes, on a bright day; would see the image
of a die with red spots upon it, as above explained.
5. On emerging from a dark cavern, where we have long continued, the light
of a bright day becomes intolerable to the eye for a considerable time,
owing to the excess of sensibility existing in the eye, after having been
long exposed to little or no stimulus. This occasions us immediately to
contract the iris to its smallest aperture, which becomes again gradually
dilated, as the retina becomes accustomed to the greater stimulus of the
daylight.
The twinkling of a bright star, or of a distant candle in the night, is
perhaps owing to the same cause. While we continue to look upon these
luminous objects, their central parts gradually appear paler, owing to the
decreasing sensibility of the part of the retina exposed to their light;
whilst, at the same time, by the unsteadiness of the eye, the edges of them
are perpetually falling on parts of the retina that were just before
exposed to the darkness of the night, and therefore tenfold more sensible
to light than the part on which the star or candle had been for some time
delineated. This pains the eye in a similar manner as when we come suddenly
from a dark room into bright daylight, and gives the appearance of bright
scintillations. Hence the stars twinkle most when the night is darkest, and
do not twinkle through telescopes, as observed by Musschenbroeck; and it
will afterwards be seen why this twinkling is sometimes of different
colours when the object is very bright, as Mr. Melvill observed in looking
at Sirius. For the opinions of others on this subject, see Dr. Priestley's
valuable History of Light and Colours, p. 494.
Many facts observable in the animal system are similar to these; as the hot
glow occasioned by the usual warmth of the air, or our clothes, on coming
out of a cold bath; the pain of the fingers on approaching the fire after
having handled snow; and the inflamed heels from walking in snow. Hence
those who have been exposed to much cold have died on being brought to a
fire, or their limbs have become so much inflamed as to mortify. Hence much
food or wine given suddenly to those who have almost perished by hunger has
destroyed them; for all the organs of the famished body are now become so
much more irritable to the stimulus of food and wine, which they have long
been deprived of, that inflammation is excited, which terminates in
gangrene or fever.
IV. OF DIRECT OCULAR SPECTRA.
_A quantity of stimulus somewhat greater than natural excites the
retina into spasmodic action, which ceases in a few seconds._
A certain duration and energy of the stimulus of light and colours excites
the perfect action of the retina in vision; for very quick motions are
imperceptible to us, as well as very slow ones, as the whirling of a top,
or the shadow on a sun-dial. So perfect darkness does not affect the eye at
all; and excess of light produces pain, not vision.
1. When a fire-coal is whirled round in the dark, a lucid circle remains a
considerable time in the eye; and that with so much vivacity of light, that
it is mistaken for a continuance of the irritation of the object. In the
same manner, when a fiery meteor shoots across the night, it appears to
leave a long lucid train behind it, part of which, and perhaps sometimes
the whole, is owing to the continuance of the action of the retina after
having been thus vividly excited. This is beautifully illustrated by the
following experiment: fix a paper sail, three or four inches in diameter,
and made like that of a smoke jack, on a tube of pasteboard; on looking
through the tube at a distant prospect, some disjointed parts of it will be
seen through the narrow intervals between the sails; but as the fly begins
to revolve, these intervals appear larger; and when it revolves quicker,
the whole prospect is seen quite as distinct as if nothing intervened,
though less luminous.
[Illustration: Fig. 3.]
2. Look through a dark tube, about half a yard long, at the area of a
yellow circle of half an inch diameter, lying upon a blue area of double
that diameter, for half a minute; and on closing your eyes the colours of
the spectrum will appear similar to the two areas, as in fig. 3.; but if
the eye is kept too long upon them, the colours of the spectrum will be the
reverse of those upon the paper, that is, the internal circle will become
blue, and the external area yellow; hence some attention is required in
making this experiment.
3. Place the bright flame of a spermaceti candle before a black object in
the night; look steadily at it for a short time, till it is observed to
become somewhat paler; and on closing the eyes, and covering them
carefully, but not so as to compress them, the image of the blazing candle
will continue distinctly to be visible.
4. Look steadily, for a short time, at a window in a dark day, as in Exp.
2. Sect. III. and then closing your eyes, and covering them with your
hands, an exact delineation of the window remains for some time visible in
the eye. This experiment requires a little practice to make it succeed
well; since, 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 spectrum, and the dark parts of the frame-work will appear luminous, as
in Exp. 2. Sect. III. And it is even difficult for many, who first try this
experiment, to perceive the spectrum at all; for any hurry of mind, or even
too great attention to the spectrum itself, will disappoint them, till they
have had a little experience in attending to such small sensations.
The spectra described in this section, termed direct ocular spectra, are
produced without much fatigue of the eye; the irritation of the luminous
object being soon withdrawn, or its quantity of light being not so great as
to produce any degree of uneasiness in the organ of vision; which
distinguishes them from the next class of ocular spectra, which are the
consequence of fatigue. These direct spectra are best observed in such
circumstances that no light, but what comes from the object, 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 side of a room, the direct spectrum was
easily produced on closing the eye without taking it from the tube; but if
the lateral light is admitted through the eyelids, or by throwing the
spectrum on white paper, it becomes a reverse spectrum, as will be
explained below.
The other senses also retain for a time the impressions that have been made
upon them, or the actions they have been excited into. So if a hard body is
pressed upon the palm of the hand, as is practised in tricks of
legerdemain, it is not easy to distinguish for a few seconds whether it
remains or is removed; and tastes continue long to exist vividly in the
mouth, as the smoke of tobacco, or the taste of gentian, after the sapid
material is withdrawn.
V. _A quantity of stimulus somewhat greater than the last mentioned
excites the retina into spasmodic action, which ceases and recurs
alternately._
1. On looking for a time on the setting sun, so as not greatly to fatigue
the sight, a yellow spectrum is seen when the eyes are closed and covered,
which continues for a time, and then disappears and recurs repeatedly
before it entirely vanishes. This yellow spectrum of the sun when the
eyelids are opened becomes blue; and if it is made to fall on the green
grass, or on other coloured objects, it varies its own colour by an
intermixture of theirs, as will be explained in another place.
2. Place a lighted spermaceti candle in the night about one foot from your
eye, and look steadily on the centre of the flame, till your eye becomes
much more fatigued than in Sect. IV. Exp. 3.; and on closing your eyes a
reddish spectrum will be perceived, which will cease and return
alternately.
The action of vomiting in like manner ceases, and is renewed by intervals,
although the emetic drug is thrown up with the first effort: so after-pains
continue some time after parturition; and the alternate pulsations of the
heart of a viper are renewed for some time after it is cleared from its
blood.
VI. OF REVERSE OCULAR SPECTRA.
_The retina, after having been excited into action by a stimulus
somewhat greater them the last mentioned falls into opposite spasmodic
action._
The actions of every part of animal bodies may be advantageously compared
with each other. This strict analogy contributes much to the investigation
of truth; while those looser analogies, which compare the phenomena of
animal life with those of chemistry or mechanics, only serve to mislead our
inquiries.
When any of our larger muscles have been in long or in violent action, and
their antagonists have been at the same time extended, as soon as the
action of the former ceases, the limb is stretched the contrary way for our
ease, and a pandiculation or yawning takes place.
By the following observations it appears, that a similar circumstance
obtains in the organ of vision; after it has been fatigued by one kind of
action, it spontaneously falls into the opposite kind.
1. Place a piece of coloured silk, about an inch in diameter, on a sheet of
white paper, about half a yard from your eyes; look steadily upon it for a
minute; then remove your eyes upon another part of the white paper, and a
spectrum will be seen of the form of the silk thus inspected, but of a
colour opposite to it. A spectrum nearly similar will appear if the eyes
are closed, and the eyelids shaded by approaching the hand near them, so as
to permit some, but to prevent too much light falling on them.
Red silk produced a green spectrum.
Green produced a red one.
Orange produced blue.
Blue produced orange.
Yellow produced violet.
Violet produced yellow.
That in these experiments the colours of the spectra are the reverse of the
colours which occasioned them, may be seen by examining the third figure in
Sir Isaac Newton's Optics, L. II. p. 1, where those thin laminæ of air,
which reflected yellow, transmitted violet; those which reflected red,
transmitted a blue green; and so of the rest, agreeing with the experiments
above related.
2. These reverse spectra are similar to a colour, formed by a combination
of all the primary colours except that with which the eye has been fatigued
in making the experiment: thus the reverse spectrum of red must be such a
green as would be produced by a combination of all the other prismatic
colours. To evince this fact the following satisfactory experiment was
made. The prismatic colours were laid on a circular pasteboard wheel, about
four inches in diameter, in the proportions described in Dr. Priestley's
History of Light and Colours, pl. 12. fig. 83. except that the red
compartment was entirely left out, and the others proportionably extended
so as to complete the circle. Then, as the orange is a mixture of red and
yellow, and as the violet is a mixture of red and indigo, it became
necessary to put yellow on the wheel instead of orange, and indigo instead
of violet, that the experiment might more exactly quadrate with the theory
it was designed to establish or confute; because in gaining a green
spectrum from a red object, the eye is supposed to have become insensible
to red light. This wheel, by means of an axis, was made to whirl like a
top; and on its being put in motion, a green colour was produced,
corresponding with great exactness to the reverse spectrum of red.
