CLOUD PHOTOGRAPHY
REFERENCE has been made in the first chapter to the fact that those
who wish to make a photographic study of clouds must follow a special
course of procedure. For every photographic purpose there is some
particular process or some special kind of apparatus which is better
fitted for the end in view than any other, and half the difficulty in
attaining success is to find out the best tools and the best methods.
There is no difficulty whatever in securing excellent photographs of
heavy grey clouds, or of clouds which stand out dark against a twilight
sky. Any camera and any plate can be used, and in an experienced hand
will ensure success after a few trials, but except under these special
conditions, cirrus, in all its varieties, the alto clouds, and even
many of the lower ones, present a real difficulty due to two causes.
In the first place, they and their surroundings are so brilliant that
a very short exposure is sufficient, far shorter than would be needed
for a sunlit landscape; and in the second place, the actinic value of
the light they reflect is very little greater than that received from
the background of blue sky. When so minute a difference comes to be
represented in the monochrome of the ordinary photograph, the eye fails
to appreciate it, and all the finer details are lost.
Now, if proper care is taken in the development of a negative,
satisfactory results may be attained even if the exposure is twice as
great, or only half as great, as it should have been to get the best
result. But if the exposure is four or more times the best duration,
the negative will generally yield but poor contrasts, if any result at
all can be coaxed out. Again, if the exposure is only a quarter or less
of the ideal time, little or no image will come out. Suppose, now, we
have a brilliant object, and the correct exposure for the plate and
aperture of lens employed should be one-fiftieth of a second; if we
make an error either in judging or in effecting the exposure, which
amounts to one twenty-fifth of a second too much, we get the negative
exposed three times as much as it should be. Suppose, again, the object
is less brilliant, and the correct exposure should be one-fifth of a
second, an equal error of one twenty-fifth will make little difference.
But in photographing cirrus and such clouds, if we used the same plates
and the same lens apertures as we employ for ordinary landscape work,
we should want exposures of the order of those given by a focal plane
shutter, and a mistake either in judging or in making the exposure, of
even the hundredth part of a second, would be fatal to good results,
and would probably completely spoil the plate. Evidently one of our
first steps must be to lengthen the correct exposure.
There are four ways in which this can be done--by using a slow-acting
plate, by lessening the aperture of the lens, by putting some
transparent screen in front of the lens to shut off some of the light,
and, finally, by pointing the camera, not at the cloud itself, but at
its image in a black mirror.
Of these, of course the slow plate and small aperture are the simplest
to adopt, and all the cloud studies shown in the illustrations to
these pages have been taken on plates prepared for photo-mechanical
purposes or for transparencies. There seems to be nothing to choose
between these two brands. Orthochromatic, isochromatic, double-coated,
and many other special types of plate had previously been tried, both
with coloured filters in front of the lens and without them, without
showing any marked superiority over an ordinary plate of low rapidity.
At last the photo-mechanical plates were tried, and the efforts made
to get satisfactory cloud portraits, which had previously been marked
only now and then with satisfactory results, became uniformly and
continuously successful.
If the slow plates are exposed in the camera without either a screen or
the black mirror, the diaphragm should be reduced to a small size and
the exposure suitably adjusted. The length of exposure may generally
be judged by looking at the image on the focusing screen, and reducing
the aperture until the picture shows its detail easily. Then, regarding
the picture as that of a sunlit sea or distant landscape, judge the
necessary exposure by the brightness of the image.
No definite rule can be given. The light varies enormously from day to
day, and hour to hour, and especially with the position occupied by
the cloud relative to the sun. Thus, working with a lens of six inches
focus and an aperture of a quarter of an inch, the exposure may vary
from the quickest snap of a Thornton-Pickard roller blind to as much as
a quarter of a second, or even more. Again, using a lens of eighteen
inches focus and an exposure of a fiftieth of a second, the necessary
aperture might vary from an eighth of an inch up to an inch and a
half. But if we suppose that we are dealing with an ordinary bright
summer sky between 9 a.m. and 5 p.m., and that the clouds are cirrus
or cirro-cumulus, an aperture of about one thirty-second of the focal
length will probably give some sort of image with a snap-shot exposure.
At first the failures will be many, but a little practice will soon
enable very respectable pictures to be taken by varying either the
diaphragm or the speed of shutter. Heavier clouds of the alto types
will need rather longer exposure or larger aperture.
The lens may be of any kind, as long as it gives a well-defined image,
but there are many advantages in using one of the rectilinear type
provided with an iris diaphragm. A rapid lens is not needed; indeed, it
has been pointed out that slowness is a very great desideratum, and if
the camera is provided with a rapid lens it must be ruthlessly stopped
down. For general cloud purposes the best kind of lens is a wide-angle
rectilinear, but many occasions will present themselves on which a
lens of longer focus will be wanted in order to give more insight into
the details of some specially delicate clouds. If the lenses are good,
and the focusing is accurate, enlargements will go a long way towards
revealing the minuter structures, but the results can never be quite so
well defined as a direct photograph in a long camera.
