IMPROVEMENTS IN ACHROMATIC OBJECT GLASSES
Year: 1866

page_text: LETTERS PATENT to William Wray, of 1, Clifton Villas, Highgate Hill,
in the County of Middlesex, Practical Optician, for the Invention of
"IMPROVEMENTS IN ACHROMATIC OBJECT GLASSES,"
Ji
Sealed the 22nd September 1866, and dated the 31st March 1866.
PROVISIONAL SPECIFICATION left by the said William. Wray at the
Office of the Commissioners of Patents, with his Petition, on the
31st March 1866.
I WILLIAM WRAY; of 1, Clifton Villas, Highgate Hill, in the County of
Middlesex; Practical Optician, do hereby declare the nature of the said
Invention for "IMPROVEMENTS IN ACHROMATIC OBJECT GLASSES," to be as
follows:
*> .I.
My improvements relate to the achromatism of object glasses, whether they
are employed for telescopes, microscopes, or any other optical purposes.'
By the use of my Invention perfect achromatism is attained, and the
secondary and all sub-spectra are destroyed; therefore in a telescope, for
instance, constructed according to my Invention, however high the power
that is applied to the image formed by the object glass, the purple or other
tint usually found in luminous objects viewed by means of ordinary achromatic
[Price 8d.]

page_text: telescopes is not produced, the said purple or other tint being caused by
what is known as the "secondary spectrum." Objects viewed by means of a
telescope that has an object glass constructed according to my Invention are
entirely free from fringes of colour, and thus practically appear as perfectly in
their natural colours, even to the very edge, as they do to the naked eye.
The presumed impracticability, by Sir Isaac Newton and others, of con-
structing an achromatic object glass led to the general adoption of the reflecting
telescope (taking the telescope as an instance of the usefulness of this
Invention). This occurred in the last century, and has even at the present
time some adherents. Dollond's discovery, about the year 1758, of the
possibility of in some degree destroying the coloured fringes presented by
objects when viewed through refracting telescopes gave them to some extent
their natural advantages over the said reflecting telescopes, butitis confidently
believed that the practical application of this Invention will so entirely and
fully develope the advantages of the refracting telescope that no doubt can
exist of its general and most efficient application.
Taking microscopes as another instance of the usefulness of this Invention,
Tulley, in 1824, was the first person in England who constructed an
achromatic object glass adapted to compound microscopes; this he did by
combining 3 simple lenses. Mr. Lister, in 1829, substituted 3 compound
lenses at determinate intervals, and Andrew Ross, in 1837, successfully
constructed a microscope object glass of the greatest possible focal distance
that would correct central as well as oblique pencils of great angle, bearing in
mind the desired degree of amplification, and providing for great magnifying
power in the eye-piece. With all these arrangements, however, it is
important to notice that the "secondary spectrum"still exists in the beautiful
butscarcely perfect instruments constructed by the above-mentioned and other
men of science. My Invention, by removing the secondary spectrum and all
tendency thereto,accomplishes the achromatism of the microscope as perfectly
as it does that of the telescope, thereby increasing its application to the arts
and to the objects of investigative science.
It is well known that the impracticability of correcting irrationality of
the spectra of the flint and crown glass was the cause of failure of Dollond's
discovery to produce a perfectly achromatic object glass, inasmuch as that
irrationality brought a "secondary" or residual spectrum into existence, and
if still another correction (by means of chromatic dispersion only) was made
another residual or "tertiary" spectrum was found to exist, and so on, whenever
somecoloursofthespectrum were neutralized by the introductionof other media,
the impossibilityof obtainingtwo spectra that should overlap when thrownupon