3. In contemplating any one or these reverse spectra in the closed and
covered eye, it disappears and re-appears several times successively, till
at length it entirely vanishes, like the direct spectra in Sect. V.; but
with this additional circumstance, that when the spectrum becomes faint or
evanescent, it is instantly revived by removing the hand from before the
eyelids, so as to admit more light: because then not only the fatigued part
of the retina is inclined spontaneously to fall into motions of a contrary
direction, but being still sensible to all other rays of light, except that
with which it was lately fatigued, is by these rays at the same time
stimulated into those motions which form the reverse spectrum.
From these experiments there is reason to conclude, that the fatigued part
of the retina throws itself into a contrary mode of action, like oscitation
or pandiculation, as soon as the stimulus which has fatigued it is
withdrawn; and that it still remains sensible, that is, liable to be
excited into action by any other colours at the same time, except the
colour with which it has been fatigued.
VII. _The retina after having been excited into action by a stimulus
somewhat greater than the last mentioned falls into various successive
spasmodic actions._
1. On looking at the meridian sun as long as the eyes can well bear its
brightness, the disk first becomes pale, with a luminous crescent, which
seems to librate from one edge of it to the other, owing to the
unsteadiness of the eye; then the whole phasis of the sun becomes blue,
surrounded with a white halo; and on closing the eyes, and covering them
with the hands, a yellow spectrum is seen, which in a little time changes
into a blue one.
M. de la Hire observed, after looking at the bright sun, that the
impression in his eye first assumed a yellow appearance, and then green,
and then blue; and wishes to ascribe these appearances to some affection of
the nerves. (Porterfield on the Eye, Vol. I. p. 313.)
2. After looking steadily on about an inch square of pink silk, placed on
white paper, in a bright sunshine, at the distance of a foot from my eyes,
and closing and covering my eyelids, the spectrum of the silk was at first
a dark green, and the spectrum of the white paper became of a pink. The
spectra then both disappeared; and then the internal spectrum was blue; and
then, after a second disappearance, became yellow, and lastly pink, whilst
the spectrum of the field varied into red and green.
These successions of different coloured spectra were not exactly the same
in the different experiments, though observed, as near as could be, with
the same quantity of light, and other similar circumstances; owing, I
suppose, to trying too many experiments at a time; so that the eye was not
quite free from the spectra of the colours which were previously attended
to.
The alternate exertions of the retina in the preceding section resembled
the oscitation or pandiculation of the muscles, as they were performed in
directions contrary to each other, and were the consequence of fatigue
rather than of pain. And in this they differ from the successive dissimilar
exertions of the retina, mentioned in this section, which resemble in
miniature the more violent agitations of the limbs in convulsive diseases,
as epilepsy, chorea S. Viti, and opisthotonos; all which diseases are
perhaps, at first, the consequence of pain, and have their periods
afterwards established by habit.
VIII. _The retina, after having been excited into action by a stimulus
somewhat greater than the last mentioned falls into a fixed spasmodic
action, which continues for some days._
1. After having looked long at the meridian sun, in making some of the
preceding experiments, till the disks faded into a pale blue, I frequently
observed a bright blue spectrum of the sun on other objects all the next
and the succeeding day, which constantly occurred when I attended to it,
and frequently when I did not previously attend to it. When I closed and
covered my eyes, this appeared of a dull yellow; and at other times mixed
with the colours of other objects on which it was thrown. It may be
imagined, that this part of the retina was become insensible to white
light, and thence a bluish spectrum became visible on all luminous objects;
but as a yellowish spectrum was also seen in the closed and covered eye,
there can remain no doubt of this being the spectrum of the sun. A similar
appearance was observed by M. Æpinus, which he acknowledges he could give
no account of. (Nov. Com. Petrop. V. 10. p. 2. and 6.)
The locked jaw, and some cataleptic spasms, are resembled by this
phenomenon; and from hence we may learn the danger to the eye by inspecting
very luminous objects too long a time.
IX. _A quantity of stimulus greater than the preceding induces a temporary
paralysis of the organ of vision._
1. Place a circular piece of bright red silk, about half an inch in
diameter, on the middle of a sheet of white paper; lay them on the floor in
a bright sunshine, and fixing your eyes steadily on the center of the red
circle, for three or four minutes, at the distance of four or six feet from
the object, the red silk will gradually become paler, and finally cease to
appear red at all.
2. Similar to these are many other animal facts; as purges, opiates, and
even poisons, and contagious matter, cease to stimulate our system, after
we have been habituated to their use. So some people sleep undisturbed by a
clock, or even by a forge hammer in their neighbourhood: and not only
continued irritations, but violent exertions of any kind, are succeeded by
temporary paralysis. The arm drops down after violent action, and continues
for a time useless; and it is probable, that those who have perished
suddenly in swimming, or in scating on the ice, have owed their deaths to
the paralysis, or extreme fatigue, which succeeds every violent and
continued exertion.
X. MISCELLANEOUS REMARKS.
There were some circumstances occurred in making these experiments, which
were liable to alter the results of them, and which I shall here mention
for the assistance of others, who may wish to repeat them.
1. _Of direct and inverse spectra existing at the same time_; _of
reciprocal direct spectra_; _of a combination of direct and inverse
spectra_; _of a spectral halo_; _rules to pre-determine the colours of
spectra_.
a. When an area, about six inches square, of bright pink Indian paper, had
been viewed on an area, about a foot square, of white writing paper, the
internal spectrum in the closed eye was green, being the reverse spectrum
of the pink paper; and the external spectrum was pink, being the direct
spectrum of the pink paper. The same circumstance happened when the
internal area was white, and external one pink; that is, the internal
spectrum was pink, and the external one green. All the same appearances
occurred when the pink paper was laid on a black hat.
b. When six inches square of deep violet polished paper was viewed on a
foot square of white writing paper, the internal spectrum was yellow, being
the reverse spectrum of the violet paper, and the external one was violet,
being the direct spectrum of the violet paper.
c. When six inches square of pink paper was viewed on a foot square of blue
paper, the internal spectrum was blue, and the external spectrum was pink;
that is, the internal one was the direct spectrum of the external object,
and the external one was the direct spectrum of the internal object,
instead of their being each the reverse spectrum of the objects they
belonged to.
d. When six inches square of blue paper were viewed on a foot square of
yellow paper, the interior spectrum became a brilliant yellow, and the
exterior one a brilliant blue. The vivacity of the spectra was owing to
their being excited both by the stimulus of the interior and exterior
objects; so that the interior yellow spectrum was both the reverse spectrum
of the blue paper, and the direct one of the yellow paper; and the exterior
blue spectrum was both the reverse spectrum of the yellow paper, and the
direct one of the blue paper.
e. When the internal area was only a square half-inch of red paper, laid on
a square foot of dark violet paper, the internal spectrum was green, with a
reddish-blue halo. When the red internal paper was two inches square, the
internal spectrum was a deeper green, and the external one redder. When the
internal paper was six inches square, the spectrum of it became blue, and
the spectrum of the external paper was red.
f. When a square half-inch of blue paper was laid on a six-inch square of
yellow paper, the spectrum of the central paper in the closed eye was
yellow, incircled with a blue halo. On looking long on the meridian sun,
the disk fades into a pale blue surrounded with a whitish halo.
These circumstances, though they very much perplexed the experiments till
they were investigated, admit of a satisfactory explanation; for while the
rays from the bright internal object in exp. a. fall with their full force
on the center of the retina, and, by fatiguing that part of it, induce the
reverse spectrum, many scattered rays, from the same internal pink paper,
fall on the more external parts of the retina, but not in such quantity as
to occasion much fatigue, and hence induce the direct spectrum of the pink
colour in those parts of the eye. The same reverse and direct spectra occur
from the violet paper in exp. b.: and in exp. c. the scattered rays from
the central pink paper produce a direct spectrum of this colour on the
external parts of the eye, while the scattered rays from the external blue
paper produce a direct spectrum of that colour on the central part of the
eye, instead of these parts of the retina falling reciprocally into their
reverse spectra. In exp. d. the colours being the reverse of each other,
the scattered rays from the exterior object falling on the central parts of
the eye, and there exciting their direct spectrum, at the same time that
the retina was excited into a reverse spectrum by the central object, and
this direct and reverse spectrum being of similar colour, the superior
brilliancy of this spectrum was produced. In exp. e. the effect of various
quantities of stimulus on the retina, from the different respective sizes
of the internal and external areas, induced a spectrum of the internal area
in the center of the eye, combined of the reverse spectrum of that internal
area and the direct one of the external area, in various shades of colour,
from a pale green to a deep blue, with similar changes in the spectrum of
the external area. For the same reasons, when an internal bright object was
small, as in exp. f. instead of the whole of the spectrum of the external
object being reverse to the colour of the internal object, only a kind of
halo, or radiation of colour, similar to that of the internal object, was
spread a little way on the external spectrum. For this internal blue area
being so small, the scattered rays from it extended but a little way on the
image of the external area of yellow paper, and could therefore produce
only a blue halo round the yellow spectrum in the center.
