IMPROVEMENTS IN THE MANUFACTURE ALSO IN MACHINERY, OF REFLECTORS, LENSES, AND OPTICAL GLASSES, AND TOOLS, AND APPARATUS EMPLOYED IN MANUFACTURING AND MOUNTING THE
Year: 1875

page_text: LETTERS PATENT to Henry Bessemer, of Denmark Hill, in the County,
of Surrey, for the Invention of "IMPROVEMENTS IN THE MANUFACTURE
ALSO IN MACHINERY,
OF REFLECTORS, LENSES, AND OPTICAL GLASSES, AND
TOOLS, AND APPARATUS EMPLOYED IN MANUFACTURING AND MOUNTING THE
"
Sealed the 3rd March 1876, and dated the 31st December 1875.
PROVISIONAL SPECIFICATION left by the said Henry Bessemer at the
Office of the Commissioners of Patents, with his Petition, on the
31st December 1875.
1, HENRY BESSEMER, of Denmark Hill, in the County of Surrey,
41 IMPROVEMENTS
do hereby declare the nature of the said Invention for
IN THE MANUFACTURE OF REFLECTORS, LENSES, AND OPTICAL GLASSES, AND ALSO IN
MACHINERY, TOOLS, AND APPARATUS EMPLOYED IN MANUFACTURING AND MOUNTING
THE SAME," to be as follows:-

page_text: .
and daylight reflectors, or for other purposes, in which light is required
to be either reflected or refracted, and which said improvements consist
in giving the general form and contour required to rough or plain
pieces of glass or speculum metal, by the operation of turning in a
lathe with a tool, in which a piece of carbon or black, or ordinary
diamond, or other sufficiently hard gem is set, and by means of which
the general form is given to the surfaces of the mass operated upon by
turning, cutting away, or abrading certain portions of the mass. For
this purpose I construct a lathe having two face plates, one at each end
of the mandril, and of such size (by preference) as will exceed in
diameter the largest lense or reflector to be turned thereon. I prefer
to make the surface of the face plate as perfectly flat by scraping and
grinding as possible, and I make certain indented concentric annular
channels on its surface, cach of which communicates by a small hole
with a central hole which passes through the mandril of the lathe, by
means of which air can be exhausted by a proper exhaust pump con-
nected by a pipe and stuffing box at one end of the mandril, so that
if a small vessel or receiver in communication with the hollow mandril
be kept in a partially exhausted state by the said air pump, air will be
drawn in at the several openings in the face plate before named; but if
a large circular disc of plate glass or other flattened piece of glass be
placed in contact with the grooved surface of the face plate it will
immediately cut off the free communication of the exhausted vessel
with the outer atmosphere; by closing the before-named holes a partial
vacuum will thus be instantly formed in the annular channels, and the
outer atmosphere pressing on the exterior surface of the glass or other
disc it will firmly adhere to the face plate, and will be so retained in
position as long as the receiver is kept in an exhausted state. A small
two-way cock on the pipe leading from the hollow mandril of the lathe
to the receiver will enable the workman at any moment to shut off all
communication with the exhausted receiver, and admit the external
atmosphere to the annular channels in the face plate, and by thus
restoring the equilibrium allow the glass or other disc to be at once.
removed. By this means the disc does not become bent or distorted
as it would do by bedding it on a soft adhesive or yielding material, as
usually practiced; the disc so operated on can thus be fastened on to or
detached from the face plate almost instantaneously.
IT

page_text: thoroughly and evenly supported as not to be easily fractured even, by
repeated heavy blows given by a wooden mallet on its surface; both the
face plates before named are to be similarly grooved. The lathe has a
long bed on which a moveable standard is secured at such a distance
from the disc of glass as the radii or focal length of the intended reflector
or lense may require. This moveable standard is furnished with a pivot
or centre on which a radius bar or frame is free to move; the opposite
end of the radius bar is formed into a rectangular frame which embraces
the lathe head and its mandril and face plates, the bar and frame lying
horizontally is supported on antifriction rollers with its upper surface
nearly level with the centre of the mandril of the lathe; the recton-
gular part is supported on three sliding pieces resting on a flat bed.
By means of a leading screw and gearing the radius bar or frame is
made to move in an are of a circle on the pivot or centre before named,
and thus forms a sort of slide rest, the cutting tool moving in a curve
whose radius is determined by the distance of its centre of motion from
the disc of glass. Now if a diamond set in the end of an iron or steel
bar or tool holder be attached to a small cross slide on the rectangular
frame, it will act in a similar manner to a common turning tool used in
ordinary slide rest turning, except that the diamond point will move in
a curved line and not in a right line, as in the common slide rest, and
will thus cut a curved hollow in the glass disc, forming what is known
as a concave lense or reflector. But if the glass disc be placed on the
|face plate furthest from the centre on which the radius frame moves,
and the diamond cutting tool be there applied, a convex surface will be
given to the disc; hence when a concave reflector is required the first
operation will be to cut the concave surface as first described, and then
by placing another disc of glass or metal on the face plate at the
opposite end of the mandril a convex grinder or polisher may be
turned to the precise same radius, care being taken that by means of a
guage rod, the cutting edge of the diamond shall be kept at precisely
the same distance from the pivot or centre on which the radius frame
moves, when cutting each of the concave and convex surfaces.
It will be understood that when convex lenses or reflectors are required
they must be turned on the face plate farthest from the centre on which
the radius bar or frame is moved. This lathe will, as in ordinary lathes,
be provided with driving gear to give different speeds, and have also the

