OPTICAL COMMS. BEFORE ELECTRONICS:
"THE HELIOGRAPH"
Published circa 1899.
Extract from 'The Sun Telegraph' by Colonel C Cooper King, FGS, RMA..
[Notes by Chris Long]: We might boast of modern DX records with lasers and exotic photodetectors, but the all-time distance record for Morse transmission by light was set one hundred and nine (109) years ago in 1896, using the human eye as the receiver! It was achieved by the United States Army Signal Sergeants, from Uncompaghre Peak, Colorado; to Mount Ellen in Utah - a distance of 183 miles. The best modern amateur laser MCW communication has only spanned three-quarters of that distance. Is this a function of increasing worldwide atmospheric air pollution, or have we just not tried hard enough?
The extract below appeared in Volume 3 of Cassell's popular anthology "Science For All" (ed: Robert Brown, London, n.d., c.1899, pps 133 - 139), under the title of "The Sun Telegraph". Its author, Colonel C Cooper King, was based at the most famous of British military training establishments, the Royal Military College at Sandhurst. The techniques described were used extensively in South Africa in the earlier part of the Anglo-Boer War (1899 - 1902). The Mance Heliograph can be seen in operation in the still frame from the Mutoscope Company's film (1899), above, where British troops are using it to communicate across Boer lines into the besieged city of Ladysmith. The simple but effective optical alignment techniques used may be of special interest to laser experimenters today. Let's review the 'forgotten' 19th century technology of 'heliography'. Over to Colonel King ...
"The principle of using a reflective mirror for signalling is neither new nor confined to civilised modern communities alone. Mr Galton notes that the North American Indians transmitted information to each other from a lofty hill range to points beyond adjacent forests by means of flashes from a piece of glass. The fleet in which Alexander the Great coasted the shores of the Persian Gulf was said to be directed in its course towards India by flashing signals from the shore.
The 'Times' of July 11, 1853, states in its Crimean correspondent's letter that "a long train of provisions came into Sebastopol today, and the mirror telegraph, which works by flashes from a mound over the Belbeck, was exceedingly busy all the forenoon." This evidently indicates telegraphic communication by some form of sun-signalling.
But the actual construction of an instrument specially designed for such work is of comparatively late origin. First of all, the Heliograph must not be confounded with the Heliostat, though the names are often used indifferently. The latter is used on great surveys to fix the points of the greater triangulation of the area, and is merely a mirror which throws a flash steadily in a certain known direction, but which is provided with no means for obscuring the reflected ray. It has been in use by, the Ordnance Survey of England for nearly fifty years. In the Cashmere survey it was used between points as much as 100 miles apart, and often when intervening clouds or haze shut out the view of the light from the observer, it could still be detected by the use of a powerful telescope. Of course this instrument could be easily used for signalling, but ordinarily is not fitted with the means for making regular and definite short or long flashes of light. The Heliograph in most general use is that of Mr Henry Mance, of the Government Persian Gulf  Telegraph Department; but an "improved" instrument has been constructed by Captain Begbie, of the Madras Engineers, and another very similar to his by M. Leseurre, in France.
They are all alike in principle, that is to say, the dash or dot is produced by a long or short exposure of the reflected ray. But in Mance's instrument, and in the "Field Heliograph"- the most portable form for field service - the obscuration of the ray is effected by moving the mirror itself; while in the two others the reflection is shut off from the distant observer by a screen interposed between him and the mirror. In the French Heliograph, moreover, this screen is like the shutter apparatus before referred to, being constructed of thin slips of metal on pivots, which can all be turned together, by a connecting rod and handle, so as either to expose or interrupt the ray of reflected light. The object of this arrangement of the screen is to increase the rapidity with which the exposure or non-exposure of the ray can be made.
