Improvements in Gratings or Screens for Photo Mechanical Printing.
Year: 1893

page_text: Date of Application, 21st Feb., 1893-Accepted, 8th Apr., 1893
Improvements in Gratings or Screens for Photo Mechanical
Printing.
I, MAX LEVY, of No. 1213, Race Street, Philadelphia, in the County of
Philadelphia, and State of Pennsylvania, United States of America, Manufacturer,
do hereby declare the nature of this invention and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:
This invention relates to improvements in gratings or screens which are intended
for use in producing so-called half-tone or cross-line engravings, the object of the
same being to provide a grating or screen in which the lines will be clear, sharp,
solid and positive, so as to be clearly defined and positive in use.
In the accompanying drawings which illustrate the invention the views represent
a small part of the article enlarged or magnified:
Figure 1 is a plan view showing a glass plate with lines ruled in one direction
through the etching ground which has been applied thereto.
Fig. 2 is a similar view showing the lines ruled so as to cross each other.
Fig. 3 is a vertical sectional view showing the appearance of the ground after
being ruled, and the glass before it is etched.
Fig. 4 is a vertical sectional view showing the plate after being etched, the
ground being removed, the furrows being filled with opaque substance and the
plate wiped or cleansed.
Fig. 5 is a sectional view showing a completed plate with a cover glass.
Fig. 6 is a sectional view showing two similar plates connected to each other.
Fig. 7 is a plan view of a portion of a completed plate.
In carrying out my invention a ground and polished plate of glass is carefully
selected, which is free from scratches or bubbles. This plate of glass is cleaned,
and is covered with an etching ground, B, which may be of any suitable composition
that will resist hydro-fluoric acid; which ground may be either flowed in solution,
rolled or dabbed on the plate.
In practice the plate of glass is covered with a coating of iodized or bromized
collodion, and prepared in a solution of nitrate of silver, exposed to light, and the
silver precipitated with a suitable deoxydizer. This affords a better basis for an
etching ground than does the glass itself, for by absorbing the resisting ground
causes it to hold more firmly to the surface of the glass during the process of
etching. The lines A' A' are ruled through the etching ground so as to expose
the glass where the composition or coating is removed by the etching implement.
The ruling, which consists of straight lines in close proximity to each other-
numbering from 120 to about 200 to the inch-are made by a dividing engine with
a diamond point. These lines, which are usually parallel, may also cross each
other at right angles. The plate, with the lines A¹ ruled through the etching
ground, is then exposed to the action of hydro-fluoric acid, either liquid or fumes,
the back of the plate being suitable protected against the action of the acid. By
means of the hydro-fluoric acid the furrows, linear depressions or lines D are
eaten into the glass to the desired depth, while the other part of the surface,
which is protected by the etching ground, is not affected by the acid and remains
in its original condition. The plate is now washed, or otherwise cleaned, and the
whole of the lined surface is covered with a suitable opaque substance, C, the body
of which may be shellac and lamp-black. This opaque substance is thoroughly
rubbed into the furrows, linear depressions or lines D which are etched into the
glass, and is allowed to partially dry and harden. The opaque substance is then
removed from the surface of the glass so as to leave the lines, furrows or linear

page_text: depressions filled with a hardened opaque paste, which is designated in the drawing
by the letter C.
What I term the lines is the base portion of the linear depressions or furrows D
which has been filled with an opaque substance, as the upper inside edges of the
linear depressions or furrows, next to the surface, are slightly roughened by the
etching, which portion of the depressions or furrows is filled with the cement D',
which has the same index of refraction as the glass.
In etching the lines, linear depressions or furrows upon the plate of glass, no
matter how fine the etching tool may be or how much care may be given to the
manipulations, the action of the hydro-fluoric acid, or other biting acid which eats
into the glass, will mar, bite or abrade the upper surface or edges, either by biting
into the glass at this point or finding its way beneath the etching ground, so that
the furrows or linear depressions instead of having sharp angular edges will be
rounded or abraded, which seriously affects the result sought to be attained by the
grating or screen; therefore, to overcome these imperfections the glass, after the
opaque substance has been placed in the lines, is wiped so that said opaque substance
lies below these rounded edges. The glass and opaque substance are then covered
with a suitable material, Di, preferably balsam of fir or Canada balsam, which has
the same index of refraction that the glass has. By employing Canada balsam the
imperfections at the upper portions of the lines, linear depressions or furrows are
filled with a substance having the same index of refraction as the glass, and the
Jiability of the aberration of the rays and unequal refrangibility is to such a degree
removed that it need not be considered in the use of such a grating or screen as is
produced by my invention.
The plate described may be protected on its ruled side by a cover glass E,
which may also be ruled in an opposite direction from the plate to which it is
attached, both plates being prepared in the same manner, and Canada balsam is
preferably used as a medium for joining together or connecting the two ruled
glasses, so as to produce, practically, a single plate in the center of which is the
grating or lines.
A plate constructed as hereinbefore described will not obstruct or deflect the
rays of light. The grating is not used to receive the picture, nor is the picture or
any part thereof at any time fixed on the grating; but said grating in use is placed
inside the camera between the lens and the sensitized plate upon which the photo-
graphic image is be impressed.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :
1. A grating or screen for the purpose set forth, consisting of a glass plate
having parallel linear depressions or furrows etched therein, the base or lower
portions of said linear depressions or furrows being filled with an opaque paste to
form lines, the portion of the linear depressions or furrows above the lines being
filled with a medium having the same index of refraction as the glass, for the
purpose set forth.
2. A grating or screen for the purpose set forth, consisting of two plates of glass
each having parallel furrows, linear depressions or lines etched into one of its
surfaces, the base portion of said furrows, linear depressions or lines being filled
with opaque material, the lined glasses being cemented to each other, so that the
lines will cross, by a substance having the same index of refraction as the glass.
3. As an improved article of manufacture, a grating or screen for photographic
purposes in the production of half-tone plates, consisting of a plate of glass having
parallel furrows, linear depressions or lines etched in one of the surfaces thereof,
Baid furrows, linear depressions or lines being partially filled with an opaque
material to form lines, the upper portions of said furrows, linear depressions or
lines above the lines being filled with a covering of Canada balsam, for the purpose
set forth.

page_text: 4. A grating or screen for photographic purposes in the production of half-tone
plates, consisting of two plates of glass, one having fine furrows, linear depressions
or lines in its surface, said furrows, linear depressions or lines containing an opaque
substance, the glass plates being cemented to each other to form one refracting
medium with a ruling or net-work in its center, substantially as shown.
5. A grating or screen for photographic purposes in the production of half-tone
plates, consisting of two plates of polished glass each having furrows, linear
depressions, or lines etched into one of its faces, said furrows linear depressions or
lines being partially filled with an opaque substance, the remaining portion of the
furrows, linear depressions or lines being filled with Canada balsam, said plates
being connected to each other so that the lined portions are adjacent to each other,
for the purpose set forth.
Deted this 20th day of February 1893.
F. BOSSHARDT & Co.,
4, Corporation Street, Manchester, Agents to Applicant.