PhotoHistory

From: PHOTOGRAPHY OF TO-DAY
SUNDRY APPLICATIONS OF PHOTOGRAPHY

By H. CHAPMAN JONES, 1913

It is common enough to photograph the clouds, for almost all out-of-doors views, whether taken for scientific purposes or merely for amusement, include a part of the sky. But it is a well known fact that the sky, including of course the clouds, is very rarely as clearly shown in the photograph as it appears to the eye. This is due to three well understood reasons. If a sufficient exposure has been given for the other part of the view, the sky will have had an excessive exposure, and over exposure, as we have shown in earlier chapters, causes a want of contrast in the picture because the brighter parts do not continue to produce a proportionally increasing effect upon the plate. This can be remedied by making the exposure suitable for the clouds and sacrificing the rest. It will often happen that blue sky and white cloud are hardly distinguishable in the photograph, because of the excessive sensitiveness of the plate to blue. That is, the plate is affected similarly whether, as in the white light from the cloud, the blue is mixed with green and red to give the white, or whether, as in the sky, the blue predominates or is alone. This is remedied by increasing the sensitiveness of the plate to green and red and using a yellow filter or screen to absorb some of the blue light. The third reason is, that there is always a certain amount of mist in the air due to the small particles in which it abounds and which can be seen in a beam of bright light. These particles reflect or scatter the light according to their size, the larger particles scattering light of a longer wave length. The smallest particles appear to be always present, and it depends upon meteorological conditions as to what extent these are mixed with particles of larger sizes. Rain “clears the air” as we say, washing down these larger particles, and the wind has a considerable effect in distributing them as they rise from fires or whatever smokes or fumes or stirs up dust. Small particles scatter ultra-violet light, rather larger particles scatter blue, larger still the green and then the red. So far as light is scattered by aerial particles, the air is misty, and as photographic plates are sensitive chiefly to the blue and violet and ultra-violet, as the particles increase in size the air becomes misty to the plate before it is misty to our eyes, because the brightest colours to us are green and yellow. So that the air may be misty photographically and clear visually, and it is invariably the case that it is more misty to the plate than to the eye. For this reason all objects at a distance, including clouds, appear in photographs as if seen through a mist, and a denser mist than we can see. This difficulty is remedied by stopping all the ultra-violet and a large portion of the blue, or it may be all the violet and blue, by fixing a suitable deep yellow filter to the lens. By observing these precautions it is possible to photograph clouds with all their detail and intensity and to produce pictures suitable for the study of them, though as they are so constantly and rapidly changing, it is impossible to make fully detailed drawings of them by hand.

The heights of clouds can be measured in different ways. Sometimes they are so low that the upper parts of high buildings are hidden, or the mountaineer or the aeronaut may pass through them on his upward journey, and then their height can be measured by direct observation. But there may be no building or mountain and no balloon or flying machine at hand, and there may be clouds above the reach of such means, so that these methods are not of general applicability. By photographing them from two points simultaneously, and measuring on the photographs their apparent alteration in position due to the different points of view, their heights can be determined by ordinary trigonometrical methods. The two cameras must be so connected that the shutters of both are released at the same moment, but this arrangement offers no practical difficulty.

There is nothing to be said of a general character as to the photography of rainbows and auroras, and by the use of screen color plates they can be shown in color. Photographs of these phenomena are of interest because the opportunities of getting them are so rare. Lightning is also comparatively rare, and the method of photographing its flashes is essentially different from the usual process, because they do not remain for long enough to permit one to focus and expose, or even to expose the plate if the focus has been previously adjusted. It is a mistake, however, to suppose that lightning is an exceedingly rapid effect, for a careful observer will frequently find that after seeing the first glare, he has time to turn his eyes in its direction and to actually see the flash itself. This appears to be due to the multiple character of many flashes. The first discharge is not complete, and a second, third, and even a fourth may take place along the same path, as if the first had made an easier way for the subsequent discharges.

