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ABCDEF represents the great speculum of glass, ground concave on one side,
and convex on the other; quicksilvered over the convex side, and of an equal
thickness all round its circumference.

The radius of concavity = a = 48 inches.

The radius of convexity = e = 40 inches.

Then putting n, the sine of incidence := 100; w?, the sign of refraction of
the least refrangible rays, out of glass into air, =154; and f*, the sine of re-
fraction of the most refrangible rays, = 1 56 ; as Sir Isaac Newton found them
by experiments; we shall have,

PB, the focal length of the speculum with regard to the most refrangible
rays ^ J 8.2926 -f, which will be somewhat increased by the thickness of the
glass, when that is considerable.

pa, the greatest aberration of the rays, occasioned by their different degrees
of refrangibility, = .05594 -|-, which quantity, in practice, should be a very
little augmented, rather than otherwise; therefore we put it here = .056 = t.

The radius of the concave surface of the lens, turned towards the speculum,
viz. of GHi, = V =: 2.8 inches.



VOL. XLI.J PHILOSOPHICAL TRANSACTIONS. SQQ

The radius of the concave surface of the lens, turned from the speculum,
viz. of KLM, = 6.7 inches.

The thickness of the lens at the vertex lh = ^ of an inch.

The aperture of the lens must be about -^ of the aperture of the speculum.

HP, the distance of the focal point p from the point h, where the abovesaid
lens is to be placed, so as to correct the errors arising from the different refran-
gibility of the mys, and also the errors of the spherical figure, =: 2^^ inches.

HR, the distance of h, the vertex of the lens, from r, the focus of the te-
lescope, = 6.8 inches.

And if we suppose the diameter of the pupil of the eye to be -i- of an inch,
though it has not one certain measure; then the diameter of the greatest
aperture of the speculum, that can ever be of use, will be 6^ inches, nearly.

The small plano-convex eye-glass o must always have one common focus
with the telescope, viz. the point r translated to r, by reflection from the base
of the prism n; for which reason it must retain, at all times, an equal and
invariable distance from the lens ghiklm ; which distance will be the focal
length of the said eye-glass + hr = hn -(- Nr, the distance of the lens from
the focus of the telescope r.

The form and position of the prism n, and the contrivance of the other
parts necessary, will be much the same as in the Newtonian telescope.

If the focal length of the eye-glass be -J- of an inch, the telescope will mag-
nify about '200 times.

This telescope may be contrived in the Gregorian way, by using, instead of
a lens and prism, a small speculum spherically concave on one side, and con-
vex on the other ; but we think it not worth while to attempt this construc-
tion, as an investigation of the proportion between the two surfaces necessary,
in this small speculum, to unite the rays proceeding from the great one, into
one point, would be intricate, and the practice also very difficult; because a
little inaccuracy will, in this case, occasion errors much more considerable than
a like imperfection in the refracting lens.

We have hitherto supposed the radius of the concavity greater than that of
the convexity; as being most convenient and useful, on several accounts, in
forming this kind of telescopes; however, it may be proper to remark, that
the same method may be used for correcting the errors of the speculum, when
the radius of its concavity is less than that of the convexity ; only the refract-
ing superficies of the lens, placed between its vertex and focus, will be convex,
and not concave, as in the former case. And there is another thing worthy
of remark, that the focus, or point p, where the most refrangible rays are col-
lected, will fall farther from the vertex of this speculum, than the focus of the



40O PHILOSOI'HICAL TRANSACTIONS. [ANNO 1740.

least refrangible a; a circumstance which never happens by refraction alone, in
glasses of any figure whatever, or however they be disposed.

Now all things being put as before, and making fig. 5, Ha = d, then the
convex superficies ghi, of a lens placed at h, that shall correct the errors
arising from the different refrangibility of rays, in this kind of speculum,
will be part of a sphere, whose radius is = - , " ^ }, = <j. And hk,

r r ' (/* — ni)d + n« '

the distance of the point r, where the rays of all sorts will unite, after this re-
fraction, from H the given point in the axis, will be = — — — — r^— - — ; which
point R being taken as a centre, describe on it the arch klm, and the figure
GHiMLK will represent the section of a meniscus-glass, or lens, which, placed
at the point h, assumed between the vertex and focus of the speculum, will
collect all sorts of rays proceeding from it, into one and the same point, or
focus K. We might also show, how this error may be rectified by one or more
glasses, placed in the axis, at a distance further from the vertex than the focal
point p ; but the former speculum is so much preferable to this, for the con-
structing of telescopes, that we think it not worth while to prosecute this
matter further. To conclude this essay ,•

