U.S. patent number 3,784,394 [Application Number 05/140,925] was granted by the patent office on 1974-01-08 for latent image composite master and method.
This patent grant is currently assigned to Minnesota Mining and Manufacturing Company. Invention is credited to Leonas L. Bildusas, Wendell J. Manske.
United States Patent |
3,784,394 |
Bildusas , et al. |
January 8, 1974 |
LATENT IMAGE COMPOSITE MASTER AND METHOD
Abstract
In the disclosed method for making a composite master capable of
producing both visible and invisible images on a copy or receptor
sheet, a first and second set of infrared absorptive images are
entered on the obverse, uncoated surface of the master and mirror
image deposits of dithiooxamide (DTO) or a derivative thereof
corresponding to the second set of images are entered on the
reverse surface of the master. The reverse surface is coated with a
dye precursor such as a gallate. A wax transfer sheet or the like
is used for providing the DTO deposits. The resulting master is
capable of providing both visible and invisible entries on a nickel
or copper salt-coated receptor sheet by passing the master and
receptor through an infrared copying machine.
Inventors: |
Bildusas; Leonas L. (Wheaton,
IL), Manske; Wendell J. (Village of Birchwood, MN) |
Assignee: |
Minnesota Mining and Manufacturing
Company (St. Paul, MN)
|
Family
ID: |
22493400 |
Appl.
No.: |
05/140,925 |
Filed: |
May 6, 1971 |
Current U.S.
Class: |
430/15;
101/DIG.37; 101/470; 250/318; 283/88; 427/145; 428/29; 430/200;
430/368; 430/395; 430/908; 434/328 |
Current CPC
Class: |
G09B
3/04 (20130101); B41M 5/48 (20130101); Y10S
430/109 (20130101); Y10S 101/37 (20130101) |
Current International
Class: |
B41M
5/40 (20060101); B41M 5/48 (20060101); G09B
3/04 (20060101); G09B 3/00 (20060101); B41m
005/18 () |
Field of
Search: |
;117/1.7,36.2,36.4
;101/426,469,470,473 ;250/65T,317,318 ;283/6 ;35/9G |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Martin; William D.
Assistant Examiner: Sofocleous; M.
Attorney, Agent or Firm: Kinney, Alexander, Sell, Steldt
& Delahunt
Claims
What is claimed is:
1. A composite master for providing images on a copy sheet
comprising:
an obverse surface having, in a first obverse position on said
obverse surface, a first infrared absorptive image, and, in a
second obverse position on said obverse surface, a second
infrared-absorptive image,
a reverse surface having a layer thereon, said layer covering at
least an area of said reverse surface opposite to said first
obverse position, said layer comprising a polyhydroxy aromatic
iron-complexing compound, said reverse surface having deposited
thereon, in a position opposite to said second obverse position, a
mirror image of said second infrared-absorptive image, said mirror
image comprising a dye precursor selected from dithiooxamide and a
derivative thereof, the area of said reverse surface opposite said
first obverse position being substantially free of said dye
precursor.
2. A master according to claim 1 wherein said layer comprising said
polyhydroxy aromatic iron-complexing compound is coextensive in
area with said reverse surface and wherein said mirror image is
deposited on said layer.
3. A master according to claim 1 wherein said poly-hydroxy aromatic
iron-complexing compound is a lower alkyl gallate and said dye
precursor is an N,N'-di-organo-substituted dithio-oxamide wherein
at least one organo radical has the formula R--CH.sub.2 --, R being
selected from the group consisting of an aliphatic, a
cycloaliphatic, and an aromatic radical.
4. A master according to claim 3 wherein said R is phenyl.
5. A master according to claim 4 wherein said lower alkyl gallate
is methyl gallate.
6. A method of forming visible and latent images on a copy sheet
comprising
a. providing a composite master sheet having on the obverse surface
in a first obverse position a first infrared absorptive image and a
second infrared absorptive image in a second obverse position, and
having on the reverse surface a layer covering at least an area of
said reverse surface opposite said first infrared absorptive image,
said layer comprising a polyhydroxy aromatic iron-complexing
compound, and having deposited on said reverse surface opposite
said second infrared absorptive image a mirror image of said second
infrared absorptive image, said mirror image comprising a dye
precursor selected from dithiooxamide and a derivative thereof, the
area of said reverse surface opposite said first infrared
absorptive image being substantially free of said dye
precursor,
b. placing the reverse surface of said master sheet in contact with
a copy sheet, said copy sheet containing a metal salt color
reactive with the dye precursor on the master sheet, and
c. exposing the master sheet to infrared radiation whereby there is
produced on said copy sheet a visible image corresponding to said
second infrared absorptive image and a latent image corresponding
to said first infrared absorptive image.
