_The minor Discoveries and Inventions of Newton—His Researches on
Heat—On Fire and Flame—On Elective Attraction—On the Structure of
Bodies—His supposed Attachment to Alchymy—His Hypothesis respecting
Ether as the Cause of Light and Gravity—On the Excitation of
Electricity in Glass—His Reflecting Sextant invented before 1700—
His Reflecting Microscope—His Prismatic Reflector as a Substitute
for the small Speculum of Reflecting Telescopes—His Method of
varying the Magnifying Power of Newtonian Telescopes—His Experiments
on Impressions on the Retina._
In the preceding chapters we have given an account of the principal
labours of Sir Isaac Newton; but there still remain to be noticed
several of his minor discoveries and inventions, which could not
properly be introduced under any general head.
The most important of these, perhaps, are his chymical researches,
which he seems to have pursued with more or less diligence from the
time when he first witnessed the practical operations of chymistry
during his residence at the apothecary’s at Grantham. His first
chymical experiments were probably made on the alloys of metals, for
the purpose of obtaining a good metallic composition for the specula
of reflecting telescopes. In his paper on thin plates he treats of the
combinations of solids and fluids; but he enters more largely on these
and other subjects in the queries published at the end of his Optics.
One of his most important chymical papers is his _Tabula quantitatum
et graduum caloris_, which was published in the Philosophical
Transactions. This short paper contains a comparative scale of
temperature from that of melting ice to that of a small kitchen
coal-fire. The following are the principal points of the scale, the
intermediate degrees of heat having been determined with great care.
Degrees Equal Parts
of Heat. of Heat.
0 0 Freezing point of water.
1 12 Blood-heat.
2 24 Heat of melting wax.
3 48 Melting point of equal parts of tin and
bismuth.
4 96 Melting point of lead.
5 192 Heat of a small coal-fire.
The first column of this table contains the degrees of heat
in arithmetical progression, and the second in geometrical
progression,—the second degree being twice as great as the first, and
so on. It is obvious from this table, that the heat at which equal
parts of tin and bismuth melt is _four_ times greater than that of
blood-heat, the heat of melting lead _eight_ times greater, and the
heat of a small coal-fire _sixteen_ times greater.
This table was constructed by the help of a thermometer, and of red-hot
iron. By the former he measured all heats as far as that of melting
tin; and by the latter he measured all the higher heats. For the heat
which heated iron loses in a given time is as the total heat of the
iron; and therefore, if the times of cooling are taken equal, the heats
will be in a geometrical progression, and may therefore be easily found
by a table of logarithms.
He found by a thermometer constructed with linseed oil, that if the
oil, when the thermometer was placed in melting snow, occupied a space
of 1000 parts, the same oil, rarefied with _one_ degree of heat, or
that of the human body, occupied a space of 10256; in the heat of water
beginning to boil, a space of 10705; in the heat of water boiling
violently, 10725; in the heat of melted tin beginning to cool, and
putting on the consistency of an amalgam, 11516, and when the tin had
become solid, 11496. Hence the oil was rarefied in the ratio of 40 to
39 by the heat of the human body; of 15 to 14 by the heat of boiling
water; of 15 to 13 in the heat of melting tin beginning to solidify;
and of 23 to 20 in the same tin when solid. The rarefaction of air
was, with the same heat, _ten_ times greater than that of oil, and
the rarefaction of oil _fifteen_ times greater than that of spirit of
wine. By making the heats of oil proportional to its rarefaction, and
by calling the heat of the human body 12 parts, we obtain the heat of
water beginning to boil, 33; of water boiling violently, 34; of melted
tin beginning to solidify, 72; and of the same become solid, 70.
Sir Isaac then heated a sufficiently thick piece of iron till it was
red-hot; and having fixed it in a cold place, where the wind blew
uniformly, he put upon it small pieces of different metals and other
fusible bodies, and noted the times of cooling, till all the particles,
having lost their fluidity, grew cold, and the heat of the iron was
equal to that of the human body. Then, by assuming that the excesses of
the heats of the iron and of the solidified particles of metal above
the heat of the atmosphere, were in geometrical progression when the
times were in arithmetical progression, all the heats were obtained.
The iron was placed in a current of air, in order that the air heated
by the iron might always be carried away by the wind, and that cold air
might replace it with a uniform motion; for thus equal parts of the
air were heated in equal times, and received a heat proportional to
that of the iron. But the heats thus found had the same ratio to one
another with the heats found by the thermometer; and hence he was right
in assuming that the rarefactions of the oil were proportional to its
heats.