page_text: a screen, or that should be rational to one another, being the seeming barrier
to the attainment of perfect achromatism in object glasses. In accordance
with these views Dr. Blair, towards the end of last century, attempted to
obtain an achromatic object glass with no residual spectrum by producing a
concave lens which should have no irrationality with convex crown glasses,
and for this purpose heemployed a solution of chloride of antimony and other
chemical solutions; the practical difficulties of this plan, however, caused it
rapidly to fall into disuse.
According to my Invention the irrationality existing between flint and
crown glass is destroyed by the introduction of a third medium, as a thin
meniscus lens placed between the flint and crowuglass lenses of the compound
lens, the said meniscus lens being composed of certain substances or compounds
herein-after mentioned, in all of which the irrationality with any given crown
or flint glass is higher in proportion than their mean dispersion. The high
irrationality of the meniscus lens is (in contradistinction to Dr. Blair's plan)
opposed to the comparatively low irrationality of the crown and flint lenses in
the method employed in my Invention. All the substances used for the said
meniscus lens have a high dispersive power; they are the essential oils of
cassia, cloves, bitter almonds, and cummin, also oil of Barbadoes Lar, castor
oil, and creosote, and the balsams of Tolu, Peru, Styrax, and Canada, also
bisulphide of carbon, and the gums copal, damar, and benzoin. The material
preferred is oil of cassia, eitber alone or slightly admixed with gum copal or
gum damar, or with Canada balsam or balsam of Styrax.
The thickness and curvature of the meniscus film are so calculated that
whilst it cements the 2 glasses firmly together, it also completely balances
the irrationality existing between any given flint and crown or plate glasses.
I have also found the boro-silicate of lead glass of density 5.45 introduced for
optical and polarization experiments by Dr. Faraday in 1845 answer as a
thin meniscus corrective lens as above set forth, but it requires a more
complex arrangement to bring out its corrective powers than the essential
oils, balsams, and gums herein-before-mentioned. The said meniscus
corrective lens may be applied to instruments constructed on the dialytic
form. The negative solution of the question of achromatism may be used when
heavy flint glass is employed; in this case the deuse glass is an internal
concave lens, and the irrationality is balanced by 2 external convexes which
differ in their irrationality.
The advantages of the perfect achromatic object glass are no chromatic
confusion, better definition, increased light and magnifying power, absolute
permanency, no deposition of dust or due upon the internal surfaces, absolute

page_text: coincidence of visual and actinic focus in photographic lenses, and the
possibility of employing telescope object glasses of short foci without dis-
advantage..
SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said William Wray in the Great Seal Patent Office on the 29th
September 1866.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, WILLIAM
WRAT, of No. 1, Clifton Villas, Highgate Hill, in the County of Middlesex,
Practical Optician, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Thirty-first day of March, in the year of our Lord
One thousand eight hundred and sixty-six, in the twenty-ninth year of Her
reign, did, for Herself, Her heirs and successors, give and grant unto me,
the said William Wray, Her special licence that I, the said William
Wray, my executors, administrators, and assigns, or such others as I, the
said William Wray, my executors, administrators, and assigns, should at
any time agree with, and no others, from time to time and at all times there-
after 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
ACHROMATIC OBJECT GLASSES," upon the condition (amongst others) that I, the
said William Wray, 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.
NOW KNOW YE, that I, the said William Wray, do hereby declare the
nature of my said Invention, and in what manner the same is to be per-
formed, to be particularly described and ascertained in and by the following
statement:
My improvements relate to the achromatism of object glasses, whether
they are employed for telescopes, microscopes, or any other optical purposes.
By the use of my Invention perfect achromatism is obtained, and the
secondary and all sub-spectra are destroyed; therefore in a telescope, for
instance, constructed according to my Invention, however high the power that

page_text: is applied to the image formed by the object glass, the purple or other tints
usually found attending luminous objects when viewed by means of ordinary.
achromatic telescopes are not produced, the said purple or other tints.being.
caused by what is known as the "secondary spectrum." Objects viewed by
means of a telescope that has an object glass constructed according to my
Invention are entirely free from fringes of colour, and thus practically appear
as perfectly in their natural colours, even to the very edge, as they do to the
naked eye.
The presumed impracticability, by Sir I. Newton and others, of constructing
the achromatic object glass led to the general adoption of the reflecting
telescope (taking the telescope as an instance of the usefulness of this Inven-
tion). This occurred in the last century, and the reflecting telescope has
even at the present time some adherents. Dollond's discovery, about the
year 175, of the possibility of (in some degree) destroying the coloured
fringes presented by objects when viewed by refracting telescopes gave them
to some extent their natural advantages over the said reflecting telescopes
but it is confidently believed that the practical application of this Invention
will so entirely and fully develope the advantages of the refracting instrument
that no doubt can exist of its general and most efficient application.
Microscopes may be taken as another instance of the usefulness of this
Ivnention. Tulley, in 1824, was the first person in England who constructed
an achromatic object glass adapted to compound microscopes; this he did by
combining three simple lenses. Mr. Lister, in 1829, substituted three
compound lenses at determinate intervals for the said simple lenses. Andrew
Ross, in 1837, successfully constructed an object glass that would correct
central as well as oblique pencils of great angle. With all these arrangements
however, it is important to notice that the "secondary spectrum" still exists
in the beautiful but scarcely perfect instruments constructed by the above-
named and other men of science. My Invention, by removing the secondary
spectrum and all tendency thereto, accomplishes the achromatism of the
microscope as perfectly as it does that of the telescope, thereby increasing its
application to the arts and to the objects of investigative science.
It is well known that the impracticability of correcting the irrationality of
the spectra of flint and crown glass was the cause of failure of Dollond's
discovery to produce a perfectly achromatic object glass, inasmuch as that
irrationality brought a secondary or residual spectrum into existence, and if
still another correction (by means of chromatic dispersion only) was made
another residual or " tertiary": spectrum was found to exist, and so on when-
euer some colours of the spectrum were neutralized by the introduction of