page_text: common strap driving drums with the necessary stopping and starting
levers.
For turning glass I prefer to use a piece of carbon or, as it is generally
called, black diamond for roughing out the mass, and a piece of ordinary
diamond split, or by preference ground, to a proper cutting angle for
use when making the last smooth finishing cuts. A fine jet of spirits
of turpentine, naptha, or alcohol is directed on the cutting edge of the
tool all the time the lathe is in motion; and, although I prefer to use
the diamond for these purposes, other gem or substance of sufficient
hardness to cut glass efficiently may be employed in lieu thereof. A
spherical figure having been thus given to the lense or reflector by the
turning tool its surface may be further perfected by grinding and
polishing in a manner similar to that by which lenses and speculum
metal reflectors have been heretofore ground and polished. The figuring
of the reflector, as it is called, whereby a more or less truly parabolic
curve is given to the reflector, may be performed in the usual way, or
by other improved means, after which the reflector may have a thin
filin of silver deposited thereon in the usual way, except when used in
the helioscope, in which case it will reflect sufficiently the rays of the
sun without silvering.
In the manufacture of very large glass reflectors, for which this system
of turning is peculiarly adapted, it is desirable that plate glass should be
employed, as that material is readily obtainable of large superficial area,
but comparatively thin in cross section, rarely exceeding one inch and
a half in thickness, hence it becomes necessary to strengthen or support
the reflector in such a manner that it is not bent or be deflected by its
own weight when made to assume different positions while in use.
The necessary strength and rigidity may be given to large plate glass
reflectors by two different modes of treatment, the first of which consists
in fusing, or what may be called soldering, together any desired number
of sheets of plate glass by the application of heat. For this purpose I
prefer to use plates as thick as can be readily obtained; they should be
ground flat, but need not be polished. I then prepare a creamy wash of
silicious and alkaline matters, such as is ordinarily used to glaze pottery-
ware, or I reduce by grinding to a very fine powder flint or other glass of
æ more fusible kind than the plates are made of, and I make this
powdered glass into a creamy fluid with water with or without the

page_text: ::
addition thereto of boracic acid, pearlash, or soda; and in either case I
dip the circular discs of glass into this mixture, or I otherwise apply a
coating of it to their surfaces, and when it is quite dry the plates are to
be piled on each other to the desired thickness, and placed in a "bung
or covered cylindrical case made of fire-clay, and gradually heated in a
pottery kiln up to such a temperature as will fuse the coating of materials,
and cause a complete adhesion or union of the surfaces of the several
plates of glass. Some lime, sand, or plumbago in the form of powder
should be spread evenly over the lower internal surface of the bung'
for the glass to rest upon, and prevent its adhesion thereto.
.
In some cases two or more plates may be united by simple fusion or
softening of the mass, or by the use of borax or alkalies only put in
Care must be taken that
small quantities between each layer of glass.
the thick mass of glass thus formed is very gradually coolel before
Or in lieu of this mode of
removing it from the "bung or case.
building up thick masses of glass a single plate or sheet may be used
as a reflector by employing a strong and light permanent cellular
backing of metal for the comparatively thin
the comparatively thin glass reflector to
rest on.
"
.
The arrangement I prefer for this purpose consists of a cellular iron
casting, cylindrical in form, one of its circular faces being scraped and
ground to a true plane, the opposite face having openings into the
various cells or interstices formed between a series of ribs, which cross
the cylinder at right angles, and serve to give great powers of resisting
The breadth of these ribs
change of form in proportion to its weight.
and the cylindrical part of the casting may be equal to about one-third
of its diameter. The face having been truly ground and fitted to the flat
back of the glass reflector, the latter is held permanently in contact with
the iron surface by exhausting the hair from between them, as herein-
before described in attaching the glass disc to the face plate of the lathe,
so that no distortion of the reflector can take place unless the large
cellular metal support is also deflected. To prevent this it will be sup-
ported by thin steel straps in different directions within a cylindrical
casting, adding suitable flanges for the purpose of attaching it to the
telescope or other instrument; or the cellular casting may be put into a
cast-iron cylinder, the bottom of which is closed, and which is of such a
diameter as will leave an annular space of about 3 of an inch all round
the cellular casting, and which space may be filled with some more or