Lightning is photographed at night by pointing the camera towards the direction where it is likely to occur, keeping the lens open. If no flash comes and it is considered that the plate is probably fogged by the general illumination of the sky or perhaps by flashes in other directions, that plate is lost, and another must take its place. As soon as a flash comes in that part of the sky represented in the camera, the lens is covered and the plate removed for development. It is not satisfactory to allow the plate to remain with the lens open with the idea of getting the picture of another flash upon it, because a feeble light gaining access to the plate after the image of the flash has acted on it, is very liable to so affect the plate that the flash is “reversed” and appears as if it had been black instead of bright. To show the multiple character of flashes, the camera, with the lens open, is continually moved from side to side. Each constituent of the flash passes so rapidly that the movement of the camera does not affect the sharpness with which it is depicted upon the plate, but there is a perceptible interval between each constituent and the next, so that the image of the next falls upon a different part of the plate. By the kindness of Dr. H. H. Hoffert we are able to give, facing page 324, a copy of a photograph of lightning taken by him on June 6, 1889, by swinging the camera from side to side as he held it in his hands. It shows the multiple character of at least three separate flashes. The most conspicuous flash is shown to consist of three consecutive discharges, and doubtless there was another before these, the image of which did not fall on the plate. This photograph shows also that the air remains continually luminous, at least in parts, during the intervals between the consecutive discharges which we associate together as a single flash.

It may be asked how is it, if we get three or four distinct images on the plate, we do not see the multiple character of the flash by our eyes, why cannot we see the three or four flashes one after the other? That is, as we have explained before, because we see nothing for less than about the one-tenth of a second the impression produced in our eyes remains for that time, and if the second flash comes before the impression of the first has died away, it appears to us to be a continuous effect, and there is no intermittency so far as our eyes are concerned. Although lightning is not a very frequent phenomenon, a great deal has been done to elucidate its character by means of photography.

The consideration of the photography of phenomena in the sky naturally leads us to think of those objects that are far beyond the clouds, beyond even the blue sky, and that range in their places away to distances that it is impossible for the mind of man to conceive of. We may talk of millions of miles, of thousands of millions of miles, and millions of millions of miles, but we cannot comprehend such distances. Anyone who has a camera and knows how to use it can photograph the sun, moon, and stars, but the photographs obtained would be of no use for astronomical purposes. The image of the sun or moon produced by such a lens as is generally used with a half-plate camera would be about the sixteenth of an inch in diameter, and this is far too minute to be of service. The moon and the stars being far inferior to the sun in brightness would require long exposures, and if the camera was fixed they would show as streaks or lines of light because of their apparent movement in the heavens. For practical work therefore we must have a camera and lens that will give a much larger image and that will move constantly and regularly so as to compensate for the earth’s rotation. Such an arrangement is an astronomical telescope with the eyepiece removed and a holder for the sensitive plate put in its place. The telescope is kept moving by a specially constructed clock, but although such mechanism is sufficiently good for eye observations, it is not perfect enough to keep the image still upon the plate for the protracted exposures that are sometimes necessary. A smaller telescope attached to the main instrument has in its eyepiece two fine wires which cross each other, and the observer makes by hand such adjustments as may be necessary to keep the image of the required star exactly on the cross wires.

If a larger image is required, the lens used must have a greater focal length, for the diameter of the image increases exactly in proportion to the focal length of the object glass. And if the focal length is great, the diameter of the lens must be increased also, because we must have more light to produce a larger image if we wish to maintain the brightness of the image. Thus astronomical telescopes are long and large, and the size of the larger instruments is limited only by their cost and the mechanical difficulties of manipulating them. It is easy to see the reason for this with regard to the sun, moon, and planets, which give images of measurable size with the smaller telescopes, but the stars are only points of light to even the largest instruments, and if their images are anything more than absolute points, it is because of the optical phenomena concerned in producing the images and the imperfections of the lens. If then we have mere points of light, what is the use of trying to get a large image? The size of the image in this case is not the image of an individual star, which indeed gives no real image at all, but of the little patch of sky that is being dealt with. The larger the image the more space will be shown on the plate between the stars, and this means that two or more stars that are so close that they seem to be joined to form one star when using a small telescope will be separated by a larger instrument. When dealing with objects of small luminosity, such as stars, nebulae, and comets, the light-gathering power of the telescope, which is represented by the size of the object glass, is of great importance. Doubling its area is equivalent to doubling the luminosity of the object, and thus stars that are not bright enough to be visible with the smaller telescopes are brought into view, and as it seems probable that the feebler stars are generally less bright than others because of their greater distance, the “penetrating” power is increased with the light-gathering power. Every telescope has its fixed limits in this direction so far as the eye is concerned, for what an observer cannot see, he will not be able to see by looking for a longer time. But in photography this limit disappears, for the plate stores up the effect the feeblest light produces in it, and it is only necessary to prolong the exposure sufficiently to get registered upon it objects that we can never hope to see because they give out so little light. The limit here, therefore, no longer depends upon the telescope directly, but upon the possible increase in the duration of the exposure, or, what is equivalent, increase in the sensitiveness of the plate used.