If any one should suspect that the scattered rays from the exterior
coloured object do not intermix with the rays from the interior coloured
object, and thus affect the central part of the eye, let him look through
an opake tube, about two feet in length, and an inch in diameter, at a
coloured wall of a room with one eye, and with the other eye naked; and he
will find, that by shutting out the lateral light, the area of the wall
seen through a tube appears as if illuminated by the sunshine, compared
with the other parts of it; from whence arises the advantage of looking
through a dark tube at distant paintings.
Hence we may safely deduce the following rules to determine before-hand the
colours of all spectra. 1. The direct spectrum without any lateral light is
an evanescent representation of its object in the unfatigued eye. 2. With
some lateral light it becomes of a colour combined of the direct spectrum
of the central object, and of the circumjacent objects, in proportion to
their respective quantity and brilliancy. 3. The reverse spectrum without
lateral light is a representation in the fatigued eye of the form of its
objects, with such a colour as would be produced by all the primary
colours, except that of the object. 4. With lateral light the colour is
compounded of the reverse spectrum of the central object, and the direct
spectrum of the circumjacent objects, in proportion to their respective
quantity and brilliancy.
2. _Variation and vivacity of the spectra occasioned by extraneous light._
The reverse spectrum, as has been before explained, is similar to a colour,
formed by a combination of all the primary colours, except that with which
the eye has been fatigued in making the experiment: so the reverse spectrum
of red is such a green as would be produced by a combination of all the
other prismatic colours. Now it must be observed, that this reverse
spectrum of red is therefore the direct spectrum of a combination of all
the other prismatic colours, except the red; whence, on removing the eye
from a piece of red silk to a sheet of white paper, the green spectrum,
which is perceived, may either be called the reverse spectrum of the red
silk, or the direct spectrum of all the rays from the white paper, except
the red; for in truth it is both. Hence we see the reason why it is not
easy to gain a direct spectrum of any coloured object in the day-time,
where there is much lateral light, except of very bright objects, as of the
setting sun, or by looking through an opake tube; because the lateral
external light falling also on the central part of the retina, contributes
to induce the reverse spectrum, which is at the same time the direct
spectrum of that lateral light, deducting only the colour of the central
object which we have been viewing. And for the same reason, it is difficult
to gain the reverse spectrum, where there is no lateral light to contribute
to its formation. Thus, in looking through an opake tube on a yellow wall,
and closing my eye, without admitting any lateral light, the spectra were
all at first yellow; but at length changed into blue. And on looking in the
same manner on red paper, I did at length get a green spectrum; but they
were all at first red ones: and the same after looking at a candle in the
night.
The reverse spectrum was formed with greater facility when the eye was
thrown from the object on a sheet of white paper, or when light was
admitted through the closed eyelids; because not only the fatigued part of
the retina was inclined spontaneously to fall into motions of a contrary
direction; but being still sensible to all other rays of light except that
with which it was lately fatigued, was by these rays stimulated at the same
time into those motions which form the reverse spectrum. Hence, when, the
reverse spectrum of any colour became faint, it was wonderfully revived by
admitting more light through the eyelids, by removing the hand from before
them: and hence, on covering the closed eyelids, the spectrum would often
cease for a time, till the retina became sensible to the stimulus of the
smaller quantity of light, and then it recurred. Nor was the spectrum only
changed in vivacity, or in degree, by this admission of light through the
eyelids; but it frequently happened, after having viewed bright objects,
that the spectrum in the closed and covered eye was changed into a third
spectrum, when light was admitted through the eyelids: which third spectrum
was composed of such colours as could pass through the eyelids, except
those of the object. Thus, when an area of half an inch diameter of pink
paper was viewed on a sheet of white paper in the sunshine, the spectrum
with closed and covered eyes was green; but on removing the hands from
before the closed eyelids, the spectrum became yellow, and returned
instantly again to green, as often as the hands were applied to cover the
eyelids, or removed from them: for the retina being now insensible to red
light, the yellow rays passing through the eyelids in greater quantity than
the other colours, induced a yellow spectrum; whereas if the spectrum was
thrown on white paper, with the eyes open, it became only a lighter green.
Though a certain quantity of light facilitates the formation of the reverse
spectrum, a greater quantity prevents its formation, as the more powerful
stimulus excites even the fatigued parts of the eye into action; otherwise
we should see the spectrum of the last viewed object as often as we turn
our eyes. Hence the reverse spectra are best seen by gradually approaching
the hand near the closed eyelids to a certain distance only, which must be
varied with the brightness of the day, or the energy of the spectrum. Add
to this, that all dark spectra, as black, blue, or green, if light be
admitted through the eyelids, after they have been some time covered, give
reddish spectra, for the reasons given in Sect. III. Exp. 1.
From these circumstances of the extraneous light coinciding with the
spontaneous efforts of the fatigued retina to produce a reverse spectrum,
as was observed before, it is not easy to gain a direct spectrum, except of
objects brighter than the ambient light; such as a candle in the night, the
setting sun, or viewing a bright object through an opake tube; and then the
reverse spectrum is instantaneously produced by the admission of some
external light; and is as instantly converted again to the direct spectrum
by the exclusion of it. Thus, on looking at the setting sun, on closing the
eyes, and covering them, a yellow spectrum is seen, which is the direct
spectrum of the setting sun; but on opening the eyes on the sky, the yellow
spectrum is immediately changed into a blue one, which is the reverse
spectrum of the yellow sun, or the direct spectrum of the blue sky, or a
combination of both. And this is again transformed into a yellow one on
closing the eyes, and so reciprocally, as quick as the motions of the
opening and closing eyelids. Hence, when Mr. Melvill observed the
scintillations of the star Sirius to be sometimes coloured, these were
probably the direct spectrum of the blue sky on the parts of the retina
fatigued by the white light of the star. (Essays Physical and Literary, p.
81. V. 2.)
When a direct spectrum is thrown on colours darker than itself, it mixes
with them; as the yellow spectrum of the setting sun, thrown on the green
grass, becomes a greener yellow. But when a direct spectrum is thrown on
colours brighter than itself, it becomes instantly changed into the reverse
spectrum, which mixes with those brighter colours. So the yellow spectrum
of the setting sun thrown on the luminous sky becomes blue, and changes
with the colour or brightness of the clouds on which it appears. But the
reverse spectrum mixes with every kind of colour on which it is thrown,
whether brighter than itself or not; thus the reverse spectrum, obtained by
viewing a piece of yellow silk, when thrown on white paper, was a lucid
blue green; when thrown on black Turkey leather, becomes a deep violet. And
the spectrum of blue silk, thrown on white paper, was a light yellow; on
black silk was an obscure orange; and, the blue spectrum, obtained from
orange-coloured silk, thrown on yellow, became a green.
In these cases the retina is thrown into activity or sensation by the
stimulus of external colours, at the same time that it continues the
activity or sensation which forms the spectra; in the same manner as the
prismatic colours, painted on a whirling top, are seen to mix together.
When these colours of external objects are brighter than the direct
spectrum which is thrown upon them, they change it into the reverse
spectrum, like the admission of external light on a direct spectrum, as
explained above. When they are darker than the direct spectrum, they mix
with it, their weaker stimulus being inefficient to induce the reverse
spectrum.
3. _Variation of spectra in respect to number, and figure, and remission._
[Illustration: Fig. 4.]
When we look long and attentively at any object, the eye cannot always be
kept entirely motionless; hence, on inspecting a circular area of red silk
placed on white paper, a lucid crescent or edge is seen to librate on one
side or other of the red circle: for the exterior parts of the retina
sometimes falling on the edge of the central silk, and sometimes on the
white paper, are less fatigued with red light than the central part of the
retina, which is constantly, exposed to it; and therefore, when they fall
on the edge of the red silk, they perceive it more vividly. Afterwards,
when the eye becomes fatigued, a green spectrum in the form of a crescent
is seen to librate on one side or other of the central circle, as by the
unsteadiness of the eye a part of the fatigued retina falls on the white
paper; and as by the increasing fatigue of the eye the central part of the
silk appears paler, the edge on which the unfatigued part of the retina
occasionally falls will appear of a deeper red than the original silk,
because it is compared with the pale internal part of it. M. de Buffon in
making this experiment observed, that the red edge of the silk was not only
deeper coloured than the original silk; but, on his retreating a little
from it, it became oblong, and at length divided into two, which must have
been owing to his observing it either before or behind the point of
intersection of the two optic axises. Thus, if a pen is held up before a
distant candle, when we look intensely at the pen two candles are seen
behind it; when we look intensely at the candle two pens are seen. If the
sight be unsteady at the time of beholding the sun, even though one eye
only be used, many images of the sun will appear, or luminous lines, when
the eye is closed. And as some parts of these will be more vivid than
others, and some parts of them will be produced nearer the center of the
eye than others, these will disappear sooner than the others; and hence the
number and shape of these spectra of the sun will continually vary, as long
as they exist. The cause of some being more vivid than others, is the
unsteadiness of the eye of the beholder, so that some parts of the retina
have been longer exposed to the sunbeams. That some parts of a complicated
spectrum fade and return before other parts of it, the following experiment
evinces. Draw three concentric circles; the external one an inch and a half
in diameter, the middle one an inch, and the internal one half an inch;
colour the external and internal areas blue, and the remaining one yellow,
as in Fig. 4.; after having looked about a minute on the center of these
circles, in a bright light, the spectrum of the external area appears first
in the closed eye, then the middle area, and lastly the central one; and
then the central one disappears, and the others in inverted order. If
concentric circles of more colours are added, it produces the beautiful
ever changing spectrum in Sect. I. Exp. 2.