page_text: less elastic material, that can be run into the cavity in a semi-fluid state,
and when cold be capable of forming an elastic cushion, giving an equal
support to the cellular casting, while its expansion and contraction by
change of temperature will not be restrained; or in lieu thereof several
flat vulcanized rubber bands may be stretched evenly over the whole
cylindrical surface of the cellular casting, and a flat disc of the same
material made to cover its lower surface. The whole may then be
temporarily suspended within the cylindrical casing, but without contact
with it, and the intermediate space between the exterior surface of the
rubber and the case be filled in with a fluid resinous matter, capable of
becoming quite solid when cold. The embedded mass will thus be
equally well supported on all sides by such a thickness of clastic material
as will allow of its expansion or contraction by change of temperature,
and admit also of change of position without subjecting the cellular
casting to such strains as will cause the reflector attached to its surface
to become distorted.
In some cases, where a short focus or deep curvature is required in
reflectors, a sheet of plate glass may be cut circular and bent in the
glass bender's oven, so as to be concave on one side and convex on the
other. A collular metal backing, having a concave face, may be
accurately ground and fitted to the convex side of the bent disc of glass,
which may then be turned true in the lathe on its concave side, and
afterwards finished in a similar manner to that herein-before described.
And I would here remark that I prefer to unite the glass disc to its
iron support previous to the final grinding operation, and in some cases
even before the turning operation takes place in the lathe, in which
case the cellular casting with its glass surface may be chucked on the
lathe in the usual way; and although I prefer to use a cellular iron
casting for large reflectors a light and strong backing may be made for
reflectors of medium size in potteryware or terra cotta, moulded with:
ribs or cells similar to the iron cellular backing before described; or for
smaller sizes a piece of marble or slate may be employed.
The lathe and diamond turning tool may also be employed for giving
to reflectors made of speculum metal a more correct figure than can be
obtained by casting alone, and thus assist in the developement of a more
accurate figure in such reflectors.

page_text: When double concave or convex lenses or reflectors are required, I
prefer to form the face plate itself, or a piece of metal or glass tem-
porally attached thereto, into a convex or concave surface, correspond-
ing precisely with the configuration of the lense or reflector to be chucked
thereon, so that the glass disc is well and properly supported while
having its second side operated on in the lathe.
1 in all cases turn the edge of the discs truly circular before removing
them from the lathe, and in some cases | form a flat sunken portion at
the edge of the disc so as to facilitate the concentric rechucking of
the glass, and insure the perfect parallelism of the two curved surfaces,
and the true concentric positions of these surfaces in reference to each
other.
When the large glass reflector of the helioscope is used in the un-
silvered state, a solid métal backing or support is inadmissable. I there-
fore in such case use a hollow double cylinder of cast iron, the inner
and outer portion being united by webs. The end of the cylinder on
which the reflector is to be fastened has channels formed in it for the
purpose of retaining the glass in firm contact by the withdrawal of air
from beneath the annular portion of the reflector which rests on the
metal cylinder; and in some cases I make a central boss, attached by a
dise or arms to the double cylinder, so as to form a support to the central
part of the reflector, and prevent its deflection when moved into
different positions.
In other cases I fasten with varnish, balsam of Canada, gold size, or
other transparent cement, two plain concave discs of glass together,
putting their two flat sides in contact, with or without a third parallel
sheet of stout glass between them for the purpose of rendering the
reflector so formed less liable to be bent or deflected while in use.
Also form reflecting or refracting surfaces by turning with a diamond
cutting tool the surface of discs of glass, forming successive annular
rings or zones thereon, capable of reflecting or refracting solar or
artificial light, such grooves or inclines being segments or portions of a
sphere, parabola cone, or other desired form, depending on their size and
intended use.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent,
filed by the said Henry Bessemer in the Great Seal Patent Office on
the 30th June 1876.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I HENRY
BESSEMER, of Denmark Hill, in the County of Surrey, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Thirty-first day of December, in the
year of our Lord One thousand eight hundred and seventy-five, in the
thirty-ninth year of Her reign, did, for Herself, Her heirs and
successors, give and grant unto me, the said Henry Bessemer Her
special license that T, the said Henry Bessemer, my executors, admini-
strators, and assigns, or such others as I, the said Henry Bessemer,
my executors, administrators, and assigns, should at any time agree
with, and no others, from time to time and at all times thereafter
during the term therein expressed, should and lawfully might make,
use, exercise, and vend, within the United Kingdom of Great Britain
and Ireland, the Channel Islands, and Isle of Man, an Invention for
'IMPROVEMENTS IN THE MANUFACTURE OF REFLECTORS, LENSES, AND OPTICAL
GLASSES, AND ALSO IN MACHINERY, TOOLS, AND APPARATUS EMPLOYED IN MANUFAC-
AND MOUNTING THE SAME," upon the condition (amongst others)
that 1, the said Henry Bessemer, my executors or administrators, by
an instrument in writing under my, or their, or one of their hands and
seals, should particularly describe and ascertain the nature of the said
Invention, and in what manner the same was to be performed, and
cause the same to be filed in the Great Seal Patent Office within six
calendar months next and immediately after the date of the said Letters
Patent.
"
TURING
4
b
NOW KNOW YE, that I, the said Henry Bessemer, do hereby declare
the nature of the said Invention, and in what manner the same is to be
performed, to be particularly described and ascertained in and by the
following statement thereof, that is to say :-
These improvements consist, firstly, in the manufacture of lenses
and reflectors for telescopes, helioscopes, cameras, lighthouses, signal
lights, and daylight-reflectors, or=for other purposes in which light is
required to be either reflected or refracted, and which said improvements

