IMPROVEMENTS IN COMPOUND LENSES WHERE ACHROMATISM IS DESIRED."
Year: 1878

page_text: PROVISIONAL SPECIFICATION left by Edmund Manasseh Tomlinson Tydeman
at the Office of the Commissioners of Patents on the 14th October 1878.
EDMUND MANASSEH TOMLINSON TYDEMAN, of Liverpool, in the County of Lan-
caster. IMPROVEMENTS IN COMPOUND LENSES WHERE ACHROMATISM IS DESIRED."
My Invention consists of improvements in the construction of compound
achromatic lenses suitable for use in telescopes, microscopes, and other optical
instruments, and is intended more completely to eliminate the secondary or out-
standing spectrum of such lenses, and to render them more perfectly achromatic.
It consists in forming such lenses, which may be double, triple, or multiple, not
as hitherto by the union of lenses made of different kinds or species of glass or other
refractive media, but of one kind or species of glass or other suitable refracting
substances only, yet of different densities and refractive powers.
Achromatic lenses have been constructed by combining lenses made of several
different kinds of glass or other refractive substance of different densities or refrac-
tive powers; for instance, there have been combinations of flint and crown glass,
of flint, crown, and plate glass, of crown or plate glass with some other more refrac-
tive substance to form achromatic lenses, but owing to the large irrationality or
irregularity or want of correspondence between the coloured spaces in the various
spectra formed by the refraction of light through such substances of different species
or kinds there is generally a considerable amount of secondary colour left uncor-
rected or outstanding in the images of objects formed by the rays of light passing
through such lenses, whereas in glass or other material of one kind or species,
although of different densities and refractive powers, the irrationality or unequal
refraction of the coloured rays is not so great.
I therefor construct my improved achromatic lenses with two or more glasses
made from materials of the same kind or species of glass (such as that known as
flint glass, which is capable of being made of various density) but of different
densities or refractive indices, and I also use flint glass lenses in lieu of the usual
crown or plate glass lenses in achromatic object lenses and object glasses. For a
microscope object glass, often composed of two or more approximately achromatic
lenses or sets or lenses, either in contact or nearly in contact with each other, I

page_text: sometimes make one of the several sets of compound lenses composing such object
glasses of one refracting medium, such as flint glass, and the other set or sets or
single lenses of a different refracting medium or media; or I use single lenses of
crown or flint or any other suitable substance in combination, though not necessarily
in contact, with an achromatic set composed wholly of flint glass or any other suitable
refracting substance.
I therefore employ glass or other suitable substance of one kind only, but of
different densities or refractive indices for the formation of (so-called) achromatic
lenses or sets of lenses and in the object glasses or other parts of telescopes and
other optical instruments; also in the formation of the individual compound lenses
used in the combinations called achromatic object glasses for microscopes, dialyte
telescopes, cameras, magic lanterns, and other optical instruments, and I do not confine
myself to any particular form or forms for such lenses, which admit of an almost
infinite variety.