From hence it would seem, that the center of the eye produces quicker
remissions of spectra, owing perhaps to its greater sensibility; that is,
to its more energetic exertions. These remissions of spectra bear some
analogy to the tremors of the hands, and palpitations of the heart, of weak
people: and perhaps a criterion of the strength of any muscle or nerve may
be taken from the time it can be continued in exertion.
4. _Variation of spectra in respect to brilliancy; the visibility of the
circulation of the blood in the eye._
1. The meridian or evening light makes a difference in the colours of some
spectra; for as the sun descends, the red rays, which are less refrangible
by the convex atmosphere, abound in great quantity. Whence the spectrum of
the light parts of a window at this time, or early in the morning, is red;
and becomes blue either a little later or earlier; and white in the
meridian day; and is also variable from the colour of the clouds or sky
which are opposed to the window.
2. All these experiments are liable to be confounded, if they are made too
soon after each other, as the remaining spectrum will mix with the new
ones. This is a very troublesome circumstance to painters, who are obliged
to look long upon the same colour; and in particular to those whose eyes,
from natural debility, cannot long, continue the same kind of exertion. For
the same reason, in making these experiments, the result becomes much
varied if the eyes, after viewing any object, are removed on other objects
for but an instant of time, before we close them to view the spectrum; for
the light from the object, of which we had only a transient view, in the
very time of closing our eyes acts as a stimulus on the fatigued retina;
and for a time prevents the defined spectrum from appearing, or mixes its
own spectrum with it. Whence, after the eyelids are closed, either a dark
field, or some unexpected colours, are beheld for a few seconds, before the
desired spectrum becomes distinctly visible.
3. The length of time taken up in viewing an object, of which we are to
observe the spectrum, makes a great difference in the appearance of the
spectrum, not only in its vivacity, but in its colour; as the direct
spectrum of the central object, or of the circumjacent ones, and also the
reverse spectra of both, with their various combinations, as well as the
time of their duration in the eye, and of their remissions or alternations,
depend upon the degree of fatigue the retina is subjected to. The Chevalier
d'Arcy constructed a machine by which a coal of fire was whirled round in
the dark, and found, that when a luminous body made a revolution in eight
thirds of time, it presented to the eye a complete circle of fire; from
whence he concludes, that the impression continues on the organ about the
seventh part of a second. (Mem. de l'Acad. des Sc. 1765.) This, however, is
only to be considered as the shortest time of the duration of these direct
spectra; since in the fatigued eye both the direct and reverse spectra,
with their intermissions, appear to take up many seconds of time, and seem
very variable in proportion to the circumstances of fatigue or energy.
4. It sometimes happens, if the eyeballs have been rubbed hard with the
fingers, that lucid sparks are seen in quick motion amidst the spectrum we
are attending to. This is similar to the flashes of fire from a stroke on
the eye in fighting, and is resembled by the warmth and glow, which appears
upon the skin after friction, and is probably owing to an acceleration of
the arterial blood into the vessels emptied by the previous pressure. By
being accustomed to observe such small sensations in the eye, it is easy to
see the circulation of the blood in this organ. I have attended to this
frequently, when I have observed my eyes more than commonly sensible to
other spectra. The circulation may be seen either in both eyes at a time,
or only in one of them; for as a certain quantity of light is necessary to
produce this curious phenomenon, if one hand be brought nearer the closed
eyelids than the other, the circulation in that eye will for a time
disappear. For the easier viewing the circulation, it is sometimes
necessary to rub the eyes with a certain degree of force after they are
closed, and to hold the breath rather longer than is agreeable, which, by
accumulating more blood in the eye, facilitates the experiment; but in
general it may be seen distinctly after having examined other spectra with
your back to the light, till the eyes become weary; then having covered
your closed eyelids for half a minute, till the spectrum is faded away
which you were examining, turn your face to the light, and removing your
hands from the eyelids, by and by again shade them a little, and the
circulation becomes curiously distinct. The streams of blood are however
generally seen to unite, which shews it to be the venous circulation,
owing, I suppose, to the greater opacity of the colour of the blood in
these vessels; for this venous circulation is also much more easily seen by
the microscope in the tail of a tadpole.
5. _Variation of spectra in respect to distinctness and size; with a new
way of magnifying objects._
1. It was before observed, that when the two colours viewed together were
opposite to each other, as yellow and blue, red and green, &c. according to
the table of reflections and transmissions of light in Sir Isaac Newton's
Optics, B. II. Fig. 3. the spectra of those colours were of all others the
most brilliant, and best defined; because they were combined of the reverse
spectrum of one colour, and of the direct spectrum of the other. Hence, in
books printed with small types, or in the minute graduation of
thermometers, or of clock-faces, which are to be seen at a distance, if the
letters or figures are coloured with orange, and the ground with indigo; or
the letters with red, and the ground with green; or any other lucid colour
is used for the letters, the spectrum of which is similar to the colour of
the ground; such letters will be seen much more distinctly, and with less
confusion, than in black or white: for as the spectrum of the letter is the
same colour with the ground on which they are seen, the unsteadiness of the
eye in long attending to them will not produce coloured lines by the edges
of the letters, which is the principal cause of their confusion. The beauty
of colours lying in vicinity to each other, whose spectra are thus
reciprocally similar to each colour, is owing to this greater ease that the
eye experiences in beholding them distinctly; and it is probable, in the
organ of hearing, a similar circumstance may constitute the pleasure of
melody. Sir Isaac Newton observes, that gold and indigo were agreeable when
viewed together; and thinks there may be some analogy between the
sensations of light and sound. (Optics, Qu. 14.)
In viewing the spectra of bright objects, as of an area of red silk of half
an inch diameter on white paper, it is easy to magnify it to tenfold its
size: for if, when the spectrum is formed, you still keep your eye fixed on
the silk area, and remove it a few inches further from you, a green circle
is seen round the red silk: for the angle now subtended by the silk is less
than it was when the spectrum was formed, but that of the spectrum
continues the same, and our imagination places them at the same distance.
Thus when you view a spectrum on a sheet of white paper, if you approach
the paper to the eye, you may diminish it to a point; and if the paper is
made to recede from the eye, the spectrum will appear magnified in
proportion to the distance.
[Illustration: Fig. 5.]
I was surprised, and agreeably amused, with the following experiment. I
covered a paper about four inches square with yellow, and with a pen filled
with a blue colour wrote upon the middle of it the word BANKS in capitals,
as in Fig. 5, and sitting with my back to the sun, fixed my eyes for a
minute exactly on the center of the letter N in the middle of the word;
after closing my eyes, and shading them somewhat with my hand, the word was
distinctly seen in the spectrum in yellow letters on a blue field; and
then, on opening my eyes on a yellowish wall at twenty feet distance, the
magnified name of BANKS appeared written on the wall in golden characters.
_Conclusion._
It was observed by the learned M. Sauvage (Nosol. Method. Cl. VIII. Ord.
i.) that the pulsations of the optic artery might be perceived by looking
attentively on a white wall well illuminated. A kind of net-work, darker
than the other parts of the wall, appears and vanishes alternately with
every pulsation. This change of the colour of the wall he well ascribes to
the compression of the retina by the diastole of the artery. The various
colours produced in the eye by the pressure of the finger, or by a stroke
on it, as mentioned by Sir Isaac Newton, seem likewise to originate from
the unequal pressure on various parts of the retina. Now as Sir Isaac
Newton has shewn, that all the different colours are reflected or
transmitted by the laminæ of soap bubbles, or of air, according to their
different thickness or thinness, is it not probable, that the effect of the
activity of the retina may be to alter its thickness or thinness, so as
better to adapt it to reflect or transmit the colours which stimulate it
into action? May not muscular fibres exist in the retina for this purpose,
which may be less minute than the locomotive muscles of microscopic
animals? May not these muscular actions of the retina constitute the
sensation of light and colours; and the voluntary repetitions of them, when
the object is withdrawn, constitute our memory of them? And lastly, may not
the laws of the sensations of light, here investigated, be applicable to
all our other senses, and much contribute to elucidate many phenomena of
animal bodies both in their healthy and diseased state; and thus render
this investigation well worthy the attention of the physician, the
metaphysician, and the natural philosopher?