Astronomers have realized the advantages that photography should offer them from the earliest days of the Daguerreotype, and with the advent of collodion plates and gelatine plates, the applications of photography in this direction have steadily increased until simple eye observations are generally of secondary importance. The images that the largest telescopes give of the planets are small, about perhaps a quarter of an inch in diameter. It is rarely if ever of use to enlarge them because the definition is not good enough. The difficulty here is not so much instrumental, but because of the constant movements of the air and the different temperatures of the various currents and moving masses. As air varies in temperature it varies in density and consequently in refractive power, as may be seen by looking across a chimney top from which hot air is rising, or over a flame, at objects beyond. This moving of the image is often sufficient to make photographic work impossible. Here, therefore, a practical limit to the duration of exposures is often imposed upon the patient astronomer. But in spite of these and other difficulties Professor Lowell recently stated of photography in connection with Mars, that “the camera has shown itself capable of rising beyond the confirmatory into the discovery stage, for one of the plates was instrumental in the detection of a new canal.”

There is one difficulty, which, however, might have been predicted and may in due time be obviated, namely that the image on photographic plates consists of metallic silver, and silver is affected by the atmosphere. The image is not permanent. We see that silver changes by the action of the air when the metal is in mass, as articles for domestic use need constant cleaning to remove the tarnish that disfigures them. When there is on the plate the feeblest deposit that can be seen, as in the images of the least brilliant stars that can under the circumstances affect the plate, we cannot be surprised that the minute amount of metal present should suffer change throughout its whole mass and so lose its visibility, for it seems probable that any change from the blackness of the original deposit must tend towards a loss of density. It is probable also that many astronomers regard photography as a simple mechanical process not worth consideration, and that they do not pay sufficient attention to the work to get as nearly as possible to an image of pure silver in clean gelatine, for any plate in which these conditions are not approximately fulfilled is likely to contain within itself the sources of its deterioration. This is not a matter to be lightly set on one side, for in some cases it has been found that as many as a third of the total number of star images have disappeared within ten years.

In the consideration of lenses we saw that whenever light is bent out of its straight path by passing into a second medium of a different kind, the constituents of light are bent to different degrees, so that the light is separated into its parts, as in the rainbow. This is a difficulty in lenses which has to be overcome as far as possible, but by taking advantage of this method of analyzing light it is often possible to tell the nature of the substance that is luminous, and for this purpose it does not matter whether the light comes from a lamp on the laboratory bench or from a star so far away that its light takes hundreds of years to travel from it to us. By this method of light analysis it is possible to tell, not only the nature of substances that can be made luminous in the laboratory, but also what substances are in the sun, in the stars, and in comets; not in the planets, for they shine only by reflecting the light of the sun, as the moon does.

