A NEW METHOD OF MAKING THE OBJECT GLASSES OF REFRACTING TELESCOPES BY COMPOUNDING MEDIUMS OF DIF- FERENT REFRACTIVE QUALITIES, WHEREBY THE ERRORS ARISING FROM THE DIFFERENT REFRANGIBILITY OF LIGHT, AS WELL AS THOSE WHICH ARE PRODUCED BY THE SPHERI- CAL SURFACES OF THE GLASSES, ARE PERFECTLY CORRECTED
Year: 1758

page_text: DOLLOND'S SPECIFICATION.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JOHN
DOLLOND, of the Parish of Saint Martin in the Fields, in the County of
Middlesex, Optician, send greeting.
WHEREAS His most Excellent Majesty King George the Second, by His
Letters Patent under the Great Seal of Great Britain, bearing date at West-
minster, the Nineteenth day of April, in the thirty-first year of His reign,
did give and grant unto me, the said John Dollond, His especial lycence, that
I, the said John Dollond, 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 NEW METHOD OF MAKING THE
OBJECT GLASSES OF REFRACTING TELESCOPES BY COMPOUNDING MEDIUMS OF DIF-
FERENT REFRACTIVE QUALITIES, WHEREBY THE ERRORS ARISING FROM THE DIFFERENT
REFRANGIBILITY OF LIGHT, AS WELL AS THOSE WHICH ARE PRODUCED BY THE SPHERI-
CAL SURFACES OF THE GLASSES, ARE PERFECTLY CORRECTED;" in which said Letters
Patent there is contained a provisoe, obliging me, the said John Dollond,
under my hand and seal, to cause a particular description of the nature of the
said Invention, and how the same is to be performed, to be inrolled in His
Majesty's High Court of Chancery within three kalender months 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 provisoe, I, the said
John Dollond, do hereby declare that my said new methood of making the
object glasses of refracting tulescopes is to be performed in the manner
following (that is to say):-
The principal glass of a refracting telescope is that which is farthest from
the eye, & is commonly called the object glass, because it is at that end of

page_text: the telescope which is directed toward the object. This glass refracts the rays
which proceed from the object in such a manner as to form an image of the
aforesaid object in the focus, which image is magnified by the eye glasses;
but as every ray of light consists of parts that differ in their degrees of refran-
gibility, an imago form by refraction is thereby rendered very defoctive, as all
opticians very well know. Now, in these new telescopes the images of objects
are formed by the difference between two contrary refractions, the object glass
being a compound of two or more glasses put close together, whereof one is
concave and the other convex; the excess of refraction by which the image
is formed is in the convex glass, which is made of a medium or substance in
which the difference of refrangibility is not so great as in the substance which
the concave is made of. Therefore, their refractions being proportioned to
their difference of refrangibility, there remains a difference of refraction by
which the image is formed, without any difference of refrangibility to disturb
the vision. The radii of the surfaces of each of these glasses are likewise
so proportioned as to make the aberrations or errors which proceed from the
spherical surfaces of these glasses respectively equal, and, being contrary, they
destroy each other.
In witness whereof, I, the said John Dollond, have hereunto set my hand
and seal, this Twenty-third day of May, One thousand seven hundred