A shutter will be essential, and it should be one which opens in the
middle, or which travels across the lens. The shutters which are
ingeniously contrived to give more exposure to the lower part of the
picture than to its upper part are useless for the purpose in view. It
should have some latitude of exposure, from about one-sixtieth of a
second up to a full second or more.
Then as to the camera. Any light-tight camera will do, and, as
the objects will all be at a great distance, it may very well be a
fixed-focus one, or may be kept set up and fixed in focus for a distant
object. If not, on setting it up it should be focused on the horizon or
most distant object possible, and not on the cloud itself. As, however,
the clouds present themselves at all heights above the horizon, even
in the zenith, it becomes necessary to have some means of pointing the
camera in such directions. To a certain extent the ordinary stand does
allow of tilting, but a special support which will allow the camera to
be fixed firmly in any position is of the greatest convenience.
If the study is meant to be at all prolonged, the best plan is to make
a suitable camera, once for all, which can be left in fixed focus, so
as to be always ready, and which can be directed with equal ease to any
part of the sky, from the horizon to the zenith. If it is intended to
use a black mirror, then a special mount becomes almost essential.
Many of the most delicate of the photographs reproduced here have been
taken with a camera of peculiar pattern, the structure of which is
shown in Plate 61. The lens is an ordinary rapid rectilinear, and the
stop used was generally one-sixteenth of the focal length. The shutter
is a light slip of aluminium, which can be drawn across from side to
side at any desired pace. The body of the camera is mahogany, with a
bellows part for getting correct focus, but when once this was obtained
the back was clamped to the tail-board and a little varnish brushed
over the clamping screws.
[Illustration: Plate 61.
CLOUD CAMERA FOR STUDIES.]
The camera swings on a couple of screws, which act as trunnions. These
pass through two upright arms, which spring on either side from the
base board, which is attached to the ordinary camera stand. This base
board can be rotated into any horizontal position desired, and the
camera can be tilted through any vertical angle by swinging it between
the uprights, and can be clamped by tightening the two trunnion screws.
These screws are so placed on the front of the camera that the lens and
its attachments on the one side nearly balance the back part of the
camera on the other side, and so lessen the danger of slipping.
Supported in front of the lens by light brass-work is the black mirror,
made of a very dark glass optically worked on the front face. It is
a curious fact that, although bits of plate-glass blackened on the
back seem to the naked eye to give a single image of sufficient truth,
if such a mirror is placed in front of the camera the second faint
image formed by reflection from the blackened surface is almost always
to be detected. Moreover, the lens with its large aperture at once
detects irregularities in the surface of the glass, which are quite
imperceptible through the narrow limits of the pupil. Black glass,
with a truly worked surface, is essential then, but the surface need
not be of the high order of excellence required for mirrors used for
telescopic work, since the first image is not, as a rule, intended to
be highly magnified.
The mirror is held so that its surface makes an angle of about 33
degrees with the axis of the lens, and the block carrying shutter and
mirror can be turned round into any position by slipping it round the
lens mount as an axis. The mirror thus always retains the correct angle.
The action of the mirror is to a large extent due to mere diminution
of brightness, but it also partly extinguishes the blue light of the
sky without exerting any such influence on the white light from a
cloud. This is due to the fact that the blue light of the sky is partly
polarized, while that reflected from the cloud is not. Now, polarized
light which falls upon a black mirror held in a particular position is
not reflected by it. This position depends upon various circumstances,
but one condition is that the reflected ray must make an angle of about
33 degrees with the surface of the glass. The amount of the polarized
component of the blue light varies greatly, but is at a maximum at all
points 90 degrees away from the sun. This, then, is the best possible
position for photographing a cloud, as the whole of this polarized
component may be suppressed by adjusting the mirror to the proper
position, and then the most delicate cirrus fibres stand out brilliant
on an almost black background.
The black mirror could with some advantage be replaced by a Nicol’s
prism mounted between the components of the lens, so that it could be
turned in any position; but Nicol’s prisms are expensive, and such an
arrangement would cost many times the sum sufficient for an excellent
mirror, and then would narrow down the field of view in a very
inconvenient way.
With this apparatus exposures of a tenth to a fifth of a second were
usually required for high clouds in bright daylight, while longer
times, up to a second, might be required under less actively actinic
conditions.
The exposure having been made, the next step is development.
Now, every practical photographer has his own pet formula, his own
particular favourite among the numerous developing compounds now on
the market. It is, therefore, rather a thankless task to offer advice
as to which should be selected. In all probability as good results may
be got by other methods and other formulæ, and the description which
follows must be understood rather as an account of the process actually
adopted, than advice as to that which should be chosen.
