An alternative method is to leave the hinge on, place
the filter on a thickness or two of blotting paper on a
level surface and place a card on top with a good-sized
weight, four pounds not being too heavy for a 12x12
cm filter. Direct heat cannot be used for drying, as this
causes the edges to dry first and gives rise to distortion.
At the end of the three weeks the exuded balsam
should be scraped off, and the glass cleaned with alcohol
and newspaper, then with bits of cloth and finally
polished. Do not try to be sparing with the cleaning
cloth, or use one large cloth; little bits and each piece
thrown away as soon as it gets sticky is the easiest way.
The final polishing should be done with tissue paper and
alcohol, following the same plan, that is, fresh pieces
continually. Benzol, xylol or chloroform should not be
used, as they are energetic solvents of balsam and will
almost inevitably creep in between the edges, in which
case the job will have to be done all over again.
Those who would like to make their own preparation
of balsam may purchase some dried Canada balsam from
a lens worker or optician. This should be roughly
powdered, which is most easily done, though it is rather
wasteful, by tying it up in a cloth and hammering it
with a heavy hammer; a fine powder is not wanted but
the big pieces should merely be broken up. Then place
this in a wide mouthed bottle, place in the water bath
and bring the latter slowly to a boil, stirring the balsam
all the time; add about one fifth of its weight of xylol,
stirring well and then letting it get cold in the water
bath. This preparation requires a much higher temperature
to melt and must be used hot. It then sets very
quickly and at a pinch a filter thus cemented may be
used the next day. The only difficulty likely to be met
with is the setting of the balsam before an even film is
obtained, but warming the glasses, or keeping them on
a hot plate for some time under pressure will soon make
the balsam spread out.
There are four possible positions for the filter; in
front of the lens; between the combinations close to the
diaphragm; behind the lens; and immediately in contact
with the sensitive surface. Between the lenses is
the very worst place to choose, although this requires
the smallest filter, as is obvious. In the first place, it
is very likely to upset the corrections of the lens, particularly
with the later forms of anastigmatic lenses, and
with these there is often not enough room to insert any
other than a film filter. Secondly, it is not easy to
change the filters in this position without some special
fitting, so that we can dismiss this at once. Either in
front of or behind the lens may be chosen, which one
being a matter of indifference, provided focusing is
always effected through the filter, a matter that we
shall have to deal with later on when talking of screen
plates (See Chapter XII). In either case some sort of
sliding fitting is advisable, although this is not conve
lent in some cases inside the camera, as not only may
the rear lens protrude beyond the lens board, but one
has to have some means of shifting the filter between
exposures, which necessitates a light-tight fitting.
It is possible in many cases to arrange a frame to
slide over the camera front, and to fit the ordinary lens
panel on this, so that the filters will be behind the lens.
The sliding frame can be made on the same lines as the
usual lantern slide carrier, and if velvet is used to line
the outer frames there will be no trouble in making it
light-tight. Or it may be possible to fit such a frame on
the lens barrel itself, but here it must be so securely
fastened that there is no chance of its slipping off.
Really the simplest plan is to obtain one of the square
slip-on cells, which, fitting on the lens hood or barrel,
may be always retained in position and the filters merely
lifted out and inserted as required. It is advisable, if
possible, to remove the lens hood and fit the holder on
the barrel, as this means not only slight reduction in
size, but as a rule a firmer hold. In order to obtain the
correct size of fitting, the diameter of the lens tube
should be taken with a pair of sliding calipers. Failing
these, the next best plan is to take a narrow strip of hard
writing paper and wrap round the lens barrel so that the
ends overlap by about half an inch, then with a sharp
penknife cut right through both pieces of paper midway
of the overlap, not at the end.
Placing the filter close to the plate means that the
filter must be of the same size as the plate. Defects in
the filter, such as want of absolute parallelism of the
surfaces, are here of the least consequence; but local
defects, such as coating striae or bubbles, are more
apparent on the negative image, though only locally.
Special sliding backs can be? obtained commercially, fitted
with the three niters and made to take three plateholders,
or with some plate-holders, particularly of the
English book-form pattern, the filter may be placed in
actual contact with the sensitive surface; then naturally
its thickness must be allowed for in focusing.
One important point in the choice of filter fittings,
particularly metal ones, is that there should be no
abnormal pressure on the glasses, as this may cause
strain and consequent degradation of definition. It
should be possible to turn the filter round, or shift it,
with the lightest pressure of the fingers. Neither is it
advisable to use cells screwing into the lens hood, as
this is almost certain to shake the camera and there
is much loss of time in changing.
For photomechanical work, in which long-focus lenses
are nearly always used with half-tone screens, the glass
must be optically worked, as carefully, in fact, as the
lenses themselves. Such glasses are known commercially
as
"
optical flats," and are very costly if of any size.
They must all be absolutely the same thickness and be
so arranged that they are always perpendicular to the
axis of the lens.
The ordinary filters may be used for making the separation
negatives for photomechanical work, as if there is
not absolute coincidence of size, this can be corrected by
the operator when making the screen negatives from the
transparencies, though he will not be pleased at having
to do this.
It may possibly be as well to interpolate here a note
as to the making of the constituent negatives for photomechanical
purposes. The use of the panchromatic
gelatine plate for this work is largely on the increase,
and in some cases the slow panchromatic plate is used
for making the color separation and the screen negatives
in one; but the usual practice is to make the separation
negatives first, from these a set of transparencies, and
then the screen negatives. It may be noted that the
transparencies for such work should be as little like a
lantern slide as possible. They should be fully exposed,
quite "soft" in character, and with practically no bare
glass except in the very deepest shadows. Full exposure
should be given to the plates, and it is better to use slow
negative rather than transparency plates for this work,
as giving a longer range of gradation and less tendency
to brilliancy. The exposure should be full and development
not pushed too far, so that the highest densities
are quite transparent.
In many commercial process establishments collodion
emulsion, and even the wet plate process, still hold their
own for the making of the separation negatives, and also
the combined separation-screen negatives. Usually the
emulsion is obtained commercially with its special sensitizers,
and the makers issue instructions for the making
of the filters, which are usually of the liquid cell type,
for use with the same. On the other hand the method of
sensitizing already advised may be adopted, or the dye
may be added to the enrulsion, and in this case 80 ccm
of sensitol violet stock solution should be added to 1000
ccm of the plain emulsion, and the plates washed in running
water or under a rose tap for fifteen minutes. The
washing increases the sensitiveness of the plates about
five times.
By some writers it has been proposed to use different
plates for the different color separations, that is to say,
an ordinary, non-color-sensitive plate for the minus
yellow negative; an orthochromatic plate for the minus
red negative; a panchromatic or red-sensitive one for
the minus blue negative. This plan may at first sight
appear to have certain advantages, but this method is
not one that should be adopted. It is a well established
fact that the degree of contrast, or gamma, differs
with different kinds of plates, and in fact with different
batches of the same kind of plate, to say nothing of the
development velocity of the plates, and one of the most
important essentials in making separation negatives is to
have them of the same degree of contrast as far as possible.
That is to say, in the three negatives the range
of densities of a black and white scale should be the
same; and with three totally different kinds of plates
this is almost an impossibility. One kind of plate
should be used for all three separation negatives, and
they should be, as already pointed out, as far as convenient,
developed together. The adoption of this plan
will save no end of after manipulation and dodging in
getting concordant results; and it may be taken as an
axiom that hand work, except for the removal of purely
mechanical defects, such as pinholes, etc., cannot be successfully
executed with color negatives.