7. The method according to claim 6 wherein said latent image is
subsequently rendered visible by means of treatment with an iron
salt.
Description
FIELD OF THE INVENTION
This invention relates to latent imaging techniques and paper-based
feedback systems. An aspect of this invention permits simultaneous
latent invisible and visible imaging of a receptor sheet. This
simultaneous imaging permits the receptor sheet to be used as a
test form, work sheet, or the like wherein invisible answers or
other data are in register with visible questions. A further aspect
of this invention relates to a composite master sheet and a method
for providing it. The composite master, placed on top of a nickel
or copper salt-coated receptor sheet will transfer both visible and
invisible entries onto the receptor sheet in single pass through an
infrared copying machine.
DESCRIPTION OF THE PRIOR ART
Composite test forms and work sheets containing both visible and
invisible entries have been developed using, for example, spirit
duplicating technology. These test forms are an important part of
an art known as the art of "paper-based feedback systems." A
typical disclosure describing a latent/visible imaged copy sheet
and a method of making it is contained in U.S. Pat. No. 3,451,143
(Thomas et al.), issued June 24, 1969. The Thomas et al patent
suggests that the invisible concealed image can be deposited alone
or in combination with visible images from the same or from
different spirit masters. The invisible image can be developed by
chemical means to provide "feedback." An obvious advantage of
depositing the visible and invisible images from the same master is
that the visible and invisible entries will be in proper register
-- a result that is difficult to achieve using two separate
masters. The obvious method for providing the combination or
composite master, suggested by Thomas et al (see column 4, line 37
et seq. of this reference), is to (1) image the master with a
heat-transferable or spirit-transferable visible dye for the
visible entry portion, and (2) image the master with a dye
precursor (e.g. one of the chemicals designated "Material A" by
Thomas et al at column 2, line 53 et seq.) to provide the invisible
entry portion or "feedback."
The difficulty with this obvious method is the likelihood that it
will be difficult to match the strength and clarity of the visible
entry on the copy sheet with the invisible entry, when the
invisible entry is developed by treating the copy sheet with, for
example, one of the compounds designated "Material B" by Thomas et
al. Good balance between the visible and invisible entries is
necessary to ensure total absence of clues to the content of the
invisible entry before its development, but good legibility of the
visible entries and the developed invisible entries. Furthermore,
it is desirable that the formation and use of the composite master
should involve thermographic duplicating rather than spirit
duplicating technology. (Very recently, the art of paper-based
feedback systems has begun to emphasize the use of thermographic
technology; see Belgian Patent 740,271.) Thermographic technology
is convenient and efficient, since it is an entirely dry process
and does not involve the use of solvents; however, it would be
expected that matching the strength and clarity of visible and
invisible entries would be even more difficult with thermographic
technology than with spirit technology.
Accordingly, this invention seeks to provide a method and a means
for utilizing thermographic technology to provide copy sheets
containing both visible and invisible entries. This invention
contemplates providing a master capable of imaging a copy sheet
with both visible and invisible entries during a single pass
through an infrared copying machine. This invention also
contemplates utilizing thermographic technology to equalize the
strength of the visible and invisible imaging capabilities of the
composite master. This invention further contemplates a paper-based
feedback system comprising copy sheets wherein visible clues to the
feedback are totally absent, where the feedback may be a complex
text rather than a color or simple indicia means, and wherein the
strength and clarity of the image that will be observed when the
feedback is developed can be evaluated before development by
observing the visible entries on the copy sheet.