The developer used has been always pyro and ammonia, made up in
accordance with the formula--
Pyro 30 grains
Potassium metabisulphite 30 „
Ammonium bromide 30 „
Water 10 ozs.
But if much work was anticipated the solution was made up in a more
concentrated form, and diluted to this strength of 3 grains of pyro per
ounce for actual use.
The ammonia solution is prepared by mixing 3 drams ammonia fortiss.
with 20 ozs. of water.
In developing it is necessary to remember that our object is to make
the most of a very small difference in effect. The plate is first
flowed over with a mixture of sufficient developer, with not more than
a quarter of its bulk of the ammonia. If the cloud should flash out in
a few seconds add more of the pyro solution, but unless the exposure
has been much overdone this will not happen. If the image begins to
appear after from thirty to forty seconds it is probable that the best
result will be reached by leaving it alone, but if there is any hanging
back of the detail another quarter bulk of ammonia should be put into
the glass, the developer mixed with it, and the whole returned to the
developing dish.
If no image appears after about forty seconds, add more ammonia as
above described, and leave for another forty seconds, and so on,
until by this method of trial the right quantity of alkali for the
particular exposure has been ascertained. The development must never be
hurried, or the background of sky will blacken too soon, and in some
cases it may take a quarter of an hour or more to get enough density on
the cloud. But as a general rule the image is fully out in about two
minutes, and the plate is then washed and fixed in the usual way.
If a black mirror is used there will seldom be any necessity for
intensification, but if not, it may frequently be required, especially
for the more delicate kinds of cirrus. Indeed, the image may sometimes
be so thin that the common process of intensification by mercury and
ammonia does not give density enough. If that seems at all likely to
be the case, it is wiser to use the formula known as Monckhoven’s,
since that simply adds silver to silver instead of replacing the silver
image by some other body, and the process can consequently be repeated
more than once, if sufficient density is not secured by the first
application. The formula does not seem to be very often used, so it may
be best to quote it.
A. Potassium bromide 100 grains
Mercuric chloride 100 „
Water 10 ozs.
B. Potassium cyanide (pure) 100 grains
Silver nitrate 100 „
Water 10 ozs.
Place the washed negative in A until it has gone white, then rinse it
well and transfer to B, in which the image turns to a velvety black.
After washing, the process can be repeated.
Intensification is, however, only a way of saving photographs which
cannot be secured again. If the first photograph of a particular
variety of cloud is not satisfactory, it ought at least to tell the
operator where he had gone wrong, and a second attempt should produce
a better result than any image built up by chemical action on an
imperfect base.
There is nothing novel in any of these methods, and there is no doubt
that other formulæ would be as good; but the one thing essential is to
have a developer whose action can be held under control, and to apply
that developer in such a way that very considerable over-exposure
will not result in the ruin of the plate. If a number of photographs
have been taken in about the same part of the sky, and within a short
time of each other, then the correct proportions of developer and
alkali will be nearly the same for all, but the first of such a batch
will always have to be attacked in the cautious step-by-step method.
Patience and perseverance, backed by a steady refusal to be discouraged
by the failures which are at first inevitable, are as certain to be
crowned by success as they are in other studies.
The workers are few, and there is much to be done; for it is mainly to
those who will photograph the higher clouds, and so trace the stages
of their growth and decay, that we must look for the data which will
enable us to solve the problems they present, and so enlarge the narrow
boundaries of our knowledge of some of the most beautiful things in
Nature.
REFERENCES
1. “International Atlas of Clouds” (Atlas International des Nuages).
Hildebrandsson, Riggenbach, and Teisserenc de Bort. Paris. 1896.
This is the atlas referred to in the text. The letter-press is short,
and is repeated in English, French, and German.
2. “Annals of the Astronomical Observatory of Harvard College.”
Vol. XXX. Observations made at the Blue Hill Meteorological
Observatory. Part III. Measurement of Cloud Heights and
Velocities. By H. H. Clayton and S. P. Fergusson. Part IV.
Discussion of the Cloud Observations. By H. H. Clayton.
This last gives a very concise account of all the different proposals
which have been made for the systematic naming of clouds.
3. “Études International des Nuages.” 1896-1897. Observations et
Mesures de la Suède. I., II. Publication de l’Observatoire
Météorologique de l’Université Roy. d’Upsala. H. H. Hildebrandsson.
An account of the Upsala observations referred to in the text.
4. _Quarterly Journal of the Royal Meteorological Society_.
Helm Wind. Marriott. 1886 and 1889.
The Thickness of Shower Clouds. Clayden. 1886.
Methods of Cloud Measurement. Ekholm. 1888.
Cirrus Formation. Clayton. 1890.