page_text: consist, firstly, in giving the general form and contour required to
rough or plain pieces of glass or speculum metal by the operation of
turning in a lathe with a tool, in which a piece of carbon or black, or
ordinary diamond, or other sufficiently hard gem is set, and by means
of which the general form is given to the surfaces of the mass operated
upon by turning, cutting away, or abrading certain portions of the mass,
and which hard cutting gems or substances are herein-after generally
referred to by the use of the word diamond, whether that particular
gem or substance or some other hard cutting material be employed. For
this purpose I construct a lathe having two face plates, one at each end
of the mandril, and of such size (by preference) as will exceed in
diameter the largest lense or reflector to be turned thereon. I prefer to
make the surface of the face plate as perfectly flat by scraping and
grinding as possible, and I make certain indented concentric annular
channels on its surface, each of which communicates by a small hole
or by a channel or passage with a central hole which passes through
the mandril of the lathe, by means of which air can be exhausted by a
proper exhaust pump connected by a pipe and staffing box at one
end of the mandril, as shewn in the detached parts of the lathe
represented at Figure 24, on Sheet F of the annexed Drawings, where
,, represent the two face plates in vertical section with their small
indented concentric grooves or channels b, all of which are connected
by means of a radial channel which crosses and connects them with the
central passage which passes through the mandril d, as shown. The
mandril d is provided with flanges d' at each end, by means of which
the face plates a are firmly bolted thereto; each face plate is strengthened
by radial webs a', and its periphery is curved to receive a driving strap
from an overhead motion by means of which it is driven. A sinall
stuffing box e and pipe f is screwed into one of the face plates, as shewn,.
while the other face plate is being used to hold on the disc of glass or
other material g to be shaped, the stuffing box e is interchangeable and
the pipe and two-way stop-cock his so arranged as to be readily con-
nected with an exhausted air vessel whichever end of the mandril it may
be attached to for the time, so that if a small vessel or receiver in
communication with the hollow mandril be kept in a partially exhausted
state by the said air pump air will be drawn in at the several openings
or channels in the face plate before named, but if a large circular disc of
plate glass or other flattened piece of glass or other material be placed
n contact with the grooved surface of the face plate it will imme-

page_text: diately cut off the free communication of the exhausted vessel with the
outer atmosphere by closing the before-named holes, grooves, or
passages, and a partial vacuum will thus be instantly formed in the
annular channels, and the outer atmosphere pressing on the exterior
surface of the glass or other disc it will firmly adhere to the face plate,
and will be so retained in position as long as the receiver is kept in an
exhausted state; the small-two-way cock on the pipe leading from the
hollow mandril of the lathe to the receiver will enable the workman at
any moment to shut off all communication with the exhausted receiver,
and admit the external atmosphere to the annulat channels in the face
plate, and by thus restoring the equilibrium allow the glass or other disc
to be at once removed. By this means the disc does not become bent or
distorted as it would do by bedding it on a soft adhesive or yielding
material as usually practiced, the disc so operated on can thus be fastened
on to or detached from the face plate almost instantaneously.
When in contact with the flat metal face plate the glass disc is so
thoroughly and evenly supported as not to be easily fractured even by
repeated heavy blows given by a wooden mallet on its surface; both the
face plates before named are to be similarly grooved.
The lathe has a
long bed on which a moveable or adjustable standard is secured at such
a distance from the disc of glass, as the radii or focal length of the
intended reflector or lense may require; this moveable standard is
furnished with a pivot or centre, on which a radius bar or frame is free
to move; the opposite end of the radius bar is formed into a rectangular
frame which embraces the lathe head and its mandril and face plates;
the bar or frame lying horizontally is supported on antifriction rollers
with its upper surface nearly level with the centre of the mandril of the
lathe; the rectangular part is supported on three sliding pieces resting
on a flat bed.
By means of a leading screw and gearing the radius bar or frame is
made to move in an arc of a circle on the pivot or centre before named,
and thus forms a sort of slide rest, the cutting tool moving in a curve,
whose radius is determined by the distance of its centre of motion from
the disc of glass; now if a diamond set in the end of an iron or steel
bar or tool holder be attached to a small cross slide on the rectangular
Frame, it will act in a similar manner to a common turning tool used in
ordinary slide rest turning, except that the diamond point will move in
a curved line and not in a right line as in the common slide rest, and

