Newark post., 1915-02-10

some way to test it. This is quite
important, for the reason that all
objectives, even though they may
be rated as first quality, are not
equally good. Hence, discrimin-
ating purchaser will be quite sure
to test an objective by examining
some microscopic object which has
extremely delicate markings of
some sort on it, as, for instance, a
fossil diatom.
A diatom is a microscopic plant
which lives in either fresh or salt
water. 1t has a skeleton in the
form of case, or shell. When the
plant dies, the shell, being free Of
living matter, settles to the bottom.
In certain parts of the world these
fossil shells have formed great
earthly deposits. In some in
stances thedeposits have hardened
into solid rock. The material Of
some of these rocks is used as a
polishing powder, as, for instance,
the Tripoi powder which is used
by jewelers.; while some Of the
earths are used as absorbents in the
manufacture of materials like dy-
Mantle.
The shells of practically all dia-
toms, and l may add that there are
over II 200 species Of them known,
are very beautiful when seen thru
a microscope. Some of them have
elaborately sculptured markings
on them which are so minute and
delicate that only the best micro
scope objectives can define them
clearly. Hence their use as test
plates, or objects, to determine the
quality of microscopic objectives.
There are other people who per-
fer to test the defining power of a
microscope objective by observing
the lines on Nobert plate. A No
bert plate is glass microscope
slide with parallel lines ruled on
it and is designed for the purpose
of testing enses. These lines are
exceedingly fine and close together,
in some instances being only the
one hundred and fifty thousandth
of an inch apart. A good objective,
however, is quite capable of resolv-
ing these lines, although the magni-
tudes involved are so small as to be
beyond our powers of conception.
A number of years ago-it was
sometime in the sixties-an En-
glishman by the name of Webb
made some marvellous test plates
which are specimens of mechanical
skill that have never been sur
passed. Webb chose to write the
Lords prayer on glass for his test
plates. An old friend of mine, Mr..
Stephen Helm, had one of them
which l have seen on several occa-
sions. On this particular plate,
which is labeled "Webb's Test
Plate No. %" the Lords Prayer
is written in a rectangular space
which measures the 1900 by the
1868 part of an inch. Multiplying
these two dimensions together, we
get the area Of the little rectangle
in which the writing is done. This
area is the 1-326,5O0 part of a
square inch. In order to get some
idea of the magnitude, or minute
ness, of this writing let us make a
comparison with something we all
appreciate.
There are in the Lords Prayer
227 letters, and if, as here, this
number occupies the 1926, 500 part
of a square inch, there would be
room in an entire square inch for
"A. .115, 800 such letters, similarly
spaced.
Now the entire Bible, Old and
New Testament. contains 1866, ,4SO
letters. Dividing this number into
"A, lis. 800 we thus find that there
would be room enough to ,Urite the
entire Bible more than t,Uenty
times Orer on one square inch Of
glass in the same manner as the
words of the Lords Prayer are
written on the specimen mentioned.
mtim ish-bs | nu ateth msog
Such statement staggers the
Imagination, but the figures are
easily verified and are certainly
correct.
The production of these plates is
a purely mechanical process. They
were written by Mr.. Webb with a
machine which was invented by a
man named Peters. The machine
is still in existence, and is the prop
erty of theRoyal Microscopical So
ciety of London.
The process of making a plate
consists simply of writing in a rec
tangle, measuring about 6 by s
Il, ID