Evening star., 1930-10-30
SCIENTISTS STUDY
CAMERA MYSTERY
Silver Bromide Crystals Yield
to Research by
Bureau.
BY THOMAS R. HENRY.

Some of the deeper mysteries Of the
behavior of the silver bromide crystal
whose peculiar properties are respon-
sible for the phenomenon OF photog-
raphy-are yielding to research In the
laboratories of the Bureau OF Standards.
Only by unearthing the fundamental
chemistry of the process, 1t Is explained
by DR.. B, H. Carroll, who is in charge
of this phase of the work, can the full
possibilities of the camera be realized,
and already some notable results have
been obtained.
Explains Fundamental Fact.
The fundamental fact Of photog-
raphy, Carroll explained, Is that grains
OF silver bromide, a crystalline sub
stance IN which atoms of silver and Of
bromide are equal in number, are quite
sensitive to different wave lengths OF
light. They are mixed in varying pro-
portions with gelatirle, and the mixture
forms the sensitive covering OF the
Photographic plate. Light falling upon
the silver bromide grains on the plate
starts to release the silver atoms from
the combination. This is responsible
for the latent image, which is brought
out In detail by chemical treatment OF
the plate with a developer
Now silver bromide, Carroll explains,
while q highly light-sensitive material,
Is equaled or surpassed in this respect
by others, such as a mixture of hydro
gen and chlorine, which explodeS as
soon as a beam of sunlight strikes 1t.
The fact essential to It's importance in
photography is that the action Of lber-
ating silver atoms from the compound
started by exposure can be continued by
chemical means The problem is to get
enough action started quickly enough
10 be continued by the developer.
The research leads into some of the
most complicated fields Of modern
physics, especially the quantum theory
of light, which holds that 1t strikes
against q surface in the form of dis
crete particles Of energy, some of which
are absorbed and some reflected. The
present indications are, DR.. Carroll
says, that for each quanta, or energy
particle, Of light absorbed by q silver
bromide crystal there is released from
the molecular compound one atom Of
silver. It requires, he believes. from
100 TO 500 quanta TO make q grain Of
the silver bromide change back into
bromide and silver far enough 80 that
It can be continued by the developer.
By study of the properties of the crystal
It may some day be possible to accom-
plish the same purpose with five or 10
quanta, thus greatly shortening the
exposure necessary.
Sensitive Spots Are Important.
The outstanding new fact revealed In
recent years, Carroll says, ts that the
Eilver-hromide crystals have sensitive
spots" about which the process of re-
leasing the silver atoms appears to start.
It has been demonstrated that these
consist Of extremely minute particles Of
an impurity, silver sulphide, which
forms In the gelatin and silver bromide
mixture because of the fact that the
cows hide or bones from which the
gelatin was made contained sulphur.
This speck Of silver sulphide may
constitute q thouSandth of q per cent
Of certain of the extremely minute
silver-bronide particles, but those
grains having It start to produce the
latent image at once. Even without
the tmpurity the silver bromide viii de-
veiop. II given b long enough exposure.
If there Is too much seilver. sllphide, it
results In fogging the plate, Just as
L the latter were overexposed. The
property Of the silver-sulphide speck
appears to be that of concentrating
the action Of the light on one point
on the surface nf the silver-bromide
grain. The objective Is then to achieve
an 0ptimum condition In which there
Is Just enough silver sulphide In the
compound to facilitate to the utmost
the absorption Of quanta and yet not
cloud the plate.
The chemistry Of various photo
graphic plates, Carroll explained, is
hidden for the most part In carefully
guarded trade secrets, and the develops
ment Of the art has been largely en-
pirical. without much general under
standing of the physical and chemical
properties underlying what happens.
Density Depends on Crystals.
The density Of an image on Q photo
graphic plate. he said, Is generally un-
derstood to depend on the number Of
silver-bromide crystals that explode
that Is, start to split up. Unless the
process has been started by the light,
nothing happens in the developer.
which alone does not have the capacity
Of breaking up the molecules. It has
been possible TO study the process by
nNEW lOsNER PRlCESn