page_text: will thus cut a curved hollow in the glass dise, forming what is known
as a concave lense or reflector; but if the glass disc be placed on the
face plate furthest from the centre on which the radius frame moves,
and the diamond cutting tool be there applied a convex surface will be
given to the disc; hence when a concave reflector is required the first
operation will be to cut the concave surface as first described, and then
by placing another disc of glass or metal. on the face plate at the
opposite end of the mandril a convex grinder or polisher may be turned
to the precise same radius, care being taken that by means of a gauge
rod the cutting edge of the diamond shall be kept at precisely the same
distance from the pivot or centre on which the radius frame moves when
cutting each of the concave and convex surfaces.
It will be understood that when convex lenses or reflectors are
required they must be turned on the face plate furthest from the centre
on which the radius bar or frame is moved. This lathe will as in
ordinary lathes be provided with driving gear to give different speeds,
and have also the common strap driving drums with the necessary
stopping and starting levers.
I however prefer to widen the external edge of the face plate as shewn
in the Drawings, and to use a leather driving belt which passes over
a wide drum above the lathe, and when turning reflectors or lenses
of large size I prefer to regulate the speed of the rotation of the lathe by
means of cone drums, so that the speed or number of revolutions of the
lathe may be increased as the cutting tool approaches the centre, and
thus diminish the time occupied in the turning operation.
or'
The form of lathe which I employ for the shaping of convex
concave reflectors or lenses is represented in several detached figures on
the Sheets of Drawings hereunto annexed, and in order that the same
may be fully understood I have shewn certain detached portions thereof
on an enlarged scale.
Figure 1, Sheet A, represents on a small scale a general block plan of
the whole machine when complete; a portion of this plan on a larger
scale and in greater detail is also shewn on Sheet B, and a further
portion on the same enlarged scale, and also in plan, is shewn at Figure 1
on Sheet C. A cross section on the line A, B, of Figure 1, Sheet A, is
also shewn at Figure 2, Sheet A; another cross section on the line C, D,
of Figure 1, Sheet A, is shewn at Figure 1, Sheet D, and a longitudinal

page_text: vertical section on the line E, F, of Figure 1; Sheet A is also shewn at
Figure 2, Sheet C.
On Sheet D, Figures 2, 3, 4, 5, 6, and 7, shew further details of the
construction of the lathe; and Figures 1, 2, 3, 4, 5, and 6, on Sheet E,
shew details of the slide rest, and the mode of driving the leading screw
on a scale four times that of the Drawing, Figure 1, Sheet A, and twice
the size of the other detailed Drawings of the lathe before referred to.
This lathe is provided with a bed A. of considerable length, one end of
which is bolted to the main bed plate B as shewn at A*, and is free to
expand or contract endwise by change of temperature, moving on
slides placed at short intervals beneath, and supporting it as shewn at
Figures 5 and 7, Sheet D; on these beds may be fixed at any
convenient distance depending on the focal length of the lease or
reflector to be turned a pivot or axis D, shewn at Figures 2, 3, and 4,
Sheet D;
this pivot projects vertically upward from a massive square
plate E, and is capable of being adjusted by set screws passing through
bosses formed on the plate F, the stops or guides F serving to retain it
in its position. The pivot is also capable of adjustment endwise of the
beds by means of sorews which pass through bosses
in the stop
plates G, G, which are fastened by bolts to the lathe bed.
By reference to Figure 5, Sheet D, it will be seen that there are
undercut grooves H. running lengthwise of the beds for the purpose of
securing the plate F and pivot D firmly in the desired position. J, J,
represents a radius bar or frame moving at one end on the pivot D, it
consists of several portions bolted together depending on the length
that may be required. At its end opposite to the pivot it is enlarged at
J so as to embrace the lathe head K with its mandril Land face
plates a. This portion of the frame is supported at three points M,
each of which consist of two castings I have hemispherical recesses,
into which a steel ball N is fitted, see Figure 2, Sheet A; the upper
pieces are bolted to the radius frame J, J, and the lower ones lie
flatly and in even contact with the flat smooth surface of the bed plate
13, thus supporting this part of the radius frame on three bearings
placed in a triangular position with reference to each other, thus giving
great steadiness of support to this part of the lathe and admitting it to
move in any arc of a circle depending on the position of its pivot or
centre of motion. Antifriction wheels P are placed at distances along
the frame for its further support, the wheels resting ou plates Q