BRIEF DESCRIPTION OF THE INVENTION
It has now been found that it is unnecessary to use a visible dye
to make a composite master if two chemically distinct,
substantially colorless dye precursors are used: an iron-complexing
compound (e.g. a gallate) for invisible or feedback information and
dithiooxamide (DTO) or a dithiooxamide derivative for the visible
information. A composite master of this invention is, initially, a
typical gallate-coated master sheet. Briefly, this invention
involves, (1) providing a first set of infrared-absorptive images
on the obverse (uncoated) surface of the master, (2) providing a
second set of infrared absorptive images on this obverse surface
and mirror image deposits of dithiooxamide (DTO) or a derivative
thereof, corresponding to this second set of images, on the coated
(reverse) surface of the master, (3) justaposing the resulting
composite master with a nickel or copper salt-coated receptor sheet
(or, less preferably, a receptor sheet coated with a cobalt or
cadmium salt) such that the coated surface of the master is next to
the coated surface of the copy sheet, and (4) exposing the
thus-imaged master to an infrared source such that infrared
radiation is directly absorbed by the obverse surface of the
composite master. The result is that the first set of infrared
absorptive images will induce the formation of latent or invisible
entries on the copy or receptor sheet, while the second set of
images induces transfer of portions of the DTO deposits to provide
visible entries. The invisible entries will comprise deposits of
the gallate (or whatever equivalent iron-complexing compound is
used) and the visible entries will comprise a DTO/nickel or
DTO/copper colored complex (or a Ni or Cu complex of a suitable DTO
derivative). Gallate or other iron-complexing deposits can be
present in the visible entries also, and this in no way interferes
with the objects of this invention. The gallate-containing
invisible entries are developed with an iron salt substantially in
the manner taught by the aforementioned Thomas et al. patent, i.e.
by the application of a marking fluid or material containing a
ferric salt. Surprisingly, it is preferred that only DTO or a
derivative thereof be used to make the visible entries in this
system and that only gallates or similar iron-complexing
polyhydroxy aromatic compounds be used for the invisible portion.
In the most preferred practice of this invention, the gallate (or
the like) and the DTO derivative are selected such that, on the
copy sheet, the strength and clarity of the invisible entries, upon
development, is the same as that of the visible entries. In short,
according to this preferred practice, if the visible print on the
copy sheet is clear and legible, the invisible print will also be
clear and legible when developed. Typically, this preferred
practice involves using methyl gallate as the iron-complexing
compound and N,N'-dibenzyldithiooxamide (DBDTO) as the DTO
derivative.
DESCRIPTION OF THE DRAWINGS
This invention can be more clearly understood with reference to the
drawing, wherein
FIG. 1 is a perspective view of a multi-sheet manifold used to make
composite masters according to this invention.
FIG. 2 is a greatly enlarged sectional view taken along line 2 --
2, showing a composite master sheet of this invention juxtaposed
with a receptor or copy sheet to provide visible and invisible
entries on the copy sheet.
DETAILED DESCRIPTION
Turning now to the drawing, it will be seen from FIG. 1 that the
composite master is initially in the form of a multi-sheet manifold
10 with perforations or the like to permit easy and convenient
separation of the master sheet 11 and the transfer sheet 13. For
shipping and storage, a protective sheet (not shown) is interposed
between master sheet 11 and transfer sheet 13. This protective
sheet can be an ordinary piece of glassine paper or the like. In
FIG. 1, the protective sheet is not shown because it has been
removed by tearing along the perforation, leaving only a stub 15.
The master sheet 11 becomes a composite master after being imaged
in accordance with the teachings of this invention. The transfer
sheet 13 is coated on its upper surface with a transferable layer
14 containing DTO or a derivative thereof, preferably in intimate
admixture with a particulate material such as wax. The use of such
wax transfer sheets for providing mirror images on the reverse
(uncoated) surface of a master is disclosed in, for example,
Newman, U.S. Pat. No. 3,459,581, issued Aug. 1969. As taught in the
Newman patent, the mirror images deposited on the reverse surface
of a master provide visible images on the surface of a coated
receptor sheet when passed through an infrared copying machine. The
master sheet 11 is provided with a coating layer 16 on its reverse
surface. For convenience in manufacturing master sheet 11, layer 16
is coextensive in area with the reverse surface of sheet 11. This
layer 16 contains a gallate such as a lower alkyl gallate or a
similar iron-complexing orthodihydroxy or trihydroxy-benzene or
-benzoic acid or ester (e.g. catechol, pyrogallol, gallic acid,
etc.). It is preferred to match, as closely as possible, the vapor
pressure of the DTO derivative in layer 14 with the gallate (or
equivalent compound) in layer 16. Typical written material provided
for visible and invisible entries is shown on the exposed, uncoated
obverse surface 12 of master sheet 11. Thus, the expression "22 +
9" will provide a visible entry on the receptor or copy sheet in
accordance with the teachings of this invention, and this visible
entry will pose an arithmetic problem for the student. The
answering or feedback entry corresponding to the arithmetic problem
("31") is visible on surface 12 because it has been typed or
written thereon. However, the copy sheet will contain this feedback
entry in latent or invisible form, and only by treating the copy
sheet with a developing fluid containing a ferric salt will this
invisible entry become visible on the copy sheet. The box around
the answer to the arithmetic problem has been drawn such that it
will show up as part of the visible matter on the copy sheet,
thereby indicating where the ferric salt should be applied.
