or pyro, or silver solution, gives his hands a wipe on the focusing
cloth, and straightway sets about making an enlargement, ending up by
blessing the manufacturer who sent him paper full of black stains and
smears. Argentic paper is capable of yielding excellent enlargements,
but it must be intelligently exposed, intelligently developed, and
cleanly and carefully handled.

* * * * *

THE MANUFACTURE AND CHARACTERISTICS OF PHOTOGRAPHIC LENSES.

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 striae 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