Another short chymical paper by Sir Isaac Newton has been published by
Dr. Horsley. It is entitled _De Natura Acidorum_, but is principally
occupied with a number of brief opinions on chymical subjects. This
paper was written later than 1687, as it bears a reference to the
Principia; and the most important facts which it contains seem to have
been more distinctly reproduced in the queries at the end of the Optics.
The most important of these queries relate to fire, flame, and electric
attractions, and as they were revised in the year 1716 and 1717, they
may be regarded as containing the most matured opinions of their
author. Fire he regards as a body heated so hot as to emit light
copiously, and flame as a vapour, fume, or exhalation heated so hot
as to shine. In his long query on elective attractions, he considers
the small particles of bodies as acting upon one another at distances
so minute as to escape observation. When salt of tartar deliquesces,
he supposes that this arises from an attraction between the saline
particles and the aqueous particles held in solution in the atmosphere,
and to the same attraction he ascribes it that the water will not
distil from the salt of tartar without great heat. For the same reason
sulphuric acid attracts water powerfully, and parts with it with great
difficulty. When this attractive force becomes very powerful, as in the
union between sulphuric acid and water, so as to make the particles
“coalesce with violence,” and rush towards one another with an
accelerated motion, heat is produced by the mixture of the two fluids.
In like manner, he explains the production of flame from the mixture
of cold fluids,—the action of fulminating powders,—the combination of
iron filings with sulphur,—and all the other chymical phenomena of
precipitation, combination, solution, and crystallization, and the
mechanical phenomena of cohesion and capillary attraction. He ascribes
hot springs, volcanoes, fire-damps, mineral coruscations, earthquakes,
hot suffocating exhalations, hurricanes, lightning, thunder, fiery
meteors, subterraneous explosions, land-slips, ebullitions of the sea,
and waterspouts, to sulphureous steams abounding in the bowels of the
earth, and fermenting with minerals, or escaping into the atmosphere,
where they ferment with acid vapours fitted to promote fermentation.
In explaining the structure of solid bodies, he is of opinion,
“that the smallest particles of matter may cohere by the strongest
attractions, and compose bigger particles of weaker virtue; and many
of these may cohere and compose bigger particles whose virtue is still
weaker; and so on for divers successions, until the progression end
in the biggest particles, on which the operations in chymistry and
the colours of natural bodies depend, and which, by adhering, compose
bodies of a sensible magnitude. If the body is compact, and bends or
yields inward to pression, without any sliding of its parts, it is
hard and elastic, returning to its figure with a force rising from the
mutual attraction of its parts. If the parts slide upon one another,
the body is malleable or soft. If they slip easily, and are of a fit
size to be agitated by heat, and the heat is big enough to keep them
in agitation, the body is fluid; and if it be apt to stick to things,
it is humid; and the drops of every fluid affect a round figure, by
the mutual attraction of their parts, as the globe of the earth and
sea affects a round figure, by the mutual attraction of its parts, by
gravity.”
Sir Isaac then supposes, that, as the attractive force of bodies can
reach but to a small distance from them, “a repulsive virtue ought
to succeed;” and he considers such a virtue as following from the
reflection of the rays of light, the rays being repelled without the
immediate contact of the reflecting body, and also from the emission of
light, the ray, as soon as it is shaken off from a shining body by the
vibrating motion of the parts of the body, getting beyond the reach of
attraction, and being driven away with exceeding great velocity by the
force of reflection.[113]
Many of the chymical views which Sir Isaac thus published in the form
of queries were in his own lifetime illustrated and confirmed by Dr.
Stephen Hales, in his book on _Vegetable Statics_,—a work of great
originality, which contains the germ of some of the finest discoveries
in modern chymistry.
Although there is no reason to suppose that Sir Isaac Newton was
a believer in the doctrines of alchymy, yet we are informed by
the Reverend Mr. Law that he had been a diligent student of Jacob
Behmen’s writings, and that there were found among his papers copious
abstracts from them in his own handwriting.[114] He states also that
Sir Isaac, together with one Dr. Newton, his relation, had, in the
earlier part of his life, set up furnaces, and were for several months
at work in quest of the philosopher’s tincture. These statements may
receive some confirmation from the fact, that there exist among the
Portsmouth papers many sheets, in Sir Isaac’s own writing, of Flammel’s
Explication of Hieroglyphic Figures, and in another hand, many sheets
of William Yworth’s _Processus Mysterii Magni Philosophicus_, and also
from the manner in which Sir Isaac requests Mr. Aston to inquire after
one Borry in Holland, who always went clothed in green, and who was
said to possess valuable secrets; but Mr. Law has weakened the force of
his own testimony, when he asserts that Newton borrowed the doctrine
of attraction from Behmen’s first three propositions of eternal nature.