A preferred method for providing the composite (i.e. visible and
invisible) imaging capability of the master is as follows: The
protective sheet is removed from the manifold, leaving only the
master and transfer sheets as shown in FIG. 1. Exposed surface 12
of manifold 10 is typed or written upon to provide all the visible
information which will later be contained on the finished copy
sheet. For best results, it is preferred to use a carbon ribbon in
the typewriter, or for long hand, a sharp pencil or ball point pen
containing infrared absorbing ink. During this first imaging step,
well-defined image deposits containing DTO, or a derivative
thereof, are transferred to the coated surface, i.e. layer 16 of
the master sheet 11. Transfer sheet 13 is in place under master
sheet 11 throughout this first imaging step; therefore, corrections
should be made by placing any separator sheet between the master
and transfer sheets 11 and 13 and erasing. The correct information
can then be entered by removing the separator sheet and retyping or
rewriting. After all the information that is to be visible on the
copy sheet has been entered, the master sheet 11 is separated from
the transfer sheet 13 in preparation for a second imaging step. In
the second imaging step, information that is to be invisible on the
finished copy sheet, e.g. an answer to the arithmetic problem, is
typed or written on appropriate areas of surface 12, preferably in
register with the line of type or writing containing the problem.
Since the transfer sheet 13 has been removed, no mirror images are
transferred onto layer 16 during the second imaging step; in fact,
it is preferred to protect layer 16 of the master with a sheet of
paper such as ordinary writing paper or, preferably, a scavenger
backup sheet coated with a suitable copper or nickel salt while
typing or writing on surface 12. The pressure of writing or typing
on surface 12, which is transmitted to the scavenger sheet, causes
an extremely small amount of the nickel or copper salt to be
transferred to layer 16 of the master sheet 11. This small amount
of coreactant salt is not sufficient to deactivate the mirror image
deposits of the DTO compound, but is sufficient to scavenge or
deactivate any contamination of the layer 16 with the DTO-type dye
precursor which may have occurred while layers 14 and 16 were in
contact.
Master sheet 11 is now a composite master, since it has now been
provided with the visible material or problem ("22 + 9," etc.) and
with the invisible or latent entry giving the answer to the problem
("31"). As an additional aid to the student, it is desirable to
replace master sheet 11 on top of transfer sheet 13 and draw a box
or circle around the answer. This box or circle will appear as a
visible entry on the copy sheet, indicating precisely the area to
be treated with a chemical pen or other means for dispensing a
coreactant iron salt capable of developing the latent entry and
making it visible.
In a further optional step of this invention, master sheet 11,
after being made into a composite master, is further treated with
an infrared copying machine to give the composite master the
ability to produce more than fifty fully legible copies. In a
suitable embodiment of this step, surface 12 of the composite
master is exposed to a line source of infrared radiation (in a
single pass through an infrared copying machine) while a sheet of
glassine paper or the like is in contact with layer 16. The
glassine paper absorbs little, if any, dye precursor vapor or
molten material from layer 16 and assists in driving this vapor
back into the body of master sheet 11.