Whoever shall think fit to put the method here proposed in execution, we
dare venture, from a trial that has been made, to assure him of success ; pro-
vided the same diligence, care, and accuracy, be applied, in choosing, figuring,
polishing and foiling, the glass, that has of late been employed for the form-
ing speculums of metal ; and let none be discouraged, though the first and
second attempt should fail; for that must be expected, if the ordinary way of
grinding and polishing be used : greater exactness is here required, than is
usually thought sufficient for the object-glasses of refracting telescopes: let it
be also considered how many essays, for a long term of years, were made by
Mr. Gregory, Sir Isaac Newton, and others, to reduce their constructions of
the reflecting telescope into practice, without answering, in any tolerable de-
gree, what their theories promised : the workmen they employed were chiefly
optical instrument-makers, and had it been left to such persons only to per-
form by themselves, we have reason to think, that it would have been pro-
nounced impracticable to this day, to make a reflecting telescope that should
equal or excel refracting ones of 10 times its length; though we now see, that
most of these artificers are capable of making them to such a degree of per-
fection as was formerly despaired of.

April 5, 1739.



I



VOL. XLI.J PHILOSOPHICAL TftAKSACTIONS. 401

Concerning an Earthquake at Naples. By the Hon. Henry Temple,

N" 436, p. 340.

Naples, Dec. 12, N.S. J 732.

— They say, the last earthquake here has made a great crack in the side of
Mount Vesuvius, above 30 yards long. But what seems much more extraor-
dinary, is, that the second shock, which was a very slight one, had a great
effect upon the nerves : I and all the company where I was, as soon as the
shock was over, were seized with a shaking, just as if we all had the palsy,
our teeth chattering in our heads to such a degree, that we could hardly speak;
and I find, that half the town felt the same effect from it. It would be natural
to imagine, that this shaking was caused by the fright, but it is easy to prove
the contrary ; because, in the first place, the first shock, which was much
more terrifying, had not that effect : 2dly, many people who were not sensible
of the earthquake, found themselves seized in the same manner : 3dly, Mr.

who used to be troubled with convulsive fits, and had got quite cured

of them here, was immediately seized with them again, after the earthquake ;
and, 4thly, every body, more or less, complained of head-achs for some
days after.

Concerning a Monstrotis Child, born of a Woman under Sentence of Trans,
portation. By Mr. Timothy Sheldrake. N° 456, p. 341.

Elizabeth Spencer, being tried at the Assizes for the city and county of
Norwich, for shop-lifting, and being found guilty of the crime, received sen-
tence for transportation ; for respiting of which sentence, she pleaded her
belly, which plea, as she was a married woman, appearing what was very pro-
bable, she was favoured by the mayor and the other magistrates, by being
allowed the full time that she said she had to go ; at the expiration of which
she was delivered of a child, which Mr. S. saw a few hours after it was born,
and was exactly, in every part, according to the following account. The
head had a rising on the top of it, and the nose was as if one nose was
on the top of another, but only 1 nostrils, and those at the bottom of the
lower nose. The arms were without the elbow-joint; the 2 bones, which
make the lower joint of the arm, in common, were in this extended to the
shoulder. Just under the ribs, and above the hips, was a deep place, as if a
cord had been tied very strait, so as to sink down below the reach of the eye :
this girding-in of the body, he believes might go almost round : he did not
turn it, to see whether it did or not, but it was continued as far about the

VOL. vin. 3 F



402 PHILOSOPHICAL TRANSACTIONS. [aNNO 1740.

boil) as he could see, without turning it. By this girding-in of the body,
the lower part of it was almost round, it being without either legs or
thighs ; but had 2 feet joined unto the lower part of the body, the heels
inward, the toes, of which it had not the full number, pointing towards the
"sides. As to sex, this creature was a female, and born alive. It was the opi-
nion of the women about her, that the midwife had injured the head in the
birth, by which the rising in the head was produced ; and this surprising crea-
ture that was born alive, was thus soon deprived of life. This woman, who
had been the mother of several children, before this strange production, and
all in perfect form, was by some free-speaking persons, charged with having
been guilty of some practices both unnatural and unlawful, which she very
positively always denied ; and said that she knew nothing that could give any
change to the natural form of this creature, but the strange apprehensions
that her sentence had put her under, from the uncommon creatures the country
to which she was sentenced might bring in her sight. These odd ideas that
she had formed to herself, was all and the only thing, that had occasioned so
great a change from the natural form the child might otherwise have had, as
she often asserted.