page_text: secured to the lathe bed where required, as shewn at Figures 5 and 6,
Slicet D.
A slide rest R and tool holder is attached to the radius frame so that
the operator (who stands in the space S) can adjust the depth of cut
and manage the turning operation.. The movement of the diamond
point of the cutting tool T is governed by the rotation of the leading
screw U, on which a small worm wheel V is fixed, see Figures 1, 2,
and 3, Sheet E. A worm W carries on its axis a small handle and a
fly wheel X. The periphery of the wheel receives a strap from an
overhead motion and moves the cutting point steadily in a curved line
from the exterior edge to the centre of the disc under operation, the
combined motion of the disc on the face plate and the radius bar pro-
|ducing an accurately spherical concave surface.
It will be understood that if the slide rest R be removed and screwed
on to the radius frame at the opposite end of the mandril at Y, that any
surface carried by the face plate a', and operated on by the cutting tool,
will acquire a convex spherical surface, and if a measuring rod be
mounted on the upper part. D# of the pivot D and the cutter set in
accordance therewith a convex surface may be formed of the precise
same radius as any concave surface that may have previously been
made on the other face plate, and thus a grinder or polisher may be
turned to fit either a concave or convex spherical lense or reflector with
great accuracy.
For turning glass 1 prefer to use a piece of carbon, or, as it is
generally called, black diamond, for roughing out the mass, and a piece
of ordinary diamond split or by preference ground to a proper cutting
angle for use when making the last smooth finishing cuts. A fine jet
of spirits of turpentine, naptha, or alcohol is directed on the cutting
edge of the tool all the time the lathe is in motion, and although I
prefer to use the diamond for these purposes other gems or substances of
suflicient hardness to cut glass efficiently may be employed in lieu
thereof. A spherical figure having been thus given to the lense or
reflector by the turning tool, its surface may be further perfected by
grinding and polishing in a manner similar to that by which lenses and
speculum metal reflectors have been heretofore ground and polished;
the figuring of the reflector as it is called, whereby a more or less truly
parabolic curve is given to the reflector may be performed in the usual

page_text: way or by other approved means, after which the reflector may have a
thin film of silver deposited thereon in the usual way, except when used
in the heleoscope, in which case it will reflect sufliciently the rays of the
sun without silvering.
In the manufacture of very large glass reflectors, for which this
system of turning is peculiarly adapted, it is desirable that plate glass
should be employed, as that material is readily obtainable. of large
superficial area, but comparatively thin in cross section, rarely exceed-
ing one inch and a half in thickness, hence it becomes necessary to
strengthen or support the reflector in such a manner that it is not bent
or deflected by its own weight when made to assume different positions
while in usc.
The necessary strength and rigidity may be given to large plate glass
reflectors by two different modes of treatment, the first of which consists
in fusing, or what may be called soldering together any desired number
of sheets of plate glass by the application of heat. For this purpose I
prefer to use plates as thick as can be readily obtained; they should be
ground flat, but need not be polished. I then prepare a creamy wash of
silicious and alkaline matters, such as is ordinarily used to glaze pottery-
ware, or I reduce by grinding to a very fine powder flint or other glass
of a more fusible kind than the plates are made of, and I make this
powdered glass into a creamy fluid with water with or without the
addition thereto of boracic acid, pearlash, or soda; and in either case 1
dip the circular discs of glass into this mixture, or I otherwise apply a
coating of it to their surfaces, and when it is quite dry the plates are to
be piled on each other to the desired thickness, and placed in a "bung"
or covered cylindrical case made of fire-clay, and gradually heated in a
pottery kiln up to such a temperature as will fuse the coating of mate-
rials, and cause a complete adhesion or union of the surfaces of the
several plates of glass. Some lime, sand, or plumbago in the form
of powder should be spread evenly over the lower internal surface
of the "
bung" for the glass to rest upon, and prevent its adhesion
thereto.
In some cases two or more plates may be united by simple fusion or
softening of the mass, or by the use of borax or alkalies only put in
small quantities between each layer of glass. Care must be taken that
the thick mass of glass thus formed is very gradually cooled before

page_text: removing it from the
1
bung
or case. Or in lieu of this mode of
building up thick masses of glass a single plate or sheet may be used
as a reflector by employing a strong and light permanent cellular
backing of metal for the comparatively thin glass reflector to
rest on.
The arrangement I prefer for this purpose consists of a cellular iron
casting, cylindrical in form, one of its circular facings being scraped and
ground to a true plane, the opposite face having openings into the
various cells or interstices formed between a series of ribs, which cross
the cylindrical casting radially or at right angles, and serve to give great
powers of resisting change of form in proportion to its weight; the
breadth of these ribs and the cylindrical part of the casting may be
The face having been truly
equal to about one-third of its diameter.
ground and fitted to the flat back of the glass reflector the latter is held
permanently in contact with the iron surface by exhausting the air from
between them, as herein-before described, in attaching the glass disc to
the face plate of the lathe, so that no distortion of the reflector can take
place unless the large cellular metal support is also deflected. To
prevent this it will be supported by thin steel straps in different direc-
tions within a cylindrical casting having suitable flanges for the purpose
of attaching it to the telescope or other instrument; and here I would
remark that I prefer to use steel straps, not only to support the reflector
in one direction as usually practiced, but to prevent its movement also
from side to side, in order that the reflector may be polished in its cell
or casc. This is the more necessary, because the cellular backing or
support for the glass is much wider than a solid reflector would be
made.
On Sheet F I have shown this mode of supporting the cellular casting
in section at Figure 20, and in elevation at Figure 21, where the screwed
ends of the steel straps D are shewn supported by bosses E formed on
the outside of the cell or case G; one of these bands is shewn detached
on a larger scale at Figure 22. There are in all S straps in sets of four
placed as near as possible at the edges of the cellular casting, and thus
governing or preventing its movement in all directions, as will be fully
understood by reference to Figure 23. Or in lieu of this mode the
cellular casting may be put into a cast-iron cylinder, the bottom of
which is closed, and is of such a diameter as will leave an annular
space
of about of an inch all round the cellular casting, and which