When light that passes through a small hole, or preferably a slit, is spread out into a band of colors, red, orange, yellow, green, blue, and violet, the result is called a spectrum, and the instrument that effects the analysis a spectroscope. A full or continuous spectrum, that is one showing all the colors and with no gaps, is obtained when the light from a white hot, solid, non-volatile substance is examined with a spectroscope. But volatile substances in general give off light of less, often very much less, complexity. The metal thallium, for example, when heated in a flame gives a green light that cannot be decomposed. However much the light is bent, there is the one green line or picture of the slit through which it is admitted to the spectroscope, just as if the light had not been bent at all. This green light is simple and cannot be decomposed, and thallium is the only substance that produces light of exactly this character, and therefore, when such light is obtained we know that thallium is there to produce it. Similarly sodium, one of the constituents of table salt, gives a yellow light, which in the same spectroscope is never bent so much as the green light due to thallium. Most substances give very complex lights when their vapors are made to glow, and their spectra consist of a number of “lines,” or images of the slit, separated from each other, and from their number and positions the substance that gives rise to them can be identified, for no two substances when their vapors are luminous have ever been found to give the same spectrum. In this way we know that the sun and stars contain iron, sodium, calcium (the metal of lime), hydrogen, and numerous other substances of which our earth largely consists. And there are many other applications of this method of light analysis to which we have not space to refer.

In eye observations of spectra it is obvious that we must be limited to light that affects the eye, that is to visible light. But we can photograph not only all the light that is visible, but a great deal more that extends in the spectrum beyond the red at one end and the violet at the other. Photography here, therefore, at the same time that it gives a permanent record that can be measured at leisure, vastly extends the range of the work, and has brought out facts that could never have been known without its aid.

In an ordinary spectroscope there is a narrow slit through which the light from the source passes, so that the constituents may be separated with as much precision as possible and without overlapping. But suppose that there is no slit, and that a flame is colored with both thallium and sodium at the same time, we can get an image of the flame as easily as of the slit by suitable adjustment of the instrument, and now we shall get a yellow image and a green image, side by side, two colored images of the flame, the one produced by the yellow light of the sodium and the other by the green light of the thallium. The two lights mixed in the flame are separated as completely as if the metals had been vaporised in two separate flames. Now the sun contains many substances, and by isolating the light from one of them it is possible to photograph the sun by means of that light alone, and so to ascertain the proportional distribution of that particular substance on the surface of the sun. The sun has been photographed by means of its calcium light, and where that light is bright, there calcium is in large quantity, and where it is dull there it is in smaller quantity.

Another application of photography in spectrum analysis is in detecting double stars that are too close together to show as two distinct stars by the most powerful telescopes ever constructed. This might seem at first a hopeless problem, but the method of attacking it can be made clear by analogy. When a whistling engine or car passes by us, the whistle giving the same note continuously as is usual, the note is quite clearly “lower” as it recedes from us than as it approaches us. It generates sound waves of the same length all the time, but as it approaches us each successive wave starts nearer to us than the preceding wave and so a greater number enter our ear in a given time; and as it goes from us each wave starts farther away from us than the preceding one, and therefore we have fewer in the same time than if the whistle was not moving. As we have said before, our eyes and ears know nothing of what goes on outside them, they are affected only by what goes into them; it is therefore not the length of the sound wave that concerns us, but the number of waves that we receive in a given time, that is the frequency of the impulses. As the sounding whistle approaches us we receive the impulses more rapidly, therefore the note is higher, and as it goes from us we receive them less rapidly and therefore the note is lower than if the whistle were stationary. Exactly the same thing happens with light, and if two stars are rolling round each other, when one is approaching us and the other receding, each line in the spectrum will be doubled, the two new lines being one on each side of the position of the single line that will be seen when both the stars are moving at right angles to the line of vision. The line that is displaced toward the red end of the spectrum will represent the increased wave length caused by the receding star, and the line displaced towards the blue end, the shortened wave length due to the approaching star. Thus the spectrum line gradually divides, the two lines separate, and they come together again to form one line according to the movement of the stars, and the period of revolution and the rate at which the stars are moving towards us or from us can be calculated from the displacement of the line.