Concerning the Mola Salu,* or Sun-Jlsh, and a Glue made of it. By the Rev.
Mr. JVilliam Barlow. N° 456, p. 343.

There was brought to this place, struck the day before in our river, a sun
iish, weighing about 500lb. The form of it nearly answers that given by Mr,
Willoughby, except that the tail of this was scolloped. It differed very much
in one thing from that described by Mr. Willoughby, whose flesh, he says, was
very soft : on the contrary, the flesh of this was hard and firm, rather a gristly
substance, than soft flesh.

A commander of a vessel told Mr. B. that his people took a sun-fish, south
of Newfoundland, which, by his description, was considerably larger than that
brought hither. They made no use of the flesh ; but he remembers it was a
gristly substance, hard and firm. ,

A piece of the flesh boiled, to try how it would look and taste, was all turned
into a jelly. Being soft and tender, it could not be taken out of the saucepan
with a fork, but only with a spoon ; in colour and consistence nearly resembling
boiled starch when cold. It had little or nothing of the fishy, but a grateful
and pleasant taste.

By the sticking together of his lips, and from what he observed by touching

* Diodon mola. Bloch.



VOL. XL!.] IfHILOSOPHlCAL TRANSACTIONS. 403

it with his fingers, Mr. B. observed that this boiled flesh was clammy and
glutinons; which brought to mind, that what the ancients made use of to serve
the purposes of glue, was made from fish. He then tried it on paper and
leather, and found it to answer the use of paste very well : and it was owing in
part to neglect, and partly to accident, that it was not also tried on wood.

From the descriptions given of the ichthyocolla, by Dioscorides and Pliny,
the glue-fish seems not to be the same as this sun-fish. Whether the fish from
which isinglass is made, be the same as the ichthyocolla of the forementioned
authors, as the name usually given to it seems to import, he cannot tell : but
neither the ichthyocolla of Rondelitius or Bellonius, nor the huso taken in the
Danube, from the bladder of which fish-glue is made, can, by the descriptions
given of them, be the same as the sun-fish.

Discovery of the Remains of a City under-ground, near Naples. Communicated
to the Royal Society by IVilliam Sloane, Esq. F. R. S. N° 455, p. 345.

At Resina, about 4 miles from Naples, under the mountain, within half a
mile of the sea-side, there is a well, down which about 30 yards is a hole, which
some people have the curiosity to creep into, and may afterwards creep a good
way under-ground, and with lights find foundations of houses and streets,
which, by some it is said, was in the time of the Romans a city called Aretina,
others say Port Hercules, where the Romans usually embarked for Africa. Mr.
S. has seen the well, which is deep, and has a good depth of water at the bot-
tom, that he never cared to venture down, being heavy, and the ropes bad.
This city, it is thought, was overwhelmed by an eruption of the mountain
Vesuvius, not sunk by earthquakes, as were Cuma, Baia, Trepergola, &c.

Of a Meteor seen in the j4ir in the Day time, Dec. 8, 1733. By Mr. Crocker.

N° 456, p. 346.

On Saturday, Dec. 8, 1733, between 11 and 12, the sun shining bright, the
weather warm, and wind at south-east, some small clouds passing, Mr. C. saw
something in the sky, which resembled a boy's paper kite, which appeared to-
wards the north, which soon vanished from sight, being intercepted by the
trees which were near the valley where he was standing. Its colour was a pale
brightness, like that of burnished or new-washed silver. It darted out of sight
with a seeming coruscation, like that of star-shooting in the night; but it had
a body much larger, and a train much longer, than any thing of that kind he
had ever seen before. On coming home, one Brown said, he had seen the
same thing, for the continuance of a minute ; and that the body and train ap-

3f 2



40\ PHILOSOPHICAL TRANSACTIONS. [aNNO 1740.

peared to be about 20 feet long, and seemed to fall to the ground somewhere
about the kennel-garden, whither Mr. C. accompanied him in expectation of
finding some of those jellies which are supposed to owe their beings to such
meteors : but we might have searched long enough, as the next day, when Mr.
Edgcombe informed him, that he and another gentleman had seen the same
appearance at the same time about 15 miles from us, steering the same course
from east to the west, and vanished from them between Walkhampton and
Oakhampton : they gave the same account of its figure, length and colour.

