Improvements relating to the Manufacture of Mirrors.
Year: 1891

page_text: Date of Application, 29th Dec., 1891-Accepted, 30th Jan., 1892
COMPLETE SPECIFICATION.
Communicated from abroad by JOHN JACOBSON, Post Office Address, Box 24,
Station A, Boston, Massachusetts, United States of America, Artist.]
Improvements relating to the Manufacture of Mirrors.
1, HENRY HARRIS LAKE, of the firm of Haseltine Lake & Co., Patent
Agents, 15 Southampton Buildings, in the County of Middlesex, do hereby
declare the nature of this invention and in what manner the same is to be
performed to be particularly described and ascertained in and by the following
statement :-
This invention relates to metallic surfaced mirrors or specula and in the method
or process employed for the manufacture of the same.
My improved method is especially valuable in the production of mirrors having
accurate surfaces, either plane concave or convex, for optical purposes, although it
may be used for producing mirrors or reflectors for ordinary uses, in which such
extreme accuracy is not necessary.
Curved mirrors or reflectors for optical instruments are now generally made by
accurately grinding a glass base which is to support the reflecting surface to the
required curvature, carefully polishing the same, and then coating the surface with
silver and finally polishing the outer exposed surface of the silver.
These operations necessitate the expenditure of a large amount of skilled labour
so that the resulting reflector is very costly.
According to the present invention a mirror is produced in which a polished
reflecting film receives its polish and curvature of whatever shape may be required
from an accurately shaped and finished form on which the said film is deposited
and from which it is detached after being first connected, while still on
the form, with a backing or support which may be of cement or some material
that can be applied while in plastic condition so that it will receive the proper
shape through the film from the form on which the film is deposited and will
then harden and adhere to the film more strongly than the latter does to the form
so that the film with its attached supporting base may be removed from the said
form.
In some cases the base may be of a rigid material such as glass, approximately
shaped, as may be done by moulding the glass, and a layer of cement material
interposed between the film while on the form and the approximately shaped
surface of the said base so that the said cement will connect the film with the base
and will also correct any inaccuracies of the surface of the said base, or in other
words the cement itself will constitute the accurate shape of surface required for the
mirror.
It is desirable in some cases in making mirrors in accordance with the present
invention, to slightly thicken the reflecting film by depositing copper upon the
silver after it has been deposited on the form as hereinafter described.
In order to produce the best results, however, such additional deposit of copper
should be extremely thin the object of the said deposit being as it were to fill or
cover the pores of the silver deposit so that the cement applied to the back cannot
work through and affect the exposed surface of the mirror.
When such an additional film is to be deposited it may be done by electro
deposition upon the back of the silver film, but in order to make an electro deposit
upon such a film it is necessary to prepare the form (before the silver is deposited
thereon) around the edges in such a way as to prevent the entrance of the electro-
lytic solution between the film and form.
Such preparation may be effected either by slightly roughening the surface of

page_text: the form around its edges to give it the characteristics of ground glass or by
applying paraffine thereto.
When silver is deposited upon a smooth polished surface without such protection
around the edges and is then subjec:ed to the action of electro deposition it
invariably happens that the electrolytic solution will creep in between the silver
film and the form, thus blistering the film and rendering it utterly worthless as an
optical reflector, but by protecting the edges as above described either by
roughening or by the application of paraffine the electro deposition can be performed,
successfully.
The improved process of making mirrors which forms the subject of this invention
consists as follows
First, a form is prepared having its surface carefully shaped and finished,
its form being the counter-part of the surface required for the mirror. That is
if a concave parabolic mirror is to be produced, the form is ground accurately to
present the curvature of the mirror on a convex surface. Such form is usually
made of glass and prepared in the usual manner of preparing lenses or mirrors;
the surface being shaped with the required degree of accuracy and highly polished.
The surface is then coated with silver by the usual process of depositing silver
from a solution, and the surface of the film so deposited receives from the form the
accuracy in shape and polish or finish or surface that is required for the finished
mirror, but such form has no strength or rigidity other than what is derived from
its contact with the form and consequently cannot without further preparation be
detached from the form to enable its surface produced by contact with the form to
be used as a reflector.
In order to enable the metallic film to be removed from the form without
bending or becoming distorted, a rigid base is connected with it while it is still on
the form the said base being composed wholly or in part of cement or material
which may be applied while in a plastic condition so that it will itself receive the
proper shape from the original form on which the film is deposited.
In some cases the base may consist mainly of a rigid material such for example,
as glass applied to and connected with the film while still on the base by a layer of
cement which secures the attachment of the film to the glass base and also by its
plasticity receives the accurate shape from the form on which the film was
deposited, this accurate shaping of the backing while in plastic condition being an
essential part of the invention.
In some cases before the cement backing is applied, the silver film is protected
by a slight additional film of baser metal, such as copper, which may be electro-
deposited thereon, in which case the form must be protected around its edges
against the entrance of the electrolytic solution, between it and the silver film, as
herein before mentioned.
In the accompanying drawing,
Figure is a sectional view of a form such as may be employed for producing a
concave mirror in accordance with this invention.
Figure 2 is a similar section showing the form with the metallic film applied
thereto, and the base or rigid support for said metallic film attached to the same
while still on the form,
Figure 3 is a similar section of the finished mirror detached, from the form,
Figure 4 is a face view of a form having its edges protected to enable an
electro-deposit to be produced on the back of a film originally deposited on said
form.
Figure 5 is a sectional view illustrating the operation for making the said
electro-deposit and
Figure 6 is a sectional view of the finished mirror, the face of which is composed
wholly of cement.
The form a should be made of very hard flint glass and have its surface a
prepared by the usual optical processes so as to give both in shape and finish a true
optical surface of high polish which is the converse or counterpart of the surface