On the 7th December, 1675, Sir Isaac Newton communicated to the
Royal Society a paper entitled _An hypothesis explaining properties
of light_, in which he, for the first time, introduces his opinions
respecting ether, and employs them to explain the nature of light, and
the cause of gravity. “He was induced,” he says, “to do this, because
he had observed the heads of some great virtuosos to run much upon
hypotheses, and he therefore gave one which he was inclined to consider
as the most probable, if he were obliged to adopt one.”[115]
This hypothesis seems to have been afterward a subject of discussion
between him and Mr. Boyle, to whom he promised to communicate his
opinion more fully in writing. He accordingly addressed to him a long
letter, dated February 28th, 1678–9, in which he explains his views
respecting ether, and employs them to account for the refraction of
light,—the cohesion of two polished pieces of metal in an exhausted
receiver,—the adhesion of quicksilver to glass tubes,—the cohesion
of the parts of all bodies,—the cause of filtration,—the phenomena
of capillary attraction,—the action of menstrua on bodies,—the
transmutation of gross compact substances into aerial ones,—and the
cause of gravity. From the language used in this paper, we should
be led to suppose that Sir Isaac had entirely forgotten that he had
formerly treated the general subject of ether, and applied it to the
explanation of gravity. “I shall set down,” says he, “one conjecture
more _which came into my mind now as I was writing this letter; it is
about the cause of gravity,_” which he goes on to explain;[116] and
he concludes by saying, that “he has so little fancy to things of this
nature, that, _had not your encouragement moved me to it_, I should
never, I think, thus far have set pen to paper about them.”
These opinions, however, about the existence of ether, Newton seems to
have subsequently renounced; for in the manuscript in the possession
of Dr. J. C. Gregory, which we have already mentioned, and which was
written previous to 1702, he states, that ether is neither obvious
to our senses, nor supported by any arguments, but is a gratuitous
assumption, which, if we are to trust to reason and to our senses, must
be banished from the nature of things; and he goes on to establish,
by various arguments, the validity of this opinion. This renunciation
of his former hypothesis probably arose from his having examined more
carefully some of the phenomena which he endeavoured to explain by it.
Those of capillary attraction, for example, he had ascribed to the
ether “standing rarer in the very sensible cavities of the capillary
tubes than without them,” whereas he afterward discovered their true
cause, and ascribed them to the reciprocal attraction of the tube and
the fluid. But, however this may be, there can be no doubt that he
resumed his early opinions before the publication of his Optics, which
may be considered as containing his views upon this subject.
The queries which contain these opinions are the 18th–24th, all of
which appeared for the first time in the second English edition of the
Optics. If a body is either heated or loses its heat when placed in
vacuo, he ascribes the conveyance of the heat in both cases “to the
vibration of a much subtiler medium than air;” and he considers this
medium as the same with that by which light is refracted and reflected,
and by whose vibrations light communicates heat to bodies, and is put
into fits of easy reflection and transmission.
This ethereal medium, according to our author, is exceedingly more rare
and more elastic than air. It pervades all bodies, and is expanded
through all the heavens. It is much rarer within the dense bodies of
the sun, stars, planets, and comets, than in the celestial spaces
between them, and also more rare within glass, water, &c. than in the
free and open spaces void of air and other grosser bodies. In passing
out of glass, water, &c. and other dense bodies into empty space, it
grows denser and denser by degrees, and this gradual condensation
extends to some distance from the bodies. Owing to its great
elasticity, and, consequently, its efforts to spread in all directions,
it presses against itself, and, consequently, against the solid
particles of bodies, so as to make them continually approach to one
another, the body being impelled from the denser parts of the medium
towards the rarer with all that power which we call gravity.
In employing this medium to explain the nature of light, Newton does
not suppose, with Descartes, Hooke, Huygens, and others, that light is
nothing more than the impression of those undulations on the retina.