Of a Luminous Appearance in the Shy, seen at London on Thursday, March 1 3,
1734-5. By John Bevis, M. D. N^ 456, p. 347.

When observing Mars near a small fixed star, then in the west, on the top
of his house in Buckingham-street, about 5 minutes after 8, equal time ; hap-
pening to turn his face southward, Dr. B. was surprised with an uncommon
bright glade of light. It was straight, about 2^ degrees broad, and 1 10 or 120
degrees long, ill defined at either end, but pretty well at the sides, much as the
common rainbow, or one of those pyramids which are used to dart up from the
horizon in an aurora borealis, which light it resembled in all respects, except in
its place and position, and that this was steady, and altogether without that
tremulous kind of motion, which usually accompanies that. Besides Saturn,
Mars, Venus, and the fixed stars, there was then no other light in the sky, nor
the least cloud, nor any of that horizontal blackness which we see northward in
the aurora. The stars were as discernible through it, as if nothing had been
there. A gentleman who was with the Doctor fancied it to be the tail of a
comet ; but as neither he nor the Doctor had ever seen one, he gave but little
heed to that conjecture: however, he carefully directed a 17-foot glass to all
parts of its western extremity, but could discern nothing like a nucleus. When
first seen, it extended from about the midway between Aldebaran, and Orion's
left shoulder, through Gemini a little under |3, and so on through Cancer and
Leo, just above Cauda Leonis, till it arrived between Vindematrix and Coma
Berenices, where it ended very dilutedly. In about half an hour it grew dim
about the middle, where in a short time it separated in two, or rather became
quite dark there ; then the disjoined parts were more luminous than before ;
but they too in a little while after grew dimmer, and shortened away, on to
their remote extremities, which remained visible the longest ; the western one
about 9 o'clock, the time of its extinction, being near Orion's right shoulder,
and the other near the left knee of Bootes ; so that this meteor seems pretty
nearly to have accompanied the earth in its diurnal motion, and to have had



'iiiln/



VOL. XLI.] PHILOSOPHICAL TRANSACTIONS. 405

little or no motion besides. The Doctor looked for this light afterwards, but
could find nothing like it.

Of a Calculus making its JVay through an old Cicatrix in the Perinceum. By
David Hartley, M. A. F. R. S. N° 456, p. 349.

William Jarman, of the parish of Bayton in Suffolk, was cut for the stone
about 15 years before, and a large stone taken from him. The patient con-
tinued easy for about 4 years after he was cut ; the wound was quite healed up,
and he made water in the natural way, without any leakage at the wound. In
July last, he felt great pain at the place where he was cut, and it was much
swelled. It looked black, and a little hole broke open there, out of which the
water came ; and a stone appearing, the hole grew wider by the force of the
water, and his frequently touching it, till at last the stone came away whole. It
was broken afterwards by a fall.

As soon as the stone was come away he became easy, and the swelling abated.
The wound was afterwards reduced to a small compass; but his water came away
through the wound, and very little the natural way. The patient was about
30 years of age. The great end of the stone came away first, which he suffered
to lie at the mouth of the wound near a fortnight ; but he applied to no
surgeon.

Of a Stone, or Calculus, making its Way out through the Scrotum. By Mr.
John Sisley, Surgeon. N° 456, p. 351.

Robert Swann, of East-Mailing, Kent, a hard working man in the woods,
sent for Mr. S. one day to see him. He found him with a large swell-
ing on his testicles ; on the upper part of the scrotum, there was a small hole
or two, and he told Mr. S. his urine oosed out sometimes. Mr. S. passed the
probe in, and found a hard substance, which seemed to be large , he told him,
he had a large stone lodged there, at which the poor man was much surprised.
He said he would make an incision and take it out ; but he refused to be cut.
Mr. S. dilated it in another manner, made the orifice pretty large : the swelling
of his testicles assuaged, he went to work, as usual ; about a week's time after,
coming home at night with a large bundle of wood at his back, he found him-
self more in pain than usual ; as soon as he got home, he complained to his
wife, and told her he was very much in pain, he went to-bed, and desired Mr.
S. to be sent for immediately ; but before he could get to him, the stone forced
its way out ; and as soon as he came, the poor man seemed much rejoiced, and
told him, (as he expressed himself) the Swan had laid an egg : its weight at



4t)6 PHILOSOPHICAL TRANSACTIONS. [aNNO 1740.

at first was Jv and jij, now almost 4^ oz. This man lived about 7 years after,
in a good state of health, and lived to the age of 6o or upwards. He said the
stone had been growing there for near 30 years ; but he never apprehended it
to be a stone, though he used to complain of a weight, as if it were half a
pound, carried between his legs.