page_text: other media, the impossibility of obtaining two spectra that should overlap
when thrown upon a screen or that should be rational to one another being
the seeming barrier to the attainment of perfect achromatism in object glasses.
In accordance with these views Dr. Blair, towards the end of last century,
attempted to obtain an achromatic object glass with no residual spectrum by
producing a concave leus which should have no irrationality with convex
crown glasses, and for this purpose he employed a solution of chloride of
antimony and other chemical solutions; the practical difficulties of this plan
however caused it rapidly to fall into disuse.
According to my Invention the irrationality existing between flint and
crown glass is destroyed by the introduction of a third medium, as a thin
meniscus lens, placed between the flint and crown glass lenses of the compound
lens, the said meniscus lens being composed of certain substances or com-
pounds herein-after mentioned, in all of which the irrationality with any
given crown or flint glass is higher in proportion than their mean dispersion
the high irrationality of the meniscus lens is (in contradistinction to Dr. Blair's
plan) opposed to the comparatively low irrationality of the crown and flint
lenses in the method employed in my Invention. All the substances used for
the said meniscus lens have a high dispersive power, they are the essential
oils of cassia, cloves, bitter almonds, and cummin; also oil of Barbadoes tar,
castor oil, and creosote, as well as the balsams of Tolu, Peru, Styrax, and
Canada, also the gums copal, damar, and benzoin, also bisulphide of carbon.
The material which is preferred is oil of cassia, either alone or admixed with
gum copal, or gum damar, or with castor oil, Canada balsam, or balsam of
Styrax. The thickness and curvature of the meniscus film are so calculated
that whilst it cements the two glasses firmly together it also completely
balances the irrationality existing between any given flint and crown or plate
glasses. I have also found the boro-silicate of lead glass of density 5-45
introduced for optical and polarization experiments by Dr. Faraday in 1845
answer as a thin meniscus corrective lens as above set forth, but it requires a
more complex arrangement to bring out its corrective powers than the essential
oils, balsams, and gums herein-before mentioned. The said meniscus
corrective lens may be applied to instruments constructed on the dialytic
form. The negative solution of the question of achromatism may be used
when heavy flint glass is employed. In this case the dense glass is an internal
concave lens, and the irrationality is balanced by two external convex lenses
which differ in their irrationality.
The advantages of the perfectly achromatic object glass are no chromatic
confusion, better definition, increased light and magnifying power, absolute

page_text: permanency, no deposition of dust or dew upon the internal surfaces,
absolute coincidence of visual and actinic foci in photographic lenses, and the
possibility (so far as the object glass is concerned) of employing telescopic
object glasses of short foci without disadvantage.
Having explained the nature of my Invention, I now proceed to describe
with reference to the accompanying Drawings the manner in which thesameis
to be performed.
The best combination for the object glass of a telescope I find to consist of
a triple arrangement of glass lenses with two cement films, one enclosed on
each side of the concave lens, as shewn in Figure I.; the next to this in
quality is a double arrangement of glass lenses and a cement film between
them, as represented in Figure II. In both cases the convex side of the
combination is presented towards the object, and the concave or posterior
surface is towards the eye. The cement film is composed of a mixture of
oil of cassia with castor oil or with Canada balsam, nearly in the proportion
of equal parts of each, but this proportion is to be adjusted until the
irrationality of the film itself exactly equals that of the flint and crown
glasses.
In Figure I., a represents the crown glass lens next to the object, a and ẞ
being the convex surfaces of the said lens; its radii of curvature are equal
and for a focal distance of 36 inches the measure of each of those radii is
10.50 inches; b is the intermediate flint glass lens, y and 8 being the concave
surfaces thereof; its radii of curvature are also equal, and for a focal distance
of 36 inches as before mentioned the measure of each radius is 9-51 inches ;
c is the crown glass lens next to the eye-piece of the telescope, the inside
convex surface of which is marked e, its radius of curvature being 10-50
inches; the convex surface next the eye-piece is marked, and should have
a radius of curvature equal to 55.50 inches. These calculations are made
upon the supposition that the crown glass lenses have a density of 2.5, and
the flint glass lenses a density of 3-25, the density of distilled water being
unity; and yare the two cement films, which being composed of the afore-
said mixture of oil of cassia with Canada balsam or with castor oil render
the compound object glass truly achromatic and free from all trace of
outstanding colour.
Figure 11. represents the best possible form of object glass with two glass
lenses, where a is the convex crown glass lens placed next the object, and b
is the flint glass lens towards the eye-piece. The radii of the surfaces of the
crown glass lens for a focal distance of 36 inches are a=9-36 inches,
B=12.60 inches, this lens is therefore a double convex lens of unequal