It will be seen at once, however, that, putting aside the many things that interfere with sun-signalling, or "heliography," such as clouds and haze, and strata of unequally-heated air, which produce irregular refraction, the single-mirror instrument is useless when signalling directly away from the sun. At early morning and towards sunset, moreover, the quarter of the horizon opposite the sun can receive only extremely feeble reflected light. Hence it is that in both the Begbie and Leseurre instruments a second, or "sun-mirror," is added, which can be turned in any direction, and which receives the sun's rays directly, and then reflects them back on the signalling mirror. In the Begbie apparatus this is supported on a separate tripod, as also is the screen interposed between the signalling mirror and the observer; but in the French instrument the sun-mirror is attached to the stand on which the signalling mirror is fixed, and a separate stand for it is not thought necessary.
It does not seem, from experiment, that the size of the mirror affects the size of the reflected image; but by increasing its dimensions greater intensity of light is produced, and therefore the reflected ray has greater penetrative power. For long distances, therefore, the size of the mirror is usually increased. One 3 inches square can signal to points 10 or 12 miles off ; one of 4 inches, 15 to 20 miles, as a rule, and up to 40 miles in exceptionally clear weather.
The only difficulty in these methods of visual signalling by sunlight, is to ensure that the pencil of light shall be thrown on the point where the opposite observer stands. How this is effected will best be understood by a description of the instruments themselves.
The Mance instrument, or "Field Heliograph," is shown in Fig. 6. It consists of a small 4.5 inch mirror, supported or pivoted in a light frame on a tripod stand. In the centre of the mirror a portion of the glass is left unsilvered, so that the signaller can look through it and direct its axis towards the required spot. In order to set it at the required angle, the mirror is fixed in a frame which admits of horizontal or lateral adjustment: by means of a tangent screw. Vertical movement is obtained by means of a steel rod, which passes through a nut in the top of the mirror, and which is also provided with a tangent screw; but it terminates in a sort of handle or key, which is kept in position, somewhat, by a spring. Thus when the handle is pressed down, movement is imparted to the mirror, and on withdrawing the pressure the spring restores the handle to its former position. About 10 yards in front of the signalling stand is placed a "lining-rod," having on it a sliding stud, and also a small cross-piece, with a stud in the centre. The former is for aligning the centre of the mirror with the object. The latter is intended to show if the reflected ray is wandering to the right or left of the true line.
A complete set of apparatus consists of two mirrors, a lining-rod, and separate cases to contain the mirrors and their frame. It is, therefore, possible to work in any direction, by utilising one of these as a "sun-mirror," to throw a reflection on the "signalling mirror."
The mode of operation is simple. The pressing back of the tangent screw, which imparts the horizontal movement to the glass, leaves the frame free to be moved by hand roughly into the required direction; and the operator then looks through the unsilvered part of the glass and aligns the sliding stud on the lining-rod with the distant station. This is first found by moving the mirror so as to throw a ray in the direction of the place, and waiting until the answering flash determines its exact position. The cross-bar on the lining-rod is adjusted at right angles to the rod, about a foot below the sliding stud; or, rather, it should be so placed that the ray of light should rest on the stud in its centre when the instrument is not moved, and should rise to and rest on the sliding stud when the key or handle is fully pressed down. Movement, for signalling, can now be given to the mirror by pressing the handle; and as the position of the sun alters, which would be shown by the ray passing to the right or left of the stud in the cross-bar of the lining-rod, a slight touch of the horizontal tangent screw will bring the ray back to the central position.
The disadvantage of this instrument is said to be that the movement of the handle, or key, disturbs the glass, and has a tendency to alter its position. This would especially be possible on sandy or soft sites.
The Begbie arrangement (Fig. 7) is designed to meet this difficulty. It consists of two mirrors and screens, each upon a tripod stand. Each mirror has a hole in the centre, in which can be fixed white or black " guiding discs " (the use of which will be seen hereafter), and has a slow-motion screw on its frame for vertical adjustments. Small horizontal adjustments are produced by a slow-motion screw at the head of the tripod. From the signalling mirror projects a horizontal bar, which can be moved in any direction, and clamped when in its proper line. At its extremity is a small circular frame, with cross-wires and a sighting-hole in the centre, which is fixed on a rod that moves up and down in a socket at the end of the bar, and which can be clamped by a screw. The sun-mirror is 5 inches, the other 4 inches square. Finally, the screen, which is in one piece, and unlike, therefore, the venetian-blind sort of arrangement of Leseurre's instrument, is provided with a key, or handle, by which it can be made to obscure or expose the ray of light from the signalling mirror. Its position, therefore, is just in front of the cross-wire frame, and the reflected ray should fall on its centre.