Another preferred method for providing the composite imaging
capability of the master is as follows: The protective sheet is
removed as described previously. The master sheet is also removed,
and placed against a suitable backing sheet or surface, e.g. the
platen of a typewriter or an ordinary sheet of writing paper. All
of the visible entries are then typed or written in one area, e.g.
the left-hand area of surface 12. If desired, the invisible entries
can be typed or written at this time in a different area, e.g. the
right-hand area of surface 12. The master sheet 11 with the visible
entries typed or written in this limited area of surface 12 is then
juxtaposed with transfer sheet 13 such that layer 16 is in contact
with layer 14; however, the juxtaposition should be selectively
arranged so that only the area of layer 16 opposite the
aforementioned limited area of surface 12 is in contact with layer
14. This can conveniently be arranged by cutting off, for example,
the right half of transfer sheet 13. The selectively juxtaposed
sheets 11 and 13 are passed through a conventional infrared copying
machine at a slow speed, i.e. a dark setting. This pass through the
copying machine transfers portions of layer 14 onto layer 16
providing mirror images corresponding to the infrared-absorptive
visible entries on the limited area of surface 12. After this vapor
and/or melt transfer step has been completed, the transfer sheet 13
can be discarded, and all the entries which are to be invisible on
the copy sheet can be typed or written onto the area of surface 12
reserved for them, e.g. the right half of surface 12, if they have
not already been provided as described previously. This alternative
method provides a composite master sheet which can be used in the
same manner as the composite master sheet produced according to the
previously described method.
Other methods of imaging the composite master will occur to the
skilled technician. For example, careful use of separator or
scavenger sheets can obviate the need to detach master sheet 11
from manifold 10 during the master imaging steps (an attached,
fully imaged composite master 11 is thus shown in FIG. 1). It is,
of course, possible to provide separately packaged master and
transfer sheets rather than the manifold as in FIG. 1.
The use of the composite master 11 is illustrated in FIG. 2. In the
greatly enlarged cross section of composite master 11 shown in FIG.
2, it can be seen that surface 12 is a paper-like substrate with
two sets of infrared-absorptive images 25 and 27 thereon. The set
of images represented by image 25 is designed to produce the
invisible entries on the copy sheet 30. Thus, image 25 is, for
example, the answers to the arithmetic problems. Image 27 includes
the arithmetic problems which are to visibly appear on copy sheet
30. The wavy lines above composite master 11 represent a line
source of infrared light provided in a known manner by a
conventional infrared copying machine having a tubular lamp with a
linear filament, wherein the lamp is mounted within a focus
reflective housing for progressive exposure of a printed surface.
Such machines are described in U.S. Pat. No. 2,740,895 (Miller) and
are available from 3M Company under the Trademark "Thermo-Fax." It
can also be seen that image 27 has a mirror image 29 precisely
corresponding thereto in intelligence content and precisely
opposite to obverse surface 12 in its position upon layer 16, i.e.
on the reverse surface of composite master 11. Image 25, however,
has not been provided with a corresponding mirror image.
Accordingly, infrared-absorptive image 27 causes a mixture of dye
precursor vapors and/or molten material from layer 16 and mirror
image 29 to transfer over to copysheet 30, while image 25 causes
only dye precursor vapors from layer 16 to transfer to copy sheet
30, i.e. only iron-complexing vapors.
It is preferred to maximize vapor transfer as opposed to melt
transfer and to use the maximum belt speed or minimum darkness
setting of the infrared copying machine in the step illustrated in
FIG. 2.
Copy sheet 30 is provided with a coating 36 which is co-reactive
with DTO-type dye precursors to provide colored complexes.
Preferably, coating 36 contains a copper or nickel salt of a
carboxylic acid. (Alternatively, copy sheet 30 could be impregnated
throughout with this color-reactive or co-reactive salt.) Coating
36 forms a stable, colored complex with DTO or derivatives thereof,
but does not react with iron-complexing compounds such as the
gallates. Paper sheets in the nature of copy sheet 30 are
commercially available. A typical commercially available sheet
comprises a paper-like substrate 32 coated with nickel rosinate in
a starch binder. The mixture of melt and/or vapor transferable dye
precursors from layer 16 and mirror image 29 which condense on
coating 36 provide visible image or entry 37. The dye precursor in
layer 16 does not react with coating 36 to provide a visible
colored complex, but the DTO-type compound in mirror image 29 does.
Since the dye precursor in layer 16 does not react with coating 36,
image 25 induces the formation of a latent image or invisible entry
35 on coating 36. This invisible entry comprises an iron-complexing
compound condensed on coating 36 in a pattern corresponding exactly
to the pattern of image 25. Upon treatment with a ferric salt
dissolved or otherwise admixed with a suitable reaction medium,
invisible entry 35 will be developed and become visible. Invisible
entry 35 can be developed in this manner by any suitable means such
as a fiber-tipped pen provided with a reservoir containing a liquid
similar to the ferric salt solution. For example, the developing
fluid or marking material can be an iron salt of an aliphatic
carboxylic acid dissolved in a solvent such as an alcohol.
