A METHOD OF IMPROVING, THE REFRACTING TELESCOPE AND OTHER DIOPTRICAL INSTRUMENTS
Year: 1791

page_text: TO ALL TO WHOM THESE PRESENTS SHALL COME, I, ROBERT
BLAIR, Surgeon in the Royal Navy, send greeting:
WHEREAS: His most Excellent Majesty King George the Third did, by
His Letters Patent under the Great Seal of great Britain, bearing date at
Westminster, the Fourth day of April, in the thirty-first year of His reign,
give and grant unto me, the said Robert Blair, His especial licence that I, the
said Robert Blair, during the term of years therein expressed, should and
lawfully might make, use, exercise, and vend, within England, Wales, and the
Town of Berwick-upon-Tweed, my Invention of "A METHOD OF IMPROVING,
THE REFRACTING TELESCOPE AND OTHER DIOPTRICAL INSTRUMENTS;" in-which
said Letters Patent thero is contained a proviso, obliging me, the said
Robert Blair, under my hand and seal, to cause a particular description of the
nature of my said Invention, and in what manner the same is to be performed,.
to be inrolled in His Majesty's High Court of Chancery within one calendar
month after the date of the said recited Letters Patent, as in and by the
same (relation being thereunto had) may more more fully and at large-
appear.
NOW KNOW YE, that in compliance with the said proviso, I, the said
Robert Blair, do hereby declare that my said Invention of a method of
improving the refracting telescope and other dioptrical instruments is described
in manner following (that is to say):-
In order to state clearly the nature of my new Invention for improving the
refracting telescopes and other dioptrical instruments, it will be necessary first

page_text: to explain the principal cause of their present imperfection, which I discovered
in the course of my experiments, as it is in removing this imperfection that
the improvement consists. The indistinctness of refracting telescopes with
single object glasses arises principally from the unequal refrangibility of the
rays of which light is composed, which prevents their being converged by
refraction to one point; it has been supposed that this imperfection is entirely
removed by composing the object glass of a convex and concave lens, which
differ in dispersive power, and that the cause why large apertures cannot be
given to telescopes with such compound object glasses is, the imperfection of
flint glass, of which the concave lens is generally made. But on constructing
object glasses with large apertures, by using fluid dispersive mediums, free
from the faults complained of in flint glass, I found that the imperfection
arose from a defect in the principle of these instruments, more than from any
fault in the execution. The defect in the principle is this: flint glass refracts
the green light considerably less than crown glass, in proportion to the whole
refraction of red and violet light, so that when the divergency of the red and
violet light, caused by the refraction of the two mediums, is equal, the diver-
gency of the red and green light is always greater in the crown glass than in
the flint glass, and the divergency of the violet and green light is always less
in the crown glass than in the flint glass. Those who are conversant in
optical studies will perceive from this, that it is impossible to unite all the
rays by any combination of these two mediums; for the correction of colour
is most perfect when the red and violet light is united, and when this is effected
the green light will always be refracted more than this united red and, violet
light, and, consequently, cannot be converged to the same point with the
united red and violet light. I found this property to obtain in most dispersive
mediums, sucli as essential oils, and metallic solutions of many different kinds,
as well as in flint glass. After a great variety of experiments, I discovered a
class of dispersive mediums with very different properties from the above. The
marine and nitrous acids, which are dispersive fluids of considerable strength,
instead of refracting the green light less than crown glass in' proportion to the
whole refraction of the red and violet light, were found to refract the green
light more than crown glass in proportion to the whole refraction of the red
and violet light; I therefore mixed these two kinds of dispersive mediums,
and thus obtained a medium which disperses the rays much more tlian crown
glass, and yet causes all of them to diverge accurately in the same proportion
in which they are made to diverge by the refraction of crown glass, which is
the desideratum required to remove entirely the aberration from the uiequal
refrangibility of light. It is for this Invention alone, of employing a dispersivo.

page_text: medium which seperates by its refraction the several kinds of rays, either
exactly in the same proportion in which they are separated by the refraction
of crown glass, or moro nearly in that proportion than flint glass separates
them, that the present Patent is granted.
And it is hereby set forth and declared, that those persons only will be
deemed to encroach on the Patent, whereof this is the Specification, who,
without my permission shall employ mediums in the construction of refracting
telescopes, refracting microscopes, or other dioptrical instruments, in order to:
remove that colour and indistinctness which arise from the dispersive and.
indispersivo mediums, not separating the several orders of rays proportionally,
the method of diminishing or removing which colour was unknown till
discovered by my experiments. Whether or not this has been done will
appear on viewing any bright object with the half of the object glass covered,
or when the eye glass is not adjusted to distinct vision; for in both cases all:
refracting telescopes constructed previous to my Invention will discover moro
or less of colour, according to tho largeness of the aperture of the object glass.
in proportion to its focal distance. In the most perfect constructions of
achromatic telescopes this colour will be purple and green, and if the spheres
of the convex and concave lenses aro not properly proportioned it will incline
more or less to red and blue; a diminution of this colour in any object glass
will appear from comparing it with one of crown and flint glass of the samo
diameter and focal length.
I have thought it requisite thus fully to describe the principle of the
Invention, that no person acquainted with the theory and practice of optics
may remain in any doubt concerning it. To dioptrical instruments con-
structed upon this principle I apply the term aplanatic, which denotes the
absence of aberration, in order to distinguish them from those which have
with impropriety been stiled achromatic. The dispersive medium which I
have found to answer best is a solution of antimony or mercury in the marine
acid, but a variety of others may be used for the same purpose, and it is
possible to remove the colour by a combination of two essential oils with glass, 1
but in a more complex and less effectual way, than by using one dispersive
medium, as above stated. It is well known that besides the aberration from
unequal refrangibility, it it necessary to correct also the aberration from the
spherical figures of lenses, but this has no connection with the Invention above
specified. The colour may be entirely removed by using only one concave
lens, formed of a dispersive medium, combined with one or with two convex
lenses formed of an indispersive medium; and if the dispersive medium be
more dense than the indispersive medium, the spherical aberration may be

page_text: also corrected without any addition; but if the dispersivo medium maile
use of be of less density than the indispersive medium, an additional lens
becomes necessary to remove the spherical aberration. The cavity which in
this case is formed between this additional lens and the former may either
remain. empty, or may be filled with spirit of wine, or any transparent
indispersive fluid, merely to prevent the loss of light which would other-
wiso arise from reflexion. The choice of the spheres to which the lenses
require to be ground admits of great variety, and no general rule can be given
respecting this proportion, because it depends entirely on the densities and
dispersive qualities of the mediums employed, which sellom turn out exactly
the samo, even when made by the same process; at different times; but the
intelligent optician, after examining the proportios of the mediums, will not be
at a loss in proportioning the spherical surfaces of the lenses to them; and in
any particular case the spheres employed may be found more accurately from
examining the instruments themselves than could be communicated by any
description or figures.
In witness whereof, I, the said Robert Blair, have hereunto set my handl
and seal, this Twenty-seventh day of April, in the year of our Lord One
that caso mad
provided.
Inrolled the samo Twenty-ninth day of April, in the year above
mentioned.