Year: 1879




Mamm 9th Telescope Lenses.
The Boston Herald conveys the intel
ligence that the Russian government
has appropriated 250,000 roubles ($1S7,
6OO) for the construction of a telescope
and that Alvin Clarke & Sons, of Cam.
bridge, Mass, have been intrusted with
the delicate task of making the object
glass, which is to be as large as can
advantageously be made-it is hoied in
the neighborhood of 36 inches in diam
ct u. The Htrald thus explains the
minute process of making large lens:


The difficulties to be overcome in
this enterprise are almost beyond the
comprehension of those who have not
given the matter special attention end
study. The lens forming an object
glass is double, one part being of crown
glass, convex on 9TH surfaces, and the
other of flint glass, concave on one sur.
face to nearly fit one convex surface OF
the first. Thus the glass surface in
front of the telescope is convex, and the
rear surface, inside the tube, nearly flat.
Light falls on the lens in exactly
parallel rays. The Office of lens in a
telescope is to collect these rays OF
light as local point of the utmost pos.
sible concentration. In the 26-inch
glass of the Washington observatory
made by the Messrs. Clark, the focal
point is 32-feet in rear of the object
glass. Now, what is to be done is to
defect-or refract-the rays which fall
parallel upon every part of the outside
of the object glass, so that after passing
through they shall exactly meet at the
focal point. No minute ray of light Is
to be unaccounted for in the shaping of
the glass, not SPOT or point must be in.
accurate to the design and final pur.
pose; but the glass must he so ground
and finished into the proper shape that
the result of perfect and the most intel.
ligent control of every ray shall be at.
tained. What marvellous work of
calculation, experiment, minute testing
and exact computation ensues! The
processes are Understandable in their
delicacy, and few imaginations can
picture the exactness of the scientific
proceeding. In the first stages of fashion.
ing the glasses machinery is used, and
the glass is ground rapidly to it's convex
form; but afterwards, for every step of
the way, the most delicate process be.
comes essential, Nothing more tangible
than the finest rouge powder. between
the human finger and the glass surface,
must come in contact, and for days the
operator sits patiently at his bench, the
glass before him, occasionally dipping
his finger in water and then in rouge,
and then rubbing alternately, from the
centre to outside, with fnger-end, liter
ally wiping down all irregularities in
the surface of the glass and rubbing it
into the shape it must assume before it
can be useful. Thus with his finger-ends
MI. Clark fashions the glass with which
ho has to do, until every ray of light
which falls upon them has been made to
obey fixed law and the behests of the
operator.


Then, too, other effects have to be &C.
counted for and controlled, No sooner
do the rays of light strike the lens than
they are decomposed, and, if not proper.
ly managed, would 60 streaming through
in their component parts, so far as color
is concerned, of red, blue, etc. Here is
presented principal reason for the use
of the double lens, and two qualities or
kinds of glass. The crown glass decom-
roses the rays, so does the flint, but the
two not equally. In travelling through
the focus red ra US are longer than blue,
and so all the colors differ in their dis
persion. Those two parts of the lens,
the crown and flint glasses, must there
fore be so finished and adjusted as to
correct this decomposition, and not only
mnst the lens be shaped to guide the
rays in the proper direction toward the
focal point, but to justify the differences
between the colors, and thus, many feet
back in the tube, bring all to a point,
which the eye can observe, so small that
four or eight of such points might be
contained upon single hair. This is
the work Mr.. Clark is atted to perform
with his finger ends, and with his keen
eye; in ways which cannot be described
here, he from time to time tests his pro.
gress, and proceeds unerringly to a sure
result.