As will be apparent to the skilled technician, the steps of the
method of this invention can be varied in their sequence so long as
the imaging of the copy sheet can be carried out as shown in FIG.
2. It is also possible to provide a master sheet 11 wherein layer
16 is not coextensive in area with the reverse surface of the
master, e.g. by piecing together portions of a gallate-coated
master (for the invisible entries) and a master coated with DTO or
one of its dirivatives (for the visible entries). This piecemeal
approach involves extensive cutting and/or taping or pasting and is
less convenient than the preferred methods.
As pointed out previously, the dye precursor in layer 16 is a
polyhydroxy aromatic iron-complexing compound. The preferred
iron-complexing compounds contain an orthodihydroxy benzene or
ortho-trihydroxy benzene nucleus substituted with an acyl,
carboxyl, or esterified carboxyl radical. The preferred polyhydroxy
benzene compounds are the lower alkyl (<C.sub.6) gallates, e.g.
methyl or propyl gallate. The layer 14 on transfer sheet 13
preferably contains a N,N'-diorgano substituted derivative of DTO.
It is preferred that the organo radical substituted on the
nitrogens of the DTO have the formula R--CH.sub.2 --, wherein R is
aliphatic (preferably alkyl), cycloaliphatic (preferably
cyclohexyl), or aromatic, e.g. phenyl. A particularly useful
N,N'-disubstituted DTO compound from the standpoint of its
color-forming ability and vapor pressure is N,N'-dibenzyl-DTO
(DBDTO). The DTO derivatives with exceptionally low vapor pressure,
such as N,N'-bis-(2-octanoyloxyethyl) dithiooxamide are not
preferred, since it is difficult to match their low vapor pressure
characteristics with the lower alkyl gallates, and it is also
difficult to obtain good vapor transfer of these compounds in
conventional thermographic copying machines. The compound
dithiooxamide itself is fully operative in this invention, but
tends to have less storage ability and less desirable color-forming
characteristics than DBDTO. It is preferred that layer 14 on the
transfer sheet 13 comprise the DTO compound intimately admixed with
a particulate material such as a wax. Suitable waxes include
paraffin wax, carnauba wax, bees wax, castor wax, polyoxyethylene
glycols and esters or ethers thereof, candellila wax,
N,N'-ethylene-bis-12-hydroxy-stearamide, and mixtures thereof. In
short, paraffin, ester-type, and synthetic polymeric waxes can be
used, as can waxy materials such as cetyl alcohol. It is also
preferred to include a minor amount of a film-forming compound such
as ethyl cellulose and a plasticizer such as tributyl phosphate,
triphenyl phosphate, dicyclohexyl phthalate, a hydrocarbon oil, or
the like.
A bright-colored dye can be included in layer 14 of transfer sheet
13 to enable those using this invention to see if the mirror images
are being properly formed on the reverse surface of master sheet 11
during the visible imaging step.
As pointed out previously, the preferred co-reactant for the
DTO-type dye precursor, i.e. the metal salt present in layer 36, is
a copper or nickel salt. The preferred copper or nickel salts are
soaps derived from carboxylic acids such as stearic or oleic acid,
or the rosin acids. If the aforementioned scavenger sheet is used,
these same salts are used to scavenge DTO contamination of the
master. Nickel rosinate/clay (e.g. 40:60) is a preferred scavenger
sheet coating. As to the coating on the scavenger sheet, and as to
layers 14 and 16, conventional coating weights commonly used in the
art are suitable. Cobalt and cadmium salts are less preferred.
Silver salts (e.g. silver behenate, silver stearate, etc.) cannot
be used in the copy sheets, since silver ions will co-react with
both the iron-complexing compounds and the DTO-type dye precursors.
However, a sheet coated with a silver salt is very useful as a
proof sheet. By passing the proof sheet through the infrared
copying machine with the composite master, a visual means for
evaluating the definition and darkness of both visible and
invisible entries is obtained as a "proof." If the proof run is
satisfactory, fifty or more subsequent copies can then be run off
as shown in FIG. 2 with the assurance that good copies are being
obtained.
Although the use of this invention has been described primarily
with reference to programmed education, other uses will occur to
the skilled technician.
* * * * *