November 1, 1785.
* * * * *
Dum, Liber! astra petis volitans trepidantibus alis,
Irruis immemori, parvula gutta, mari.
Me quoque, me currente rotâ revolubilis ætas
Volverit in tenebras,--i, Liber, ipse sequor.
* * * * *
INDEX TO THE SECTIONS OF PART FIRST.
A.
Abortion from fear, xxxix. 6. 5.
Absorbent vessels, xxiii. 3. xxix. 1.
---- regurgitate their fluids, xxix. 2.
---- their valves, xxix. 2.
---- communicate with vena portarum, xxvii. 2.
Absorption of solids, xxxiii. 3. 1. xxxvii.
---- of fluids in anasarca, xxxv. 1. 3.
Accumulation of sensorial power, iv. 2. xii. 5. 2.
Activity of system too great, cure of, xii. 6.
---- too small, cure of, xii. 7.
Age, old, xii. 3. 1. xxxvii. 4.
Ague-fit, xii. 7. 1. xxxii. 3. 4. xxxii. 9.
---- how cured by bark, xii. 3. 4.
---- periods, how occasioned, xii. 2. 3. xxxii. 3. 4.
Ague cakes, xxxii. 7. xxxii. 9.
Air, sense of fresh, xiv. 8.
---- injures ulcers, xxviii. 2.
---- injected into veins, xxxii. 5.
Alcohol deleterious, xxx. 3.
Alliterations, why agreeable, xxii. 2.
Aloes in lessened doses, xii. 3. 1.
American natives indolent, xxxi. 2.
---- narrow shouldered, xxxi. 1.
Analogy intuitive, xvii. 3. 7.
Animals less liable to madness, xxxiii. 1.
---- less liable to contagion, xxxiii. 1.
---- how to teach, xxii. 3. 2.
---- their similarity to each other, xxxix. 4. 8.
---- their changes after nativity, xxxix. 4. 8.
---- their changes before nativity, xxxix. 4. 8.
---- less liable to contagious diseases, why, xxxiii. 1. 5.
---- less liable to delirium and insanity, why, xxxiii. 1. 5.
---- easier to preserve than to reproduce, xxxvii.
---- food, distaste of, xxviii. 1.
---- appetency, xxxix. 4. 7.
Antipathy, x. 2. 2.
Aphthæ, xxviii.
Apoplexy, xxxiv. 1. 7.
---- not from deficient irritation, xxxii. 2. 1.
Appetites, xi. 2. 2. xiv. 8.
Architecture, xxii. 2. xvi. 10.
Arts, fine, xxii. 2.
Asparagus, its smell in urine, xxix.
Association defined, ii. 2. 11. iv. 7. v. 2.
---- associate motions, x.
---- stronger than irritative ones, xxiv. 2. 8.
---- formed before nativity, xi. 3.
---- with irritative ones, xxiv. 2. 8.
---- with retrograde ones, xxv. 7. xxv. 10. xxv. 15.
---- diseases from, xxxv.
Asthma, xviii. 15.
Attention, language of, xvi. 8. 6.
Atrophy, xxviii.
Aversion, origin of, xi. 2. 3.
B.
Balance ourselves by vision, xx. 1.
Bandage increases absorption, xxxiii. 3. 2.
Barrenness, xxxvi. 2. 3.
Battement of sounds, xx. 7.
Bath, cold. See Cold Bath.
Beauty, sense of, xvi. 6. xxii. 2.
Bile-ducts, xxx.
---- stones, xxx. 1. 3.
---- regurgitates into the blood, xxiv. 2. 7.
---- vomiting of, xxx. 1. 3.
Birds of passage, xvi. 12.
---- nests of, xvi. 13.
---- colour of their eggs, xxxix. 5.
Biting in pain, xxxiv. 1. 3.
---- of mad animals, xxxiv. 1. 3.
Black spots on dice appear red, xl. 3.
Bladder, communication of with the intestines, xxix. 3.
---- of fish, xxiv. 1. 4.
Blood, transfusion of in nervous fevers, xxxii. 4.
---- deficiency of, xxxii. 2. and 4.
---- from the vena portarum into the intestines, xxvii. 2.
---- its momentum, xxxii. 5. 2.
---- momentum increased by venesection, xxxii. 5. 4.
---- drawn in nervous pains, xxxii. 5. 4.
---- its oxygenation, xxxviii.
Breasts of men, xiv. 8.
Breathing, how learnt, xvi. 4.
Brutes differ from men, xi. 2. 3. xvi. 17.
Brutes. See Animals.
Buxton bath, why it feels warm, xii. 2. 1. xxxii. 3. 3.
C.
Capillary vessels are glands, xxvi. 1.
Catalepsy, xxxiv. 1. 5.
Catarrh from cold skin, xxxv. 1. 3. xxxv. 2. 3.
---- from thin caps in sleep, xviii. 15.
Catenation of motions defined, ii. 2. 11. iv. 7.
---- cause of them, xvii. 1. 3.
---- described, xvii.
---- continue some time after their production, xvii. 1. 3.
---- voluntary ones dissevered in sleep, xvii. 1. 12. xvii. 3. 7.
Cathartics, external, their operation, xxix. 7. 6.
Causation, animal, defined, ii. 2. 11. iv. 7.
Cause of causes, xxxix. 4. 8.
Causes inert and efficient, xxxix. 8. 2.
---- active and passive, xxxix. 8. 3.
---- proximate and remote, xxxix. 8. 4.
Chick in the egg, oxygenation of, xxxviii. 2.
Child riding on a stick, xxxiv. 2. 6.
Chilness after meals, xxi. 3. xxxv. 1. 1.
Cholera, case of, xxv. 13.
Circulation in the eye visible, xl. 10. 4.
Cold in the head, xii. 6. 5.
---- perceived by the teeth, xxxii. 3. 1. xiv. 6.
---- air, uses of in fevers, xxxii. 3. 3.
---- feet, produces coryza, xxxv. 2. 3. xxxv. 1. 3.
---- bath, why it strengthens, xxxii. 3. 2.
---- short and cold breathing in it, xxxii. 3. 2.
---- produces a fever-fit, xxxii. 3. 2.
---- fit of fever the consequence of hot fit, xxxii. 9. 3.
---- bathing in pulmonary hæmorrhage, xxvii. 1.
---- fits of fever, xxxii. 4. xxxii. 9. xvii. 3. 3.
Colours of animals, efficient cause of, xxxix. 5. 1.
---- of eggs from female imagination, xxxix. 5. 1.
---- of the choroid coat of the eye, xxxix. 5. 1.
---- of birds nests, xvi. 13.
Comparing ideas, xv. 3.
Consciousness, xv. 3. 4.
---- in dreams, xviii. 13.
Consent of parts. See Sympathy.
Consumption, its temperament, xxxi. 1. and 2.
---- of dark-eyed patients, xxvii. 2.
---- of light-eyed patients, xxviii. 2.
---- is contagious, xxxiii. 2. 7.
Contagion, xii. 3. 6. xix. 9. xxxiii. 2. 6. and 8. xxii. 3. 3.
---- does not enter the blood, xxxiii. 2. 10. xxii. 3. 3.
Contraction and attraction, iv. 1.
---- of fibres produces sensation, iv. 5. xii. 1. 6.
---- continues some time, xii. 1. 5.
---- alternates with relaxation, xii. 1. 3.
Convulsion, xvii. 1. 8. xxxiv. 1. 1. and 4. iii. 5. 8.
---- of particular muscles, xvii. 1. 8.
---- periods of, xxxvi. 3. 9.
Coryza. See Catarrh.
Cough, nervous, periods of, xxxvi. 3. 9.
Cramp, xviii. 15. xxxiv. 1. 7.
Critical days from lunations, xxxvi. 4.
D.
Darkish room, why we see well in it, xii. 2. 1.
Debility sensorial and stimulatory, xii. 2. 1.
---- direct and indirect of Dr. Brown, xii. 2. 1. xxxii. 3. 2.
---- See Weakness.
---- from drinking spirits, cure of, xii. 7. 8.
---- in fevers, cure of, xii. 7. 8.
Deliberation, what, xxxiv. 1.
Delirium, two kinds of, xxxiii. 1. 4. xxxiv. 2. 2.
---- cases of, iii. 5. 8.
---- prevented by dreams, xviii. 2.
Desire, origin of, xi. 2. 3.
Diabetes explained, xxix. 4.
---- with bloody urine, xxvii. 2.
---- in the night, xviii. 15.
Diarrhoea, xxix. 4.
Digestion, xxxiii. 1. xxxvii.
---- strengthened by emetics, xxxv. 1. 3.
---- strengthened by regular hours, why, xxxvi. 2. 1.
Digitalis, use of in dropsy, xxix. 5. 2.
Distention acts as a stimulus, xxxii. 4.
---- See Extension.
Distinguishing, xv. 3.
Diurnal circle of actions, xxv. 4.
Doubting, xv. 3.
Dreams, viii. 1. 2. xiv. 2. 5.