page_text: radii of curvature with the deepest side towards the object. The flint glass
lens has radii -9.36 inches, and 8-35-00 inches, and is therefore a double
concave lens of unequal radii of curvature with the shallowest side towards
the eye-piece. The cement film must be so proportioned in irrationality as
to correct the whole of the outstanding colour. In this case the density of
the crown glass lens is supposed to be 2.5, and that of the flint glass 3-65.
The radii of curvature of the surfaces of the various lenses for other focal
distances may be obtained by simple proportion. For small variations in the
density of the glass lenses a corresponding variation is made in the power of
the cement film. The power of correcting irrationality and the dispersion of
the cement film increases in direct proportion to the amount of the oil
of cassia used. This adjustment of the optical power of the semi-fluid
meniscus lens or lenses to produce a perfectly achromatic object glass is an
important feature in the Invention. The same materials and a similar modi-
fication of lenses and proportionate curves answer for the object glasses of
microscopes, the radii being so proportioned as to give the focal power desired.
In a double object glass the radii of curvature for a glass of one inch in focal
length (reckoning from the outer curve of the crown glass, which is placed
towards the eye-piece) are respectively, 1st, 0-26250 inches; 2nd, 0-35625
inches; 3rd, 0.25625 inches; 4, 1-00000 inch. For a triple object
glass also with a focal length of one inch (reckoning the sequence of curves
in a similar manner to that observed in the double object glass) the radii are
respectively, 1st, 0.277 inches; 2nd, 0-277 inches; 3rd, 0-250 inches;
4th, 0-250 inches; 5th, 0-277 inches; 6th, 1-544 inches. These object
glasses for the microscope may be used either separately or combined, and
their radii and correcting film may be varied to suit different densities of glass
so as to make the final result truly achromatic. Although all the materials
and combinations herein-before set forth may be used for the cement film.
practice and experience has shown the extreme value of the mixture of oil of
cassia with Canada balsam or with castor oil for that purpose; therefore it is
much preferred to carry out the Invention by means of one of the latter
mixtures. Similarly, Dr. Faraday's or other heavy glass may be employed
as a material for producing perfect achromatism as herein-before set forth,
but it is not preferred, owing to the objections to its use that were believed to
exist, and which have since been confirmed. To keep the cement film in its
place, and to prevent the formation of bubbles between the lenses of this
achromatic combination by hard usage, &c., each edge of the intermediate
concave lens in the triple object glass and the inner edge of the concave lens
in the double object glass is finely chamfered in the lathe. This provision

page_text: enables the two or three lenses (as the case may be) to bear equally upon
each other throughout their circumferences, and to be in such perfect
contact when fixed in their mounting as to seal hermetically the film or films
of cement.
Having now described the nature of my Invention, and the manner in
which the same is to be performed, I wish it to be understood that I do not
limit myself to the precise sizes, dimensions, densities, and curves herein set
forth and shewn in the accompanying Drawings, as the same may be varied
without departing from the nature of my Invention; but I claim as my
Invention of "Improvements in Achromatic Object Glasses" the processes
and methods herein-before described and illnstrated by the Figures in the
Drawings accompanying as examples of the application of my Invention.
In witness whercof, I, the said William Wray, have hereunto set my
hand and seal, this Twenty-eighth day of September, in the year of
our Lord One thousand eight hundred and sixty-six.
Witness,
J. TURNBULL.
W WRAY. (L.S.)