He regards light as a peculiar substance, composed of heterogeneous
particles thrown off with great velocity, and in all directions, from
luminous bodies; and he supposes that these particles while passing
through the ether, excite in it vibrations or pulses which accelerate
or retard the particles of light, and thus throw them into their
alternate fits of easy reflection and transmission.
Hence, if a ray of light falls upon a transparent body, in which the
ether consists of strata of variable density, the particles of light
acted upon by the vibrations which they create will be urged with an
accelerated velocity in entering the body, while their velocity will
be retarded in quitting it. In this manner he conceives the phenomena
of refraction to be produced, and he shows how in such a case the
refraction would be regulated by the law of the sines.
In order that the ethereal medium may produce the fits of easy
reflection and transmission, he conceives that its vibrations must be
swifter than light. He computes its elasticity to be 490,000,000,000
times greater than that of air, in proportion to its density, and about
600,000,000 times more rare than water, from which he infers that the
resistance which it would oppose to the motions of the planets would
not be sensible in 10,000 years. He considers that the functions of
vision and hearing may be performed chiefly by the vibrations of this
medium, executed in the bottom of the eye, or in the auditory nerve
by the rays of light, and propagated through the solid, pellucid, and
uniform capillamenta of the optic or auditory nerves into the place of
sensation; and he is of opinion that animal motion may be performed by
the vibrations of the same medium, excited in the brain by the power
of the will, and propagated from thence by the solid, pellucid, and
uniform capillamenta of the nerves into the muscles for contracting and
dilating them.
In the registers of the Royal Society there exist several letters[117]
on the excitation of electricity in glass, which were occasioned
by an experiment of this kind having been mentioned in Sir Isaac’s
hypothesis of light. The society had ordered the experiment to be tried
at their meeting of the 16th December, 1675; but, in order to secure
its success, Mr. Oldenburg wrote to Sir Isaac for a more particular
account of it. Sir Isaac being thus “put upon recollecting himself a
little farther about it,” remembers that he made the experiment with a
glass fixed at the distance of the 1/3d of an inch from one end of a
brass hoop, and only the 1/8th of an inch from the other. Small pieces
of thin paper were then laid upon the table; when the glass was laid
above them and rubbed, the pieces of paper leaped from the one part of
the glass to the other, and twirled about in the air. Notwithstanding
this explicit account of the experiment, it entirely failed at the
Royal Society, and the secretary was desired to request the loan of
Sir Isaac’s apparatus, and to inquire whether or not he had secured
the papers from being moved by the air, which might have somewhere
stole in. In a letter, dated 21st December, Sir Isaac recommended to
the society to rub the glass “with stuff whose threads may rake its
surface, and, if that will not do, to rub it with the fingers’ ends
to and fro, and knock them as often upon the glass.” These directions
enabled the society to succeed with the experiment on the 13th January,
1676, when they used a scrubbing brush of short hog’s bristles, and the
heft of a knife made with whalebone.
Among the minor inventions of Sir Isaac Newton, we must enumerate
his reflecting instrument for observing the moon’s distance from
the fixed stars at sea. The description of this instrument was
communicated to Dr. Halley in the year 1700; but, either from having
mislaid the manuscript, or from attaching no value to the invention,
he never communicated it to the Royal Society, and it remained among
his papers till after his death in 1742, when it was published in
the Philosophical Transactions. The following is Sir Isaac’s own
description of it as communicated to Dr. Halley.
“In the annexed figure PQRS denotes a plate of brass, accurately
divided in the limb DQ, into ½ degrees, ½ minutes, and 1/12 minutes, by
a diagonal scale; and the ½ degrees, and ½ minutes, and 1/12 minutes,
counted for degrees, minutes, and 1/6 minutes. AB is a telescope three
or four feet long, fixed on the edge of that brass plate. G is a
speculum fixed on the brass plate perpendicularly as near as may be
to the object-glass of the telescope, so as to be inclined forty-five
degrees to the axis of the telescope, and intercept half the light
which would otherwise come through the telescope to the eye. CD is a
moveable index turning about the centre C, and, with its fiducial edge,
showing the degrees, minutes, and 1/6 minutes on the limb of the brass
plate PQ; the centre C must be over against the middle of the speculum
G. H is another speculum, parallel to the former, when the fiducial
edge of index falls on 0° 0′ 0″; so that the same star may then appear
through the telescope in one and the same place, both by the direct
rays and by the reflexed ones; but if the index be turned, the star
shall appear in two places, whose distance is showed on the brass limb
by the index.