Account of the Petrifactions near Matlock Baths in Derbyshire; with Conjectures
concerning Petrifaction in General. By Mr. Moreton Gilhs, F. R. S.
N° 456, p. 352.

In the mountainous part of Derbyshire, about Cromford, is a valley of at
least a mile and half long, walled on each side with high craggy rocks ; the east
side cliffy, the west more reclining, but extremely rough and difficult of ascent ;
being composed of large loose pieces of the lime-stone rock, of 5, 10, or 20
ton weight ; that seem some time to have broken off from the top of the cliffs,
and fallen down into the valleys. At the bottom of the valley, which seems to
be a great gaping fissure of the rock, runs the river Derwent harshly along its
rocky bottom. About the middle of the valley, at near 50 feet perpendicular
height from the river, issue forth several rivulets of a lukewarm water, falling
into the Derwent below. Some of this water, being collected in a reservoir, on
account of its agreeable warmth, has of late years been much used for bathing,
and is called Matlock Bath. Now for about the compass of 50O or 600 yards,
near where this water gushes out, the stone appears of a very different texture
and complexion ; and proves, on examination, to be a perfect incrustation,
formed on the original rock ; composing a fictitious stone of earth, vegetables,
&c. of various kinds, such as usually grow in rocky places, as polypody, trico-
manes, and other species of the capillary tribe, mosses, brambles, ivy, hazle,
&c. There are several large grottos, at about 15 feet above the level of the
river, lined most curiously with the stalactitae, lapides stillatitii, &c. Some of
them nearly resemble large bunches of grapes, and other clusters of fruit, and
very beautiful. The farther you penetrate into this mountain, the closer and
more compact the stone appears ; the interstices in the petrified matter being at
the depth of 15 or l6 feet, almost filled up, and nearly as solid as the lime-
stone, of which the original rock is composed ; and even within 4 or 5 feet of
the surface, though very open and porous ; yet is it so hard, as to be used in
the building about the bath.

The mountain in several places jets out almost over the brink of the river ;
under these protuberances are the grottos, very dangerous and difficult to get
at ; but it is impossible to give an idea of the natural beauty of the place. The



VOL. XLI.] PHILOSOPHICAL TRANSACTIONS. 407

frost-work, and incrusted plants, are some of them so very delicate and tender,
as to make it impracticable to bring them away with half their beauty, by the
most careful conveyance. In one place there is an ivy creeping along the rock,
part of it entirely petrified, another part only incrusted, and a third still
vegetating. In another place is a hazle-tree, its root composing part of this
petrified mountain, the branches some petrified, and some tenderly incrusted.
As these are changed, others spring up, and in time will undergo the same fate.
In short, nothing in nature can give a more clear idea, or more beautiful re-
presentation, of the whole business of petrifaction, than a curious observer will
see, and frame in his mind from this mountain. He will see, that not only the
water, as it distils out of the rocks, is capable of incrusting and petrifying the
bodies it meets with in its passage, but that even the streams and exhalations,
being highly saturated with these mineral particles, will produce the same efl^ect;
as is evident in the place under consideration, and will generally best account
for the supply of petrifying matter, brought to fill up the vacuities left by the
decay and waste of vegetables incrusted over; and which in course of time are
constantly filled with it. For though the water of some springs may be so
loaded with mineral matter, as, perhaps by penetrating the pores of wood and
other lax bodies, to increase greatly their specific gravities ; yet surely it is con-
trary to the laws of matter, and absurd to say, there is any hidden property in
such waters, capable of changing the parts of one body into another, specifically
different. It may in time, no doubt, lose its texture and coherency, by the
admittance of heterogeneous particles of different attractions ; but the cause of



Online LibraryRoyal Society (Great Britain)The Philosophical transactions of the Royal society of London, from their commencement in 1665, in the year 1800 (Volume 8) → online text (page 47 of 85)