---- their inconsistency, xviii. 17.
---- no surprise in them, xviii. 17.
---- much novelty of combination, xviii. 9.
Dropsies explained, xxix. 5. 1.
Dropsy cured by insanity, xxxiv. 2. 7.
---- cure of, xxix. 5. 2.
Drunkards weak till next day, xvii. 1. 7.
---- stammer, and stagger, and weep, xii. 4. 1. xxi. 4.
---- see objects double, why, xxi. 7.
---- become delirious, sleepy, stupid, xxi. 5.
Drunkenness. See Intoxication, xxi.
---- diminished by attention, xxi. 8.
Dyspnoea in cold bath, xxxii. 3. 2.
E.
Ear, a good one, xvi. 10.
---- noise in, xx. 7.
Eggs of frogs, fish, fowl, xxxix. 2.
---- of birds, why spotted, xxxix. 5.
---- with double yolk, xxxix. 4. 4.
Electricity, xii. 1. xiv. 9.
---- jaundice cured by it, xxx. 1. 2.
Embryon produced by the male, xxxix. 2.
---- consists of a living fibre, xxxix. 4.
---- absorbs nutriment, receives oxygen, xxxix. 1.
---- its actions and sensations, xvi. 2.
Emetic. See Vomiting.
Emotions, xi. 2. 2.
Ennui, or tædium vitæ, xxxiv. 2. 3. xxxiii. 1. 1. xxxix. 6.
Epileptic fits explained, xxxiv. 1. 4. xxvii. 2.
---- in sleep, why, xviii. 14. & 15.
Equinoxial lunations, xxxii. 6.
Excitability perpetually varies, xii. 1. 7.
---- synonymous to quantity of sensorial power, xii. 1. 7.
Exercise, its use, xxxii. 5. 3.
Exertion of sensorial power defined, xii. 2. 1.
Existence in space, xiv. 2. 5.
Extension, sense of, xiv. 7.
Eyes become black in some epilepsies, xxvii. 2.
F.
Face, flushing of after dinner, xxxv. 1. 1.
---- why first affected in small-pox, xxxv. 1. 1.
---- red from inflamed liver, xxxv. 2. 2.
Fainting fits, xii. 7. 1. xiv. 7.
Fear, language of, xvi. 8. 1.
---- a cause of fever, xxxii. 8.
---- cause of, xvii. 3. 7.
Fetus. See Embryon, xvi. 2. xxxix. 1.
Fevers, irritative, xxxii. 1.
---- intermittent, xxxii. 1. xxxii. 3.
---- sensitive, xxxiii. 1.
---- not an effort of nature for relief, xxxii. 10.
---- paroxysms of, xii. 7. 1. xii. 2. 3. xii. 3. 5.
---- why some intermit and not others, xxxvi. 1.
---- cold fits of, xxxii. 4. xxxii. 9. xvii. 3. 3.
---- periods of, xxxvi. 3.
---- have solar or lunar periods, xxxii. 6.
---- source of the symptoms of, xxxii. 1.
---- prostration of strength in, xii. 4. 1. xxxii. 3. 2.
---- cure of, xii. 6. 1.
---- how cured by the bark, xii. 3. 4.
---- cured by increased volition, xii. 2. 4. xxxiv. 2. 8.
---- best quantity of stimulus in, xii. 7. 8.
Fibres. See Muscles.
Fibres, their mobility, xii. 1. 7. xii. 1. 1.
---- contractions of, vi. xii. 1. 1.
---- four classes of their motions, vi.
---- their motions distinguished from sensorial ones, v. 3.
Figure, xiv. 2. 2. iii. 1.
Fish, their knowledge, xvi. 14.
Foxglove, its use in dropsies, xxix. 5. 2.
---- overdose of, xxv. 17.
Free-will, xv. 3. 7.
G.
Gall-stone, xxv. 17.
---- See Bile-stones.
Generation, xxxiii. 1. xxxix.
Gills of fish, xxxviii. 2.
Glands, xxiii. 2.
---- conglobate glands, xxiii. 3.
---- have their peculiar stimulus, xi. 1.
---- their senses, xiv. 9. xxxix. 6.
---- invert their motions, xxv. 7.
---- increase their motions, xxv. 7.
Golden rule for exhibiting wine, xii. 7. 8.
---- for leaving off wine, xii. 7. 8.
Gout from inflamed liver, xxxv. 2. 2. xviii. 16. xxiv. 2. 8.
---- in the stomach, xxiv. 2. 8. xxv. 17.
---- why it returns after evacuations, xxxii. 4.
---- owing to vinous spirit only, xxi. 10.
---- periods of, xxxvi. 3. 6.
Grinning in pain, xxxiv. 1. 3.
Gyration on one foot, xx. 5. and 6.
H.
Habit defined, ii. 2. 11. iv. 7.
Hæmorrhages, periods of, xxxvi. 3. 11.
---- from paralysis of veins, xxvii. 1. and 2.
Hair and nails, xxxix. 3. 2.
---- colour of, xxxix. 5. 1.
Harmony, xxii. 2.
Head-achs, xxxv. 2. 1.
Hearing, xiv. 4.
Heat, sense of, xiv. 6. xxxii. 3. 1.
---- produced by the glands, xxxii. 3.
---- external and internal, xxxii. 3. 1.
---- atmosphere of heat, xxxii. 3. 1.
---- increases during sleep, xviii. 15.
Hemicrania, xxxv. 2. 1.
---- from decaying teeth, xxxv. 2. 1.
Hepatitis, cause of, xxxv. 2. 3.
Hereditary diseases, xxxix. 7. 6.
Hermaphrodite insects, xxxix. 5.
Herpes, xxviii. 2.
---- from inflamed kidney, xxxv. 2. 2.
Hilarity from diurnal fever, xxxvi. 3. 1.
Hunger, sense of, xiv. 8.
Hydrophobia, xxii. 3. 3.
Hypochondriacism, xxxiii. 1. 1. xxxiv. 2. 3.
I.
Ideas defined, ii. 2. 7.
---- are motions of the organs of sense, iii. 4. xviii. 5. xviii. 10.
xviii. 6.
---- analogous to muscular motions, iii. 5.
---- continue some time, xx. 6.
---- new ones cannot be invented, iii. 6. 1.
---- abstracted ones, iii. 6. 4.
---- inconsistent trains of, xviii. 17.
---- perish with the organ of sense, iii. 4. 4.
---- painful from inflammation of the organ, iii. 5. 5.
---- irritative ones, vii. 1. 4. vii. 3. 2. xv. 2. xx. 7.
---- of resemblance, contiguity, causation, viii. 3. 2. x. 3. 3.
---- resemble the figure and other properties of bodies, xiv. 2. 2.
---- received in tribes, xv. 1.
---- of the same sense easier combined, xv. 1. 1.
---- of reflection, xv. 1. 6. ii. 2. 12.
Ideal presence, xv. 1. 7.
Identity, xv. 3. 5. xviii. 13.
Iliac passion, xxv. 15.
Imagination, viii. 1. 2. xv. 1. 7. xv. 2. 2.
---- of the male forms the sex, xxxix. 6.
Imitation, origin of, xii. 3. 3. xxxix. 5. xxii. 3. xvi. 7.
Immaterial beings, xiv. 1. xiv. 2. 4.
Impediment of speech, xvii. 1. 10. xvii. 2. 10.
Infection. See Contagion.
Inflammation, xii. 2. 3. xxxiii. 2. 2.
---- great vascular exertion in, xii. 2. 1.
---- not from pains from defect of stimulus, xxxiii. 2. 3.
---- of parts previously insensible, xii. 3. 7.
---- often distant from its cause, xxiv. 2. 8.
---- observes solar days, xxxii. 6.
---- of the eye, xxxiii. 3. 1.
---- of the bowels prevented by their continued action in sleep, xviii.
2.
Inoculation with blood, xxxiii. 2. 10.
Insane people, their great strength, xii. 2. 1.
Insanity (see Madness) pleasurable one, xxxiv. 2. 6.
Insects, their knowledge, xvi. 15. and 16.
---- in the heads of calves, xxxix. 1.
---- class of, xxxix. 4. 8.
Instinctive actions defined, xvi. 1.
Intestines, xxv. 3.
Intoxication relieves pain, why, xxi. 3.
---- from food after fatigue, xxi. 2.
---- diseases from it, xxi. 10.
---- See Drunkenness.
Intuitive analogy, xvii. 3. 7.
Invention, xv. 3. 3.
Irritability increases during sleep, xviii. 15.
Itching, xiv. 9.
J.
Jaundice from paralysis of the liver, xxx. 1. 2.
---- cured by electricity, xxx. 1. 2.
Jaw-locked, xxxiv. 1. 5.
Judgment, xv. 3.
K.
Knowledge of various animals, xvi. 11.
L.
Lachrymal sack, xvi. 8. xxiv. 2. 2. and 7.
Lacteals, paralysis of, xxviii.
---- See Absorbents.
Lady playing on the harpsichord, xvii. 2.
---- distressed for her dying bird, xvii. 2. 10.
