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