[Illustration: _Fig. 12._]
“By this instrument the distance of the moon from any fixed star is
thus observed: view the star through the perspicil by the direct
light, and the moon by the reflexed (or on the contrary); and turn the
index till the star touch the limb of the moon, and the index shall
show on the brass limb of the instrument the distance of the star from
the moon’s limb; and though the instrument shake by the motion of the
ship at sea, yet the moon and star will move together as if they did
really touch one another in the heavens; so that an observation may be
made as exactly at sea as at land.
“And by the same instrument may be observed exactly the altitudes of
the moon and stars, by bringing them to the horizon; and thereby the
latitude and times of observation may be determined more exactly than
by the ways now in use.
“In the time of the observation, if the instrument move angularly about
the axis of the telescope, the star will move in a tangent of the
moon’s limb, or of the horizon; but the observation may notwithstanding
be made exactly, by noting when the line described by the star is a
tangent to the moon’s limb, or to the horizon.
“To make the instrument useful, the telescope ought to take in a large
angle; and to make the observation true, let the star touch the moon’s
limb, not on the outside, but on the inside.”
This ingenious contrivance is obviously the very same invention as
that which Mr. Hadley produced in 1731, and which, under the name of
Hadley’s Quadrant, has been of so great service in navigation. The
merit of its first invention must therefore be transferred to Sir Isaac
Newton.
In the year 1672, Sir Isaac communicated to Mr. Oldenburg his design
for a microscope, which he considered to be as capable of improvement
as the telescope, and perhaps more so, because it requires only one
speculum. This microscope is shown in the annexed diagram, where AB is
the object-metal, CD the eye-glass, F their common focus, and O the
other focus of the metal in which the object is placed. This ingenious
idea has been greatly improved in modern times by Professor Amici, who
makes AB a portion of an ellipsoid, whose foci are O and F, and who
places a small plain speculum between O and AB, in order to reflect
the object, which is placed on one side AP, for the purpose of being
illuminated.
[Illustration: _Fig. 13._]
In another letter to Mr. Oldenburg, dated July 11th in the same year,
he suggests another improvement in microscopes, which is to “illuminate
the object in a darkened room with the light of any convenient colour
not too much compounded: for by that means the microscope will, with
distinctness, bear a deeper charge and larger aperture, especially
if its construction be such as I may hereafter describe.”[118]
This happy idea I have some years ago succeeded in realizing, by
illuminating microscopic objects with the light of a monochromatic
lamp, which discharges a copious flame of pure yellow light of definite
refrangibility.[119]
[Illustration: _Fig. 14._]
[Illustration: _Fig. 15._]
In order to remedy the evils arising from the weak reflecting power of
speculum metal, and from its tarnishing by exposure to the air, Sir
Isaac proposed to substitute for the small oval speculum a triangular
prism of glass or crystal ABC. Its side AB _ba_ he supposes to perform
the office of that metal, by reflecting towards the eye-glass the light
which comes from the concave speculum DF, _fig. 13_, whose light he
supposes to enter into this prism at its side CB _bc_, and lest any
colours should be produced by the refraction of these planes, it is
requisite that the angles of the prism at A_a_ and B_b_ be precisely
equal. This may be done most conveniently, by making them half right
angles, and consequently the third angle at C_c_ a right one. The plane
AB _ba_ will reflect all the light incident upon it; but in order to
exclude unnecessary light, it will be proper to cover it all over with
some black substance excepting two circular spaces of the planes A_c_
and B_c_, through which the useful light may pass. The length of the
prism should be such that its sides A_c_ and B_c_ may be square, and
so much of the angles B and _b_ as are superfluous ought to be ground
off, to give passage for as much light as is possible from the object
to the speculum.