Nomenclature of Clouds. Hildebrandsson. 1887.
„ „ Abercromby. 1887.
„ „ Wilson-Barker. 1890.
„ „ Gaster. 1893.
„ „ Scott. 1895.
A New Instrument for Cloud Measurement. Ekholm. 1893.
Calculation of Photographic Cloud Measurements. Olsson. 1894.
The Motion of Clouds. Shaw. 1895.
5. Reports of the British Association. Reports of the Committee on
Meteorological Photography. Clayden. 1891 to 1900.
The reports for 1896 and 1900 refer mainly to the measurements
described in the text.
6. “Cloudland.” Clement Ley.
The work in which Mr. Ley set forth his proposed scheme.
7. “A Popular Treatise on the Winds.” Ferrel.
Not a “popular” work in the usual sense, but contains lucid
descriptions of the mechanics of the atmosphere.
8. There are many excellent text-books on meteorology, all of which
deal more or less with the movements of the atmosphere and the
formation of clouds.
INDEX
Abercromby, 10
Aitken, 3, 91
Altitude of clouds, 19, 149
---- of rain-clouds, 76
Alto clouds, 59, 159
Alto-cumulus communis, 161
---- castellatus, 66, 160
---- glomeratus, 67, 161
---- informis, 64, 160
---- nebulosus, 65, 160
---- stratiformis, 65, 161
Alto-strato-cumulus, 70, 160
Alto-stratus maculosus, 68, 160
Ascent of vapour, 124
Atlas, International, 11
Band cirrus, 41
Bidwell, Shelford, 92
Black mirror, 14, 173
Blue Hill, 20, 138
Cameras, 140, 171
Change cirrus, 37
---- of velocity, 35
Cirriform top of thunder-cloud, 110
Cirro-cumulus, 45, 159
---- nebulosus, 52, 159
Cirro-filum, 33, 156
Cirro-macula, 53, 159
Cirro-nebula, 26, 27, 155
Cirro-stratus, 45, 157
---- communis, 47, 158
Cirro-stratus cumulosus, 48, 158
---- nebulosus, 45, 158
---- vittatus, 158
Cirro-velum, 24
Cirrus, 21, 155
---- altitudes, 30, 149
---- communis, 40, 157
---- caudatus, 34, 156
---- excelsus, 31, 156
---- inconstans, 37, 157
---- nebulosus, 36, 156
---- ripples, 51
---- ventosus, 32, 156
---- vittatus, 41, 156
Cloud altitudes, 149
---- nuclei, 3
---- photography, 17, 165
Condensation, 93
Cooling by contact, 89
---- by expansion, 92
---- by mixture, 90
---- by radiation, 88
Development, 175
Differential currents, 133
Diffraction, 63
Dimensions of clouds, 112
Dry fog, 73
Equilibrium, stable and unstable, 106
Exeter measurements, 137
Exposure, 167
Fall cloud, 81, 88
False cirrus, 110
Fog particles, 60
Fracto, 164
Gravitation, 4
Great waves, 129
Halos, 21, 22, 23
Heat cumulus, 85
---- thunderstorms, 107
Hildebrandsson, 10
Hoar frost, 60
Howard, Luke, 9, 10
Intensification, 177
Interconvection, 125
International Code, 11
---- Committee, 10
---- System, 12, 71
Ions as nuclei, 92
Irregularities of ground, 39
Kepler, 4
Kew, 138
Ley, Clement, 24, 34, 55
Lightning, 114
Lower clouds, 71
Mammato-cumulus, 99
Meteorological Conference, 9
Methods of computing altitudes, 143
Munich, 9
Newton, 4
Nimbus, 74, 163
Nuclei, 92
Photographic methods, 17, 165
Rain-clouds, altitude, 76
----, thickness, 81
Rate of fall of temperature, 94
Sandstorms, 117
Saturation, 87
Scotch mist, 73
Slow plates, 167
Spread of condensation, 57
Stable equilibrium, 106
Strato-cumulus, 77
Stratus, 72, 161
---- communis, 77, 161
---- lenticularis, 80, 98, 162
---- maculosus, 78, 162
---- radius, 79, 162
Subsidence of cloud top, 111
Surfusion, 61
Swing stand, 171
Turreted cloud, 66
Tycho Brahe, 4
Types, 8
Umbra and penumbra, 29
Undatus, 164
Unsaturated cloud, 101
Upsala, 10, 138
Wave clouds, 47, 119, 133
Wilson, C. T. R., 92
THE END
PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES.
* * * * *
Transcriber’s Note
Variations in hyphenation (i.e. thunderstorm and thunder-storm) have
been retained. The following apparent typographical errors were
corrected:
Page 171, “focussed” changed to “focused.” (it should be focused on the
horizon)
Page 173, “aperature” changed to “aperture.” (the lens with its large
aperture)