page_text: space may be filled with some more or less elastic material that can be
run into the cavity in a semi-fluid state, and when cold be capable of
forming an elastic cushion, giving an equal support to the cellular
casting, while its expansion and contraction by change of temperature
will not be restrained; or in lieu thereof several flat vulcanized rubber
bands may be stretched evenly over the whole cylindrical surface of the
cellular casting, and a flat dise of the same material made to cover its
lower surface. The whole may then be temporarily suspended within
the cylindrical casing, but without contact with it, and the intermediate
space between the exterior surface of the rubber and the case be filled
in with a fluid resinous matter capable of becoming quite solid.
When cold the embedded mass will thus be equally well supported on
all sides by such a thickness of clastic material as will allow of its
expansion or contraction by change of temperature, and admit also
of change of position without subjecting the cellular casting to such
strains as will cause the reflector attached to its surface to become
distorted.
•
When this last-named mode of supporting the reflector and cellular
casting is employed it will be preferable to turn a series of grooves or
rings on the sides and bottom of the casting, into which the bands of
rubber may to some extent expand, and thus yield to the pressures that
may be brought to bear upon them, and the space outside the rubber
may be filled in with fluid pitch, rosin, or turners' cement. In those
cases where steel straps are employed to support the cellular casting
to which the reflector is attached it will also be found advantageous to
fill in the space between it and the box in which it is so supported with
some soft elastic yielding matter, which will tend to prevent vibratory
motion in the mass so suspended, and by interposing a slow or bad con-
ductor of heat prevent rapid changes in the temperature of the metal
casting to which the reflector is attached.
A mixture of glue and treacle, such as is in general use for making
the elastic inking rollers used in printing machines may be used for this
purpose, or in lieu thereof very soft marine glue may be employed.
One of these cellular supports for a plate glass reflector is represented
on Sheet F of the annexed Drawings, where Figure 1 represents an
elevation of its flat face, showing the annular grooves on its surface and
the cross groove or passage, by means of which all the grooves com-
municate with a central hole; Figures 2 and 3 are sections through the

page_text: cells, shewing the central exhausted chamber i closed by a plate j, to
which a small stop-cock is affixed, seen better in the enlarged section,
The surface having
Figure 5, where these parts are more fully shewn.
been carefully scraped flat and ground to fit a disc of plate glass ka
flanged ring of iron is screwed on to the cellular casting, and when the
circular plate of glass is placed on the prepared surface, some fluid or
semi-fluid matter capable of becoming nearly solid when cold is poured
into the annular space m between the ring or hoop and the glass disc,
an elastic tube is then placed on the nozzle n of the stop-cock and the
chamber partially exhausted by an air pump; the stop-cock is then
closed, the elastic pipe removed, and the space round about it filled with
pitch or other like substance, which will effectually prevent the ingress
of air, and thus permanently retain the glass disc in contact with its
cellular support; a very thin film of olive oil may be laid on the iron
surface before the glass disc is brought in contact, as it will facilitate the
extremely small amount of sliding motion which the difference in the
expansion of cast iron and glass by change of temperature may render
necessary; in some cases the back of the cellular casting may be
supported by three levers or plates 9, as shewn in Figure 4; small
knobs on the ends of these levers press against the cellular casting
opposite the spaces marked p, Figure 2; the plates 9 are themselves
supported at their centres by adjusting screws, which pass through
the bottom of the box or cylinder in which the cellular casting is
enclosed.
In some cases where a short focus or deep curvature is required in
reflectors a sheet of plate glass may be cut circular and bent in the glass
bender's oven so as to be concave on one side and convex on the other.
A cellular metal backing having a concave face
be accurately
ground and fitted to the convex side of the bent disc of glass, which
may then be turned true in the lathe on its concave side and afterwards
finished in a similar manner to that herein-before described.
may
cross
On Sheet F of the annexed Drawings Figure 6 represents a
section through a concave reflector so made, and Figure 7 is a section
through the cells; and I would here remark that I prefer to unite the
glass disc to its iron support previous to the final grinding operation, and
in some cases even before the turning operation takes place in the lathe,
in which case the cellular casting with its glass surface may be chucked
on the lathe in the usual way; this mode of chucking the cellular

