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The 2nd photo in row 20 of the thumbnails on this page shows four standing soldiers facing right with the third one operating a Mance Mark III 5″ heliograph in single mirror (“simplex”) mode. If you click on the thumbnail, you will see a 800×695 pixel version.
Others will be far more knowledgeable than I on the uniforms, etc. – I’ll confine myself to my specialty – the heliograph.
The heliograph is the tripod device with the circular mirror being operated by the third soldier. The round 5″ mirror is on an azimuth-elevation gimbal,
and aimed by the shadow cast by an unsilvered spot in its centre and the vertical sighting rod at right. The book held out by the second soldier is likely the
message book where the message being transmitted is recorded.
We can tell that this is a Mance Mark III heliograph by the latch under the arm supporting the sighting rod. The latch locks the hinged arm in place. The Mark IV and V arms are not hinged. A detailed photo of a Mark III arm is at  . The strap about 1 foot up from the bottom of the leftmost tripod leg was used to secure the tripod legs during transport (later models used a cup attached to a tripod sling). The vertical sighting rod is characteristic of the Mark III – the Mark V had a double-hinged rod for more rapid adjustments of azimuth and elevation. This unit probably had the blade sight rather than a crosshairs sight. The object dangling by a chain from the leftmost tripod leg is the protective cap that replaced the gimballed mirror head atop the tripod when the head was removed for transport,
to protect the male threads atop the tripod. Between the heliograph operator’s feet is the the leather transport case for the heliograph hardware, and atop that
is the second mirror, which replaced the sighting rod when the sun was at the signaller’s back. The function of the second mirror was to reverse the sunlight
and deliver it to the gimballed mirror that then reflected it forward to the target.
Note the pipe in the fourth soldier’s mouth and the spurs on the boots of the first figure. Can anyone identify the object on the ground in front of the
rightmost tripod leg?
 Henry Mance invented his heliograph in 1869, a wireless sun-telegraph that sent Morse messages as flashes of reflected sunlight. By depressing a ring on a four-bar mechanism behind the mirror, the mirror was tilted intermittently to produce flashes of light. A demonstration of the sighting and signaling process is on YouTube, here: https://youtu.be/f8nVkVaHZ4Q
For more information, see the Wikipedia article: https://en.wikipedia.org/wiki/Heliograph and its references.
 Detailed photo of Mark III arm (mounted to a Mark IV/V gimbal head) at the York Digital Library
 A diagram of a Mance Mark III heliograph is here: https://flic.kr/p/ncpBL6 and of a Mance Mark V heliograph, here: https://flic.kr/p/ecRWyr
Note that the threaded rod at the top of the Mark III mirror has a spherical nut, and that the Mark V has a cylindro-conical nut, which is why I say the York
photo of the Mark III arm has a Mark IV/V gimbal head.
 Mance Mark V Heliograph at the York Digital Library – note:
– rigid T-cross-section arm
– double-hinged sighting rod with crosshairs sight
– conispherical nut at top
Another Heliograph Photo:
The third photo in the 26th row also also shows a heliograph in action. Click on the thumbnail for a larger image. In contrast to the prior image I commented on above, in this photo the operator is using the heliograph on double-mirror (or “duplex”) mode.
The gimballed mirror is still oriented to catch the sun (as you can see by the shadows), but the light reflected from the gimballed mirror heads away from the target, and is reversed to hit the target by the second mirror.
This appears to be a Mance Mark III heliograph – the characteristic hinge on the mirror arm seems to be (barely) visible on top.
One rarely seen point of interest that shows clearly in this image is the hook mounted below the tripod head. This feature was provided so that a weight could be suspended from the hook to stabilize the tripod.
The 1896 Signalling Instructions said that “It is always desirable to suspend a sandbag, or other weight between the legs”, but it seems that operators did not confuse the adjective “desirable” with “required”. I’ve seen photographs with a weight suspended … but rarely.
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