[Illustration: _Fig. 16._]
One great advantage of this prism, which cannot be obtained from the
oval metal, is, that without using two glasses the object may be
erected, and the magnifying power of the telescope varied at pleasure,
by merely varying the distances of the speculum, the prism, and the
eye-glass. This will be understood from _fig. 16_, where AI represents
the great concave speculum, EF the eye-glass, and BCD the prism of
glass, whose sides BC and CD are not flat, but spherically convex. The
rays which come from G, the focus of the great speculum AI, will, by
the refraction of the first side BC, be reduced to parallelism, and
after reflection from the base CD, will be made by the refraction of
the next side BD to converge to the focus H of the eye-glass EF. If we
now bring the prism BCD nearer the image at G, the point H will recede
from BD, and the image formed there will be greater than that at G, and
if we remove the prism BCD from G, the point H will approach to BD, and
the image at H will be less than that at G. The prism BCD performs the
same part as a convex lens, G and H being its conjugate foci, and the
relative size of the images formed at these points being proportional
to their distance from the lens. This construction would be a good one
for varying optically the angular distance of a pair of wires placed in
the focus of the eye-glass EF; and by bisecting the lenticular prism
BCD, and giving the halves a slight inclination, we should be able
to separate and to close the two images or disks which would thus be
produced, and thus form a double image micrometer.
Among the minor and detached labours of Sir Isaac, we must not omit his
curious experiments on the action of light upon the retina. Locke seems
to have wished his opinion respecting a fact stated in Boyle’s Book
on Colours, and in a letter from Cambridge, dated June 30th, 1691, he
communicated to his friend the following very remarkable observations
made by himself.
“The observation you mention in Mr. Boyle’s book of colours I once
made upon myself with the hazard of my eyes. The manner was this; I
looked a very little while upon the sun in the looking-glass with my
right eye, and then turned my eyes into a dark corner of my chamber,
and winked, to observe the impression made, and the circles of colours
which encompassed it, and how they decayed by degrees, and at last
vanished. This I repeated a second and a third time. At the third time,
when the phantasm of light and colours about it were almost vanished,
intending my fancy upon them to see their last appearance, I found, to
my amazement, that they began to return, and by little and little to
become as lively and vivid as when I had newly looked upon the sun. But
when I ceased to intend my fancy upon them, they vanished again. After
this, I found, that as often as I went into the dark, and intended my
mind upon them, as when a man looks earnestly to see any thing which
is difficult to be seen, I could make the phantasm return without
looking any more upon the sun; and the oftener I made it return, the
more easily I could make it return again. And at length, by repeating
this without looking any more upon the sun, I made such an impression
on my eye, that, if I looked upon the clouds, or a book, or any bright
object, I saw upon it a round bright spot of light like the sun, and,
which is still stranger, though I looked upon the sun with my right eye
only, and not with my left, yet my fancy began to make an impression
upon my left eye, as well as upon my right. For if I shut my right
eye, or looked upon a book or the clouds with my left eye, I could
see the spectrum of the sun almost as plain as with my right eye, if
I did but intend my fancy a little while upon it; for at first, if I
shut my right eye, and looked with my left, the spectrum of the sun
did not appear till I intended my fancy upon it; but by repeating,
this appeared every time more easily. And now, in a few hours’ time, I
had brought my eyes to such a pass, that I could look upon no bright
object with either eye but I saw the sun before me, so that I durst
neither write nor read; but to recover the use of my eyes, shut myself
up in my chamber made dark, for three days together, and used all means
to divert my imagination from the sun. For if I thought upon him, I
presently saw his picture, though I was in the dark. But by keeping in
the dark, and employing my mind about other things, I began in three or
four days to have some use of my eyes again; and, by forbearing to look
upon bright objects, recovered them pretty well, though not so well but
that, for some months after, the spectrum of the sun began to return
as often as I began to meditate upon the phenomena, even though I lay
in bed at midnight with my curtains drawn. But now I have been very
well for many years, though I am apt to think, if I durst venture my
eyes, I could still make the phantasm return by the power of my fancy.
This story I tell you, to let you understand, that in the observation
related by Mr. Boyle, the man’s fancy probably concurred with the
impression made by the sun’s light to produce that phantasm of the sun
which he constantly saw in bright objects. And so your question about
the cause of this phantasm involves another about the power of fancy,
which I must confess is too hard a knot for me to untie. To place this
effect in a constant motion is hard, because the sun ought then to
appear perpetually. It seems rather to consist in a disposition of the
sensorium to move the imagination strongly, and to be easily moved,
both by the imagination and by the light, as often as bright objects
are looked upon.”
These observations possess in many respects a high degree of interest.
The fact of the transmission of the impression from the retina of the
one eye to that of the other is particularly important; and it deserves
to be remarked, as a singular coincidence, that I had occasion to
observe and to describe the same phenomena above twenty years ago,[120]
and long before the observations of Sir Isaac were communicated to the
scientific world.