A METHOD TO IMPROVE THE REFRACTING TELLE- SCOPE AND OTHER OPTICAL INSTRUMENTS
Year: 1785

page_text: TO ALL TO WHOM THESE PRESENTS SHALL COME, I, ROBERT
BLAIR, Surgeon in the Royal Navy, send grecting.
WHEREAS His most Excellent Majesty King George the Third did, by
His Letters Patent under the Great Seal of Great Britain, bearing date the
Twenty-sixth day of April, in the twenty-fifth 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 use, exercise, and vend, within England, Wales, and Town of Berwick-
upon-Tweed, my Invention of "A METHOD TO IMPROVE THE REFRACTING TELLE-
SCOPE AND OTHER OPTICAL INSTRUMENTS;" in which said Letters Patent there 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
how 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 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 to
Improve the Refracting Telescope and other Optical Instruments is described
in the manner following (that is to say):-
What is chiefly wanting to bring the refracting telescope to greater perfec-
tion is, a method of encreasing the aperture or size of the largest glass called
the object glass, so as to admit a sufficient quantity of light to render objects
seen through them sufficiently luminous and distinct when greatly magnified.

page_text: From certain errors which arise from the constitution of light, and the
spherical figure of the lenses by which it is collected, this encrease of aperture
could only be effected in tellescopes of the old construction by encreasing their
lengths to a very inconvenient degree. The discovery of a certain quality in
flint glass, namely, that it disperses light refracted through it in a greater
degree than other kinds of glass, opened a way to shorten and improve those
telescopes in which object glasses of a large size are not required. But from
the imperfections of the glass in which this property was discovered, and its
unfitness for optical purposes, it has been found impossible to make these
compound object glasses larger or even equal in size to those of the old
construction. Many attempts have been made both in this country and abroad
to get over this difficulty, which is deemed of so much consequence that a
public reward has been offered to whoever shall succeed. By an extensive
course of experiments- it hath been discovered that there are a variety of
transparent fluids which possess this dispersive property in an equal and some
of them in a much superior degree to flint glass, and, being free from those
imperfections which occasion irregularity in the refraction of light, afford a
method of bringing the refracting telescope to equal a greater perfection than
could be done by means of flint glass, even if it could be had in the degree
of perfection which is required, for, if opposite refractions are made use of in
the common way, the great dispersive power discovered in some of the fluids
gives them the advantage of flint glass for this purpose. But this method of
opposite refractions is less perfect than could be wished, for by the first refrac-
tion the heterogeneal rays are necessarily separated, and the most that can
be done even in plane surfaces is to bring them parrallel by the second
refraction; but in lenses even this cannot be effected throughout the whole
of the aperture, as the degrees of curvature of the convex and concave lenses
do not correspond at different distances from the centre in the manner
necessary to produce this effect, and from these causes it is evident that no
telescope of the common construction can be rendered truly and accurately
ackromatic and unite the rays of light with that perfection which is required
when eye glasses of great power are made use of. Now though it be contrary
to the common doctrine concerning light that in a
rays should be equally refracted, yet it is demonstrable, and has been found by
experience, that where two mediums are in contact whose dispersion is the
same, and their mean refractive density different, light, whatever be its
obliquity of incidence, will in passing from the one medium into the other
be bent from its rectilinear course equally and regularly as in reflexion without
any greater refraction of one kind of rays than another, a fact of much
single refraction all the

page_text: consequence in dioptrics, as it was from a supposition of the impossibility of
attaining this point that telescopes constructed on the principle of refraction
were despaired of, and recourse had to reflexion. Besides this advantage,
which can only be attained by making use of dispersive fluids, they have that
of being more transparent and homogencal than glass of any kind, and by
coming into immediate contact with the glass lenses prevent the loss of light
by reflexion, which is inseparable from the common ackromatic telescope, and
by rendering a concave lens unnecessary obviate any errors which might
arise from imperfections in its figure as well as its texture. The present
improvement consists in removing the impediments arising from the imperfec-
tion of flint glass to the construction of refracting telescopes with large
apertures and high magnifying powers, in correcting the errors arising from
a difference of refrangibility of the rays of light and those from the spherical
figures of glasses more perfectly, and transmitting more light where the
apertures are equal, than can be done in the common achromatic telescope, by
rejecting the imperfect kind of glass called white flint glass from the com-
pound object glass, and using in lieu of it a dispersive fluid. And in general
the improvement consists in refracting the rays of light without dispersion or
colour in dioptrical instruments, whether composed of lenses or prisms, whero
refraction without colour is wanted by a combination of crown glass, or any
other kind of glass except flint glass, with a fluid of different dispersive power
from that of the glass with which it is combined, and that either by opposite
refractions as at present practised with different kinds of glass, or by a single
refraction in the manner above explained. By accurate experiments most
metallic solutions were found to possess this dispersive power; or solution of
corrosive sublimate mercury, either alone in spirit of wine, or in water with the
addition of crude sal ammoniacum, disperses in a very considerable degree,
but is greatly inferior in this respect to a chemical preparation called the
caustic or butter of antimony, which, in its strongest state, disperses three
tines more than crown glass. Some essential oils were also found to possess
this property in a sufficient degree to be useful for optical purposes. Those
who are conversant in optics will not be at a loss in adapting the curvature
of the lenses to the dispersive power of the fluid whose properties they have
examined, so as to produce a colourless refraction, and will also perceive
that, from having a choice of so great a variety of mediums of different
refractive and dispersive powers, the errors of homogeneal rays arising from
the spherical figure of the lenses may be more accurately corrected than
when two kinds of glass of nearly the same density are made use of; this
subject having been treated of long since by opticians of the greatest eminence.