page_text: casting with its plate glass surface permanently fixed is represented in
the Drawings of the lathe shewn on Sheets A, B, and C, and although I
prefer to use a cellular iron casting for large reflectors a light and strong
backing may be made for reflectors of medium size in potteryware or
terra cotta moulded with ribs or cells similar to the iron cellular backing
before described, or for smaller sizes a piece of marble or slate may he
employed, and these substances may be reduced in weight by removing
a portion of the material hy boring or turning cells therein, as shewn at
Figures S, 9, and 10, Sheet F; this mode of making cellular support
by boring numerous holes may in some cases be preferable in lieu of
forming the cells by casting when iron or other metal is employed for
such purposes.
The lathe and diamond turning tool may also he employed for giving
to reflectors made of speculum metal a more correct figure than can be
obtained by casting alone, and thus assist in the development of a more
accurate figure in such reflectors.
When double concave or convex lenses or reflectors are required 1
prefer to form the face plate itself or a piece of metal or glass temporarily
attached thereto into a convex or concave surface corresponding precisely
with the configuration of the lense or reflector to be chucked thereon, so
that the glass disc is well and properly supported while having its second
side operated on in the lathe.
1 in all cases turn the edge of the discs truly circular before removing
them from the lathe, and in some cases I form a flat sunken portion at
the edge of the dise, so as to facilitate the concentric rechucking of the
glass, and insure the perfect parallelism of the two curved surfaces, and
the true concentric positions of these surfaces in reference to each other.
This will be readily understood by reference to Sheet F, where Figure 11.
is a cross section of a double convex lense so formed, and Figure 12 is a
section through a double concave lense, with a rebate or recess formed
at its external edge, as before described.
I
When the large glass reflector of the helioscope is used in the un-
silvered state a solid metal backing or support is inadmissible.
therefore in such a case use a hollow double cylinder of cast iron, the
inner and outer portion being united by webs. The end of the cylinder
on which the reflector is to be fastened has channels formed in it for the
purpose of retaining the glass in firm contact by the withdrawal of air

page_text: from beneath the annular portion of the reflector which rests on the
metal cylinder, and in some cases 1 make a central boss attached by a
disc or arms to the double cylinder, so as to form a support to the
central part of the reflector, and prevent its deflection when moved into
different positions. This mode of mounting a transparent reflector for
the heleoscope is shown on Sheet F, where Figure 18 is a side elevation;
Figure 14, a vertical section through the centre; and Figure 15, a section
on the line A, B, of Figure 14. Two plano-concave unsilvered glass
reflectors s, &, are cemented together so as to form a double concave
lense; their curved surfaces do not extend quite so far as their external
edges in order to leave a flat bearing surface to rest on the hollow ring
or double cylindrical part of the cast-iron frame or cylinder t. The
top surface of the part * is grooved, and small holes communicate with
the hollow part, from which air is pumped out. In order to retain the
lense in contact with its support inside the cylindrical casting t a second
casting is fixed; it is formed with a conical mound or projection,
whose sides are curved and are to be electroplated with silver and
highly polished, so that heat rays passing through the lenses s, s, and
falling thereon will be reflected and dispersed radially, passing out
which
through the openings left between the thin vertical bars &²,
connect the upper and lower parts of the casting t. The central support
thus given to the lense will prevent that deflection which a large thin
lense would suffer by alteration of position.
A modification of this plan of supporting thin glass reflectors of large
area is also shewn on Sheet F, where Figure 16 is a cross section, and
Figure 17 a plan of the same. A cast-iron circular frame A has a hollow
channel formed around its upper angle, and is provided with numerous
small holes for the purpose of holding on the plano-concave glass
reflector B by atmospheric pressure, as before described. The frame A
has radial arms A' uniting in a hollow central boss or chamber, which is
also exhausted of air. The upper surface of the arms, and also the
ring A must be ground flat, so that the plain side of the glass may fit
accurately, and come in contact with them, after which the flat face of
the arms and ring is to be filed or cut away so as to form a very thin or
knife edge, in order not to intercept any notable quantity of the light
which would otherwise pass through the glass. A narrow black line
may be drawn on the concave side opposite to the supporting arms to
prevent their interfering with the action of the reflector,

page_text: It will be understood that in all cases the central support of the
reflector will not interfere with the image formed, because the plane
diagonal mirror used in the Newtonian mode of construction shields the
centre of the reflector from parallel rays.
In other cases I fasten with varnish, balsam of Canada, gold size, or
other transparent" cement two plain concave discs of glass together,
putting their two flat sides in contact with or without a third parallel
sheet of stout glass between them for the purpose of rendering the
reflector so formed less liable to be bent or deflected while in use.
I form reflecting or refracting surfaces also by turning with a
diamond cutting tool the surface of discs of plate or other glass forming
successive annular rings or zones thereon capable of reflecting or
refracting solar or artificial light, such grooves or inclines being segments
or portions of a sphere, parabola, cône, or other desired form, depending
on their size and intended use. Two lenses so formed from the solid by
turning with a diamond tool on the lathe are shown in section at Figures
18 and 19 on Sheet F of the annexed Drawings, and it will be obvious
that lenses may thus be formed of large area on .comparatively thin
sheets of plate glass.

page_text: 5. The means for mounting a reflector with its backing in the telescope
cell, substantially as shown in respect to Figures 21, 22, and 23.
6. The construction of glass reflectors from a built up mass of sheets
fused together, substantially as described.
7. The construction of glass reflectors of helioscopes by building them
up of two or more discs truly fashioned and cemented the one to the
other, substantially as described.
In witness whereof, I, the said Henry Bessemer, have hereunio
set my hand and seal, this Thirtieth day of June, in the year of
our Lord One thousand eight hundred and seventy-six.
HENRY BESSEMER. (L.S.)