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<h2>THE MANUFACTURE AND CHARACTERISTICS OF PHOTOGRAPHIC LENSES.</h2>
<p>At a recent meeting of the London and Provincial Photographic
Association Mr. J. Traill Taylor, formerly of New York, commenced
his lecture by referring to the functions of lenses, and by
describing the method by which the necessary curves were computed
in order to obtain a definite focal length. The varieties of
optical glass were next discussed, and specimens (both in the rough
and partly shaped state) were handed round for examination. The
defects frequently met with in glass, such as striæ and
tears, were then treated upon; specimens of lenses defective from
this cause were submitted to inspection, and the mode of searching
for such flaws described. Tools for grinding and polishing lenses
of various curvatures were exhibited, together with a collection of
glass disks obtained from the factory of Messrs. Ross & Co.,
and in various stages of manufacture--from the first rough slab to
the surface of highest polish. Details of polishing and edging were
gone into, and a series of the various grades of emery used in the
processes was shown. The lecturer then, by means of diagrams which
he placed upon the blackboard, showed the forms of various makes of
photographic lenses, and explained the influence of particular
constructions in producing certain results; positive and negative
spherical aberration, and the manner in which they are made to
balance each other, was also described by the aid of diagrams, as
was also chromatic aberration. He next spoke of the question of
optical center of lenses, and said that that was not, as had been
hitherto generally supposed, the true place from which to measure
the focus of a lens or combination. This place was a point very
near the optical center, and was known as the "Gauss" point, from
the name of the eminent German mathematician who had investigated
and made known its properties, the knowledge of which was of the
greatest importance in the construction of lenses. A diagram was
drawn to show the manner of ascertaining the two Gauss points of a
bi-convex lens, and a sheet exhibited in which the various kinds of
lenses with their optical centers and Gauss points were shown. For
this drawing he (Mr. Taylor) said he was indebted to Dr. Hugo
Schroeder, now with the firm of Ross & Co. The lecturer
congratulated the newly-proposed member of the Society, Mr. John
Stuart, for his enterprise in securing for this country a man of
such profound acquirements. The subject of distortion was next
treated of, and the manner in which the idea of a non distorting
doublet could be evolved from a single bi-convex lens by division
into two plano-convex lenses with a central diaphragm was shown.
The influence of density of glass was illustrated by a description
of the doublet of Steinheil, the parent of the large family of
rapid doublets now known under various names. The effect of
thickness of lenses was shown by a diagram of the ingenious method
of Mr. F. Wenham, who had long ago by this means corrected
spherical aberration in microscopic objective. The construction of
portrait lenses was next gone into, the influence of the negative
element of the back lens being especially noted. A method was then
referred to of making a rapid portrait lens cover a very large
angle by pivoting at its optical center and traversing the plate in
the manner of the pantoscopic camera. The lecturer concluded by
requesting a careful examination of the valuable exhibits upon the
table, kindly lent for the occasion by Messrs. Ross & Co.</p>
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