Language, natural, its origin, xvi. 7. & 8.
---- of various passions described, xvi. 8.
---- artificial, of various animals, xvi. 9.
---- theory of, xxxix. 8. 3.
Lapping of puppies, xvi. 4.
Laughter explained, xxxiv. 1. 4.
---- from tickling, xvii. 3. 5. xxxiv. 1. 4.
---- from frivolous ideas, xxxiv. 1. 4. xviii. 12.
Life, long, art of producing, xxxvii.
Light has no momentum, iii. 3. 1.
Liquor amnii, xvi. 2. xxxviii. 2.
---- is nutritious, xxxviii. 3.
---- frozen, xxxviii. 3.
Liver, paralysis of, xxx. 1. 4.
---- large of geese, xxx. 1. 6.
Love, sentimental, its origin, xvi. 6.
---- animal, xiv. 8. xvi. 5.
Lunar periods affect diseases, xxxii. 6.
Lust, xiv. 8. xvi. 5.
Lymphatics, paralysis of, xxviii.
---- See Absorbents.
M.
Mad-dog, bite of, xxii. 3. 3.
Madness, xxxiv. 2. 1. xii. 2. 1.
Magnetism, xii. 1. 1.
Magnifying objects, new way of, xl. 10. 5.
Male animals have teats, xxxix. 4. 8.
---- pigeons give milk, xxxix. 4. 8.
Man distinguished from brutes, xi. 2. 3. xvi. 17.
Material world, xiv. 1. xiv. 2. 5. xviii. 7.
Matter, penetrability of, xiv. 2. 3.
---- purulent, xxxiii. 2. 4.
Measles, xxxiii. 2. 9.
Membranes, xxvi. 2.
Memory defined, ii. 2. 10. xv. 1. 7. xv. 3.
Menstruation by lunar periods, xxxii. 6.
Miscarriage from fear, xxxix. 6. 5.
Mobility of fibres, xii. 1. 7.
Momentum of the blood, xxxii. 5. 2.
---- sometimes increased by venesection, xxxii. 5. 4.
Monsters, xxxix. 4. 4. and 5. 2.
---- without heads, xxxviii. 3.
Moon and sun, their influence, xxxii. 6.
Mortification, xxxiii. 3. 3.
Motion is either cause or effect, i. xiv. 2. 2.
---- primary and secondary, i.
---- animal, i. iii. 1.
---- propensity to, xxii. 1.
---- animal, continue some time after their production, xvii. 1. 3.
---- defined, a variation of figure, iii. 1. xiv. 2. 2. xxxix. 8.
Mucus, experiments on, xxvi. 1.
---- secretion of, xxvi. 2.
Mules, xxxix. 4. 5. and 6. xxxix. 5. 2.
Mule plants, xxxix. 2.
Muscæ volitantes, xl. 2.
Muscles constitute an organ of sense, xiv. 7. ii. 2. 4.
---- stimulated by extension, xi. 1. xiv. 7.
---- contract by spirit of animation, xii. 1. 1. and 3.
Music, xvi. 10. xxii. 2.
Musical time, why agreeable, xii. 3. 3.
N.
Nausea, xxv. 6.
Nerves and brain, ii. 2. 3.
---- extremities of form the whole system, xxxvii. 3.
---- are not changed with age, xxxvii. 4.
Nervous pains defined, xxxiv. 1. 1.
Number defined, xiv. 2. 2.
Nutriment for the embryon, xxxix. 5. 2.
Nutrition owing to stimulus, xxxvii. 3.
---- by animal selection, xxxvii. 3.
---- when the fibres are elongated, xxxvii. 3.
---- like inflammation, xxxvii. 3.
O.
Objects long viewed become faint, iii. 3. 2.
Ocular spectra, xl.
Oil externally in diabætes, xxix. 4.
Old age from inirritability, xxxvii.
Opium is stimulant, xxxii. 2. 2.
---- promotes absorption after evacuation, xxxiii. 3. 1.
---- in increasing doses, xii. 3. 1.
Organs of sense, ii. 2. 5. and 6.
Organs when destroyed cease to produce ideas, iii. 4. 4.
Organic particles of Buffon, xxxvii. 3. xxxix. 3. 3.
Organ-pipes, xx. 7.
Oxygenation of the blood, xxxviii.
P.
Pain from excess and defect of motion, iv. 5. xii. 5. 3. xxxiv. 1. xxxv.
2. 1.
---- not felt during exertion, xxxiv. 1. 2.
---- from greater contraction of fibres, xii. 1. 6.
---- from accumulation of sensorial power, xii. 5. 3.
---- from light, pressure, heat, caustics, xiv. 9.
---- in epilepsy, xxxv. 2. 1.
---- distant from its cause, xxiv. 2. 8.
---- from stone in the bladder, xxxv. 2. 1.
---- of head and back from defect, xxxii. 3.
---- from a gall-stone, xxxv. 2. 1. xxv. 17.
---- of the stomach in gout, xxv. 17.
---- of shoulder in hepatitis, xxxv. 2. 4.
---- produces volition, iv. 6.
Paleness in cold fit, xxxii. 3. 2.
Palsies explained, xxxiv. 1. 7.
Paralytic limbs stretch from irritation, vii. 1. 3.
---- patients move their sound limb much, xii. 5. 1.
Paralysis from great exertion, xii. 4. 6.
---- from less exertion, xii. 5. 6.
---- of the lacteals, xxviii.
---- of the liver, xxx. 1. 4.
---- of the right arm, why, xxxiv. 1. 7.
---- of the veins, xxvii. 2.
Particles of matter will not approach, xii. 1. 1.
Passions, xi. 2. 2.
---- connate, xvi. 1.
Pecking of chickens, xvi. 4.
Perception defined, ii. 2. 8. xv. 3. 1.
Periods of agues, how formed, xxxii. 3. 4.
---- of diseases, xxxvi.
---- of natural actions and of diseased actions, xxxvi.
Perspiration in fever-fits, xxxii. 9. See Sweat.
Petechiæ, xxvii. 2.
Pigeons secrete milk in their stomachs, xxxix. 4. 8.
Piles, xxvii. 2.
Placenta a pulmonary organ, xxxviii. 2.
Pleasure of life, xxxiii. 1. xxxix. 5.
---- from greater fibrous contractions, xii. 1. 6.
---- what kind causes laughter, xxxiv. 1. 4.
---- what kind causes sleep, xxxiv. 1. 4.
Pleurisy, periods of, xxxvi. 3. 7.
---- cause of, xxxv. 2. 3.
Prometheus, story of, xxx. 3.
Prostration of strength in fevers, xii. 4. 1.
Pupils of the eyes large, xxxi. 1.
Pulse quick in fevers with debility, xii. 1. 4. xii. 5. 4. xxxii. 2. 1.
---- in fevers with strength, xxxii. 2.
---- from defect of blood, xxxii. 2. 3. xii. 1. 4.
---- weak from emetics, xxv. 17.
Q.
Quack advertisements injurious. Preface.
Quadrupeds have no sanguiferous lochia, xxxviii. 2.
---- have nothing similar to the yolk of egg, xxxix. 1.
R.
Rhaphania, periods of, xxxvi. 3. 9.
Reason, ix. 1. 2. xv. 3.
Reasoning, xv. 3.
Recollection, ii. 2. 10. ix. 1. 2. xv. 2. 3.
Relaxation and bracing, xxxii. 3. 2.
Repetition, why agreeable, xii. 3. 3. xxii. 2.
Respiration affected by attention, xxxvi. 2. 1.
Restlessness in fevers, xxxiv. 1. 2.
Retrograde motions, xii. 5. 5. xxv. 6. xxix. 11.
---- of the stomach, xxv. 6.
---- of the skin, xxv. 9.
---- of fluids, how distinguished, xxix. 8.
---- how caused, xxix. 11. 5.
---- vegetable motions, xxix. 9.
Retina is fibrous, iii. 2. xl. 1.
---- is active in vision, iii. 3. xl. 1.
---- excited into spasmodic motions, xl. 7.
---- is sensible during sleep, xviii. 5. xix. 8.
Reverie, xix. 1. xxxiv. 3.
---- case of a sleep-walker, xix. 2.
---- is an epileptic disease, xix. 9.
Rhymes in poetry, why agreeable, xxii. 2.
Rheumatism, three kinds of, xxvi. 3.
Rocking young children, xxi. 3.
Ruminating animals, xxv. 1.
S.
Saliva produced by mercury, xxiv. 1.
---- by food, xxiv. 1. 1.
---- by ideas, xxiv. 1. 2. and 5.
---- by disordered volition, xxiv. 1. 7.
Schirrous tumours revive, xii. 2. 2.
Screaming in pain, xxxiv. 1. 3.
Scrophula, its temperament, xxxi. 1.
---- xxviii. 2. xxxix. 4. 5.
Scurvy of the lungs, xxvii. 2.
Sea-sickness, xx. 4.
---- stopped by attention, xx. 5.
Secretion, xxxiii. 1. xxxvii.
