In 1848 Bontemps was obliged to leave France for political reasons and came to England, where he initiated the optical glass manufacture at Chance's glass works near Birmingham, and this firm ultimately attained a considerable reputation in the production of optical glass, especially of large disks for telescope objectives.
Efforts at improving optical glass had, however, not been confined to the descendants and successors of Guinand and Fraunhofer.
Very perfectly annealed optical glass is now, however, readily obtainable.
An originally unanticipated difficulty has arisen from the fact that the reseau-lines have not been ruled on plates of optical glass with optical surfaces, and that, in consequence of irregular refraction in the glass plate, the rays do not always pass through the silver film-lines in a direction strictly normal to the silvered surface; therefore, if the sensitive surface of the photographic plate is not in contact with the silver film of the reseau, the undeveloped photographic copy of the reseau may in such a case not be an exact reproduction of the silvered reseau.
The next and most important forward step in the progress of optical glass manufacture was initiated by Ernst Abbe and carried out jointly by him and 0.
In spite of the improvements in themanufacture of optical glass (see Glass) practically the same crown and flint glasses as used by John Dollond in 1758 for achromatic objectives are still used for all the largest of the modern refracting telescopes.
The furnace used for the production of optical glass is generally constructed to take one crucible only, so that the heat of the furnace may be accurately adjusted to the requirements of the particular glass under treatment.
From the large masses great lenses and mirrors may be produced, while the smaller pieces are used for the production of the disks and slabs of moderate size, in which the optical glass of commerce is usually supplied.
The first step towards overcoming this vital defect in optical glass was taken by P. L.