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.
* * * * *