---- increased during sleep, xviii. 16.
Seeds require oxygenation, xxxviii. 2.
Sensation defined, ii. 2. 9. v. 2. xxxix. 8. 4.
---- diseases of, xxxiii.
---- from fibrous contractions, iv. 5. xii. 1. 6.
---- in an amputated limb, iii. 6. 3.
---- affects the whole sensorium, xi. 2.
---- produces volition, iv. 6.
Sensibility increases during sleep, xviii. 15.
Sensitive motions, viii. xxxiii. 2. xxxiv. 1.
---- fevers of two kinds, xxxiii. 1. 2.
---- ideas, xv. 2. 2.
Sensorium defined, ii. 2. 1.
Senses correct one another, xviii. 7.
---- distinguished from appetites, xxxiv. 1. 1.
Sensorial power. See Spirit of Animation.
---- great expence of in the vital motions, xxxii. 3. 2.
---- two kinds of excited in sensitive fevers, xxxiii. 1. 3.
---- powers defined, v. 1.
---- motions distinguished from fibrous motions, v. 3.
---- not much, accumulated in sleep, xviii. 2.
---- powers, accumulation of, xii. 5. 1.
---- exhaustion of, xii. 4. 1.
---- wasted below natural in hot fits, xxxii. 9. 3.
---- less exertion of produces pain, xii. 5. 3.
---- less quantity of it, xii. 5. 4.
Sensual motions distinguished from muscular, ii. 2. 7.
Sex owing to the imagination of the father, xxxix. 7. 6. xxxix. 6. 3.
xxxix. 6. 7. xxxix. 5.
Shingles from inflamed kidney, xxxv. 2. 2.
Shoulders broad, xxxi. 1. xxxix. 7. 6.
Shuddering from cold, xxxiv. 1. 1. and 2.
Sight, its accuracy in men, xvi. 6.
Skin, skurf on it, xxvi. 1.
Sleep suspends volition, xviii. 1.
---- defined, xviii. 21.
---- remote causes, xviii. 20.
---- sensation continues in it, xviii. 2.
---- from food, xxi. 1.
---- from rocking, uniform sounds, xxi. 1.
---- from wine and opium, xxi. 3.
---- why it invigorates, xii. 5. 1.
---- pulse slower and fuller, xxxii. 2. 2.
---- interrupted, xxvii. 2.
---- from breathing less oxygene, xviii. 20.
---- from being whirled on a millstone, xviii. 20.
---- from application of cold, xviii. 20.
---- induced by regular hours, xxxvi. 2. 2.
Sleeping animals, xii. 2. 2.
Sleep-walkers. See Reverie, xix. 1.
Small-pox, xxxiii. 2. 6. xxxix. 6. 1.
---- eruption first on the face, why, xxxv. 1. 1. xxxiii. 2. 10.
---- the blood will not infect, xxxiii. 2. 10.
---- obeys lunations, xxxvi. 4.
Smell, xiv. 5. xvi. 5.
Smiling, origin of, xvi. 8. 4.
Solidity, xiv. 2. 1.
Somnambulation. See Reverie, xix. 1.
Space, xiv. 2. 2.
Spasm, doctrine of, xxxii. 10.
Spectra, ocular, xl.
---- mistaken for spectres, xl. 2.
---- vary from long inspection, iii. 3. 5.
Spirit of animation. See Sensorial Power.
---- of animation causes fibrous contraction, iv. 2. ii. 2. 1. xiv. 2. 4.
---- possesses solidity, figure, and other properties of matter, xiv. 2.
4.
Spirits and angels, xiv. 2. 4.
Stammering explained, xvii. 1. 10. xvii. 2. 10.
Stimulus defined, ii. 2. 13. iv. 4. xii. 2. 1.
---- of various kinds, xi. 1.
---- with lessened effect, xii. 3. 1.
---- with greater effect, xii. 3. 3.
---- ceases to produce sensation, xii. 3. 6.
Stomach and intestines, xxv.
---- inverted by great stimulus, xxv. 6.
---- its actions decreased in vomiting, xxxv. 1. 3.
---- a blow on it occasions death, xxv. 17.
Stools black, xxvii. 2.
Strangury, xxxv. 2. 1.
Sucking before nativity, xvi. 4.
Suckling children, sense of, xiv. 8.
Suggestion defined, ii. 2. 10. xv. 2. 4.
Sun and moon, their influence, xxxii. 6.
Surprise, xvii. 3. 7. xviii. 17.
Suspicion attends madness, xxxiv. 2. 4.
Swallowing, act of, xxv. 1. xvi. 4.
Sweat, cold, xxv. 9. xxix. 6.
---- in hot fit of fever, xxxii. 9.
---- in a morning, why, xviii. 15.
Sweaty hands cured by lime, xxix. 4. 9.
Swinging and rocking, why agreeable, xxi. 3.
Sympathy, xxxv. 1.
Syncope, xii. 7. 1. xxxiv. 1. 6.
T.
Tædium vitæ. See Ennui.
Tape-worm, xxxix. 2. 3.
Taste, sense of, xiv. 5.
Tears, secretion of, xxiv.
---- from grief, xvi. 8. 2.
---- from tender pleasure, xvi. 8. 3.
---- from stimulus of nasal duct, xvi. 8. xxiv. 2. 4.
---- by volition, xxiv. 2. 6.
Teeth decaying cause headachs, xxxv. 2. 1.
Temperaments, xxxi.
Theory of medicine, wanted. Preface.
Thirst, sense of, xiv. 8.
---- why in dropsies, xxix. 5.
Tickle themselves, children cannot, xvii. 3. 5.
Tickling, xiv. 9.
Time, xiv. 2. 2. xviii. 12.
---- lapse of, xv. 3. 6.
---- poetic and musical, why agreeable, xxii. 2.
---- dramatic, xviii. 12.
Tooth-edge, xvi. 10. iii. 4. 3. xxii. 3. 3.
Touch, sense of, xiv. 2. 1.
---- liable to vertigo, xxi. 9.
---- of various animals, xvi. 6.
Trains of motions inverted, xii. 5. 5.
Transfusion of blood in nervous fever, xxxii. 4.
Translations of matter, xxix. 7.
Typhus, best quantity of stimulus in, xii. 7. 8.
---- periods of observe lunar days, xxxii. 6.
U.
Ulcers, art of healing, xxxiii. 3. 2.
---- of the lungs, why difficult to heal, xxviii. 2.
Uniformity in the fine arts, why agreeable, xxii. 2.
Urine pale in intoxication, xxi. 6.
---- paucity of in anasarca, why, xxix. 5.
---- its passage from intestines to bladder, xxix. 3.
---- copious during sleep, xviii. 15.
V.
Variation, perpetual, of irritability, xii. 2. 1.
Vegetable buds are inferior animals, xiii. 1.
---- exactly resemble their parents, xxxix.
---- possess sensation and volition, xiii. 2.
---- have associate and retrograde motions, xiii. 4. xxix. 9.
---- their anthers and stigmas are alive, xiii. 5.
---- have organs of sense and ideas, xiii. 5.
---- contend for light and air, xxxix. 4. 8.
---- duplicature of their flowers, xxxix. 4. 4.
Veins are absorbents, xxvii. 1.
---- paralysis of, xxvii. 1.
Venereal orgasm of brutes, xxxii. 6.
Venesection in nervous pains, xxxii. 5. 4.
Verbs of three kinds, xv. 3. 4.
Verses, their measure, xxii. 2.
Vertigo, xx.
---- defined, xx. 11.
---- in looking from a tower, xx. 1.
---- in a ship at sea, xx. 4.
---- of all the senses, xxi. 9.
---- by intoxication, xxxv. 1. 2.
Vibratory motions perceived after sailing, xx. 5. xx. 10.
Vinegar makes the lips pale, xxvii. 1.
Vis medicatrix of nature, xxxix. 4. 7.
Vision, sense of, xiv. 3.
Volition defined, v. 2. xxxiv. 1.
---- affects the whole sensorium, xi. 2.
---- diseases of, xxxiv.
Voluntarity, xi. 2. 4.
Voluntary motions, ix. xxxiv. 1.
Voluntary ideas, xv. 2. 3.
---- criterion of, xi. 2. 3. xxxiv. 1.
Vomiting from vertigo, xx. 8.
---- from drunkenness, xx. 8. xxi. 6.
---- by intervals, xxv. 8.
---- by voluntary efforts, xxv. 6.
---- of two kinds, xxxv. 1. 3.
---- in cold fit of fever, xxxii. 9. 1.
---- stopped by quicksilver, xxv. 16.
---- weakens the pulse, xxv. 17.
W.
Waking, how, xviii. 14.
Walking, how learnt, xvi. 3.
Warmth in sleep, why, xviii. 15.
Weakness defined, xii. 1. 3. xii. 2. 1. xxxii. 3. 2.
---- cure of, xii. 7. 8.
---- See Debility.
Wit producing laughter, xxxiv. 1. 4.
World generated, xxxix. 4. 8.
* * * * *
END OF THE FIRST VOLUME.