Pittsburg dispatch. [volume] (Pittsburg [Pa.]) 1880-1923, 1892-02-07

are made visible and palpable in the cam
era, the question suggests itself, why may
not the broad, blank interval between light
and sound be spanned in the same way? IL
would be hardly more surprising than the
process by which a bar of warmed iron
paints Its portrait in dark room, or
chemical ray registers itself beyond the
verge of visible color sent forth by lncan-
descent metal Just here it Is worth re-
membering how mechanical pressure may
directly produce visible chemical change.
Where stylus has sharply indented o bit
Of silvered paper an image can be developed
exactly such as light itself might have i'm
printed.
The master problem of photography IL
the seizure Of color as well as of form in
the camera. In approaching this problem
experimenters have availed themselves OF
the sensitiveness Of various dyes to req,
greenland violet RAYS. Although six or seven
leading distinctions of color can be discrimi-
mated by the eye, it is held that red, green
and violet underlie them all. In this process
are exposed, each of which has been sensi-
tized for one of the elemental colors. Having
obtained positivcs of corresponding t1nts,
producing picture with colors much re-
sembling those of nature.
The Question OF Permanency.
By an ingenious application OF methods
similar to those Of chromo-lithogra-
ply, an artist can produce permanent pos
ftives of great beauty. That is certainly
roundabout way of making the rainbow
paint itself. but no other attempt is either
SO satisfactory or so promising. Thus 1s
would seem that the germs of success in
catching color, as well as form, in the cam
era may lie in the same gift from DR.. Vogel,
which was made the message of light
%I,, lo-i1, Ia
and given new truth ard beauty to every
Photographic transcript from nature. When
certain German chemists sought few years
ago to dye some yards of silk and woolen
piece of rare good fortune that they failed.
In its early days Photographic printing
was restricted to slow and costly chemical
methods. negative, as now IN ordinsry
portraiture, imprinted Its positive, and had
to take its course through series OF ton1ng,
fixing and cleaning baths. Was there not
some feasible way by which light could give
ing press where it could impress itself as
speedily as type and in ink both cheap and
permanent? Niepce's process, one OF the
as to how the task might be accomplished.
Its plates were coated with bitumen, and
that was rendered insoluble by the solar
ray.
A Property OF Gelatine.
A substance much easier to use and pos
sessed OF the same property was found to be
the very gelatine to which in other apph-
cations photography is so much indebred.
Combined with bichromate OF potash
luminous beam renders it insoluble in water,
upon which simple fact turns wide variety
of photo-mechanicai processes. These In
their last refinements give US reproductions
hardly inferior to originals and compositions
with AL the mezzotints delicacy of tone.
In the simplest processes . sketch in pen
and ink, or a line engraving, is placed face
downward upon sheet OF sensltiZed gela-
tine. After few minutes action Of sun
light the geiat1ne ea can ssh be washed s out % OF
solar beam by the black lines. By stereotypy
or eectrotypy the gelatine used as mold
yields relief plate in metal which can pc
printed from In an ordinary press. In
another process the unhardened gelatfne is
not washed out, but swollen with water, and
from the resulting projections metallic
cover co.. m s plate of zinc or copper with a film Of
gelatine or other similar material. After
exposure the film is washed away, except in
the lines OF its picture. At all points unpro;
tected by the film the plate Is then etched
or bitten in by an acid bath. A recent
adaptati tion of the sand blast has produced
good work on such plate. The
acid, in that It does not burrow at the sides
of the thin walls beneath the lines Of
picture.
The Half-Tone Processes.
The simplest OF these processes renders
only such lines rs those of an arohitect's
plan Of line engraving or of pen-and-ink
sketch How can the difficulty of express
ing half tone, of graduated shadow, be over
come? Usually by interposing between the
gelatine and the picture to be copied net
work of fine lines ruled closely together,
with the result that, if inspection be not too
close O faithful Transcripts in dots is con-
veyed to the plate. The half-tone process
devised by Mr.. F. z. Ives, of Philadelphia,
follows an entirely different principle in
very ingenious way. From gelatine relief
plate he takes a cast in plaster. An linked
pad, or film, of elastic V-shaped lines or dots
is pressed against the plaster until the lines
or dots are completely fiattened out where
they meet the highest parts OF the cast.
When the cast is removed there remains i'm
pressed upon it an ink picture having the
appearance Off photograph, but made up
of sharply defined lines and dots, graduated
In size like those of q wood engraving. That
can be photo-engraved as II IL were draw
ing, or transferred to Zinc or copper and
etched into relief.
The finest issue of the marriage Of the
camera with the printing press is the photo
gravure. Its delicate grain is derived from
the carbon, blended with the gelatine OF its
plate. To confer jnk-holding power upon
the copper 1t is busted while hot with finely
powdered resin. 1t is probable that in the |
near future we shall have one or two of the
processes still further simplihed, which will
mean to the amateur almost az valuable . |
gift as that of the gelatino-bromide plate.
GEORQE IIEa