page_text: required for the mirror, being concave if convex mirrors are to be made, and
convex as shown in Figure 1, if concave mirrors are to be made therefrom.
The said surface a' has then deposited upon it the film, usually of metallic silver,
which is represented at and which may be deposited chemically from a solution
in the well known manner.
In some cases it is desirable to slightly thicken or protect the film before the
supporting backing is applied thereto, by an additional deposit of metal which
fills the pores thereof and prevents the plastic material of which the back is
formed from entering the said pores and affecting the reflecting surface of the
film.
When such step is to be provided for, the form a has to be protected around its
edges as shown at a³, Figure 4, either by slightly roughening it giving the surface
the character of ground glass, or by applying paraffine around the edges before the
silver deposit is produced thereon, after which the form having its edges so
protected and the reflecting film deposited thereon, as before described, is subjected
to the operation of electro-deposition as indicated in Figure 5, by which a slight
additional amount of metal preferably copper, is deposited thereon, the protection a
before described, preventing the electrolytic solution from entering between the
silver film and the glass form as it would do if the latter had a complete polished
surface without such peripheral protection above described.
The film while still on the form has then connected with it, the backing which is
to form the support of the said film when used as a reflector, the said backing
being composed wholly or in part of cement, which term is intended to include any
suitable material which may be applied while in plastic condition and will
subsequently set or become rigid, having assumed while in its plastic condition the
accurate shape of the form, and also become sufficiently adherent to the film to
cause the latter to remain fixed on the backing and be detached therewith from the
form.
In some cases the backing may be composed wholly of cement as shown at c,
Figure 6, which may however, for large surfaces have stiffening pieces embedded
in it, or it may be composed in part of a rigid piece d such as shown in Figures 2
and 3, which may be of glass having its surface adjacent to the film shaped roughly
or approximately like the surface required for the reflector.
Such a base could be cheaply formed of pressed glass and could be connected
with the film while the latter is still on the form, as shown in Figure 2, by an
interposed layer of cement, which by its adhesiveness will connect the film securely
with the main body of the base and which owing to its plasticity will assume the
accurate shape required for the reflecting surface.
Gum shellac applied in a molten condition has been found to answer well either
as a cement to connect the film with a backing such as shown at d or to compose
the entire backing as shown at c Figure 6, but other analogous materials which
are plastic under certain conditions and subsequently set or become rigid may
be used.
After the backing has been applied to the metallic film as shown in Figure 2,
and the cement of the backing is thoroughly set or hardened, the said backing and
connected film may be detached from the form, when the film will have the
accurate and highly polished surface of exactly the same shape and quality as
that of the form a the outer exposed surface of the film however having the
opposite curvature to the outer exposed surface of the form, when curved surfaces
are used.
The great advantage of this process is that after a single form has been produced
with the required accuracy any number of mirrors may be produced therefrom
which have all the required properties of accuracy and durability necessary
for use in optical instruments and may thus be produced in numbers at a
very small cost in comparison with the cost of the mirrors made by the old
process.
Moreover the cost of production is so small that mirrors having an exposed

page_text: metallic surface of the kind represented in Figure 6, may be used for ordinary
looking glasses, or reflectors, that is, as a substitute for those mirrors in which the
film is deposited and remains supported upon glass, and in which the light passes to
the reflecting surface through the glass.
For such uses, the polished reflecting surface of the film may be covered with a
transparent film of some suitable colorless material, or varnish, to protect the same
from the air, as such a protecting film will not injure the reflecting surface for
anything except accurate optical work.
By this process moreover, any peculiar character of surface may be produced,
such for example, as the finely striated optical gratings. Such gratings are now
made by ruling the surface of a piece of speculum metal and are very expensive.
Such a ruled grating may however, be employed as the form in carrying out the
present invention, if used precisely as the form a herein described and from it may
be reproduced an indefinite number of gratings having the same optical surface,
which is produced on the film deposited on the original grating, which film may be
of any suitable material capable of such deposition, silver being the most desirable,
and by the term "Inirror is meant any article having an optical surface or one
capable of reflecting or dispersing or producing other optical effects upon light or
other rays.
I am aware that it has been proposed to produce reflectors by depositing a film
of silver on a form and subsequently coating the same by electrically depositing a
baser metal on the back of the silver film, to a sufficient thickness to support the
said film but such a process is impracticable owing to the fact that if a sufficient
amount of baser metal is deposited to afford sufficient rigidity the resulting mirror
is very heavy and furthermore, the expansion and contraction of such a metallic
body by changes of temperature or by unequal heating of the same, produces
distortion which renders the mirror unsuitable for optical purposes while it would
be altogether too expensive to be used for ordinary reflectors.
Having now particularly described and ascertained the nature of this invention
and in what manner the same is to be performed as communicated to me by my
foreign correspondent I declare that what I claim is:
First. The herein described method or process of manufacturing metallic
surfaced mirrors which consists in depositing upon a perfectly finished mould or
form, a film of metal from solution, the reflecting face of which is completed by
contact with the said form, and applying a backing composed wholly or in part of
cement to said film and finally removing the film and connected backing from the
mould, substantially as described.
Second. A form adapted to receive a metallic film the edges of which form are
rendered proof against the entrance of a liquid between the said form and film by
roughening the same or applying paraffine thereto, whereby the said form may be
used to receive a deposit of metallic silver or other metal, precipitated thereon
from a solution and may subsequently receive a further deposit of metal, as for
example, copper by electro deposition on said previously deposited film.
Dated this 29th day of December 1891.
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, W.C., Agents for the Applicant.