Of course the machine can be worked like the Mance pattern, that is to say, when the position is favourable the sun-mirror can be dispensed with. In this latter case, then, the mode of operation is as follows:- While an assistant stands at the end of the bar to move it horizontally, or else raise or depress the wire frame as required, the observer looks through the hole in the mirror and aligns the centre of the cross-wires with the object. The arm and wire frame are then clamped, and a white disc is placed in the hole in the latter, and a dark one in the hole in the glass. The mirror is then unclamped at the tripod-head and moved till the ray of light rests near the sighting arrangement, and then being re-clamped, the small-motion screws are used to bring the reflection of the black disc of the mirror on to the white disc of the cross-wires. As, owing to the motion of the sun, the black disc shadow passes over the white one, it is brought back by the small-motion screws.
In using the second or sun-mirror, the tripod on which it is fixed is placed on one side of, and about a foot in front of, the signalling glass. After aligning the latter with the object, as before described, a white disc is placed both in the cross-wires and the signal mirror. Then looking through the hole in the sun-glass, the operator should alter the position of the other glass until the reflection of the hole in the cross-wires coincides with the white disc on it. Next, by the small-motion screws, the position of the sun-mirror should be altered, until the black disc, which now fills the hole in it, is reflected on the white disc in the other glass. Then the screen tripod is placed in position, and the signalling mirror should not be moved again, any alteration rendered necessary by the sun's movement being remedied by the sun-glass.
Though this obviates the possible disadvantage of moving the signalling glass itself, it has still disadvantages. First, the quantity of apparatus is greater, and the weight, 12 lbs., as compared with the 5 lbs. of a simple Mance instrument, is therefore greater. Next, it requires two operators to manipulate the signals - one at the screen, the other at the mirror. Lastly, the working of the screen through an arc of 90 takes a relatively longer time than moving the mirror itself through 2; that is to say, the Begbie instrument is slower in operation than that introduced by Mance.
At Ekowe, in the campaign in Zululand, the whole apparatus had to be improvised, a common looking-glass belonging to one of the staff being utilised. The great difficulty experienced was in directing the flash on to the position occupied by the signalling party on the hills above the Tugela. A common gun-barrel resting on bags of "mealies" {Indian corn), and provided with brightened bullets for sights, was first tried, but it was not very successful. Then two wires with the upper parts bent into the form of a semi-circle, and with cross-wires uniting the bent end with the upright part, were tried. The sights were composed of brightened bullets in the centre of the cross-wires, and when these rod~ were set in the ground and aligned with the flash, no difficulty was experienced in communicating (Fig. 8).
In India, in some of the Cashmere surveys, the native assistants dispense with the sighting-rod, using only a piece of straight bamboo with a piece of cotton wound round it at the proper altitude. And there are many ways in which means of aligning the ray, which is the only practical difficulty, could be improvised.
Sun-signalling has great advantages over all other methods of visual telegraphy. Messages can be transmitted to great distances, and the clearness with which the signals can be made renders background of but little importance. In flag-signalling [(semaphore)] the distinctness of the signal depends materially on this question, and one of the first points that requires the operator's attention in this case is that he should place himself in the exact line in which he is going to telegraph, and then turning his back upon the distant station, notice against what materials his signals will be shown. On this depends his choice of the flag for the work. Unbroken backgrounds are better than more varied ones; thus trees and the sky would represent the darkest and lightest classes. And this kind of signalling can be most favourably carried out when there is a clear atmosphere and a clouded sun. Heliography, on the other hand, is naturally most effective on a clear day and with a bright sun. Given these conditions, there seems to be no practical limit to the distance over which a signal can be transmitted. The ray of sunlight is not lessened materially in intensity by reflection from the mirror's surface, for it is merely bent or altered in direction. But for great distances a considerable altitude for the signal station is essential, and this is not always obtainable under ordinary conditions of ground.
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