But it is impossible to describe the various applications of photography adequately without writing a treatise on each of the subjects concerned. We must therefore be content with mentioning only a few other subjects with which it is inseparably connected. Stereoscopic views show objects in their full solidity by giving each eye that view of the object that it would receive if the solid object were being looked at. The difference in the view as seen by the two eyes is often very small because the points of view of the two eyes differ by only a little, they are so near to each other. It would be hopeless by hand drawing to represent these differences with any approach to sufficient accuracy, but here photography is perfect. And the stereoscope is not a mere plaything or interesting toy; a stereoscopic view is to a single photograph just the difference between binocular and monocular vision if the photographs are properly made : it reveals the true shape of things, and the comparative distances of things or of their various parts. It is applicable to microscopic objects as well as to larger views, and even the moon has been subjected to similar treatment by photographing it in two different aspects, for a pair of stereoscopic views may be made from the same point, if the object is turned a little so as to present the same appearance that it would have from another point of view. The stereoscopic photographs of the moon show it as it would appear to an enormous giant with eyes a vast distance apart.

In making observations with scientific instruments it often happens that the field of the instrument is but feebly luminous, and that the adjustments of it alternate with the reading of a scale. The scale must be rather brightly illuminated that its fine divisions may be seen, and the constant changing of the eye from a dull to a bright object not only strains it but may impair the accuracy of the observations. Cameras have been made to photograph the scale whenever a reading is desired, and this not only spares the eye and facilitates the adjustments, but gives a record free from bias or possible misreading. Sixty readings can be recorded on a small plate, so that the expense for photographic materials is negligible. Similarly, cameras have been designed for photographing gas, water, and electric meters, watchman’s clocks, &c., and for the continuous and automatic recording of the readings of meteorological instruments, such as barometers and thermometers.

We began our consideration of this subject by stating that photography is writing or drawing by means of light. We have seen how almost every thing and every movement can be recorded and often investigated by its means, and how it is possible in some cases to deal with and investigate the light itself as well as the objects that the light may illuminate. In leaving light we are really going away from photography, yet we may refer to the use of photographic methods in other connections, for the general method of dealing with photographic plates is the same whether they have been affected by light or by any other force that produces a similar change in them. Light so changes the silver salt of a gelatine-bromide plate that a developer is enabled to remove the bromine and leave the metal silver as direct evidence of the change. The silver salt is rendered amenable to the action of the developer by other agencies than light. It was by putting a piece of a uranium mineral near to a plate and leaving it there for some time, that it was discovered that the mineral affected the plate, and this led eventually to the discovery of radium. Since then a great many other substances have been found that give out a something, either rays similar to light rays, or gases, or something of the kind, that affect a photographic plate. It seems that so common a substance as potassium, which is the essential constituent of potash, gives out something of this sort. The photographic method of testing for these emanations is not the only method, but it has the great advantage that an exceedingly feeble action may be detected by simply leaving the substance to be tested near to a plate in the dark for the necessary time, and this may be extended as may be necessary, for the action is cumulative. If we compare, the millionth of a second, which is sufficient exposure to the light of an electric spark to affect a plate, with the months that may be necessary for a potassium salt to produce a similar change, we get some idea of the extreme feebleness of the potassium compound in this direction and might suppose that it is self-luminous to that extremely small extent. But whatever it is that is given off by potassium salts and substances that act similarly, it will pass through various opaque media, and presumably is not what we generally understand as “light.”

It must not be supposed that forces are similar because they can produce one similar effect, and it does not follow that because radium glows and affects a photographic plate as light does, that other forces that affect the plate are comparable to light in a general sense. The Roentgen rays certainly affect the plate and might perhaps be supposed to be a kind of light, but every known kind of energy is able to render the silver bromide less stable and therefore amenable to the action of the developer. Heat, mechanical force such as pressure, electrical energy, and the contact of substances which appear to act in a chemical way, will all render silver bromide developable under suitable conditions, and it seems not impossible that these effects may eventually be utilized as the action of light has been. One fact to bear in mind is that the relationship between the photographic plate and light is not so exceptional as it used to be thought to be, and that it is possible that we are only on the threshold of the applications of methods that we at present associate almost exclusively with light. But looking at photography even as it is, we do not hesitate to say that the growing importance of it from every point of view, educational, commercial, and scientific, is not realized as it should be, or photography would not be left to a few specialists, and a comparatively large number of those who regard its practice merely as a pastime or an amusement.