heliograph

There are two heliographs visible - one towards the front, and one further back. I think this is one panel of a stereograph - the sterograph is here at the National Galleries of Scotland:
www.nationalgalleries.org/art-and-artists/80944/heliograp...

It is very hard to tell from this photo, but I think the angle between the heliograph rear rod and the mirror is too large for the heliograph to be the British Mance pattern (right example in image below), and is rather than India pattern (left example in image below), as one might expect from Indian troops in WWI.

The "mirror and disc" apparatus at the right is a 5" Mance heliograph. The heliograph was used to signal by Morse code using flashes of reflected sunlight created by tilting the mirror intermittently. This one is set up in the "duplex" (twin mirror) arrangement. The duplex configuration was used when the sun was on the opposite side of the signaller from the target. The leather carrying case dangling below the tripod by the shoulder strap was use to carry the parts atop the tripod for travel, and the tripod itself was slung over the shoulder by the strap attached to the left leg of the tripod, with the cylindrical cap at the end of the sling holding the legs of the tripod together. The disc dangling just above the case is a protective cap (with female threads) that protected the male threads atop the tripod. As noted at left, the heliograph saw little use during the trench warfare on the Western front in WWI, but rather more so in the mobile warfare towards the end. Another photograph of heliographs in use on the western front is this one at the Imperial War Museum: www.iwm.org.uk/collections/item/object/205072698 .For more information on heliographs, see the Wikipedia article on heliographs and its references.

British troops operating from Nordia Hotel building overlooking Jaffa Road and Barclays Bank, showing heliograph operator signaling to troops on Tower of David, Jerusalem, Palestine - circa 1938

The heliograph was used to signal using dots and dashes of reflected sunlight, and was used at ranges up to 20 miles and more. It consists of two tripods at center. The left tripod holds a mirror and a foresight to reflect and aim the light, and the right tripod a shutter to break up the reflected sunlight into dots and dashes. The thumbnail image below is a clickable link to a photo I took of a surviving example of this type of heliograph:

Seuss. The diagram you posted is of the 1888 model - some slight changes were made between that and the 1905 model shown here, though the parts and functions were, for the most part, the same. The big change was from the single-flap shutter to the six-blade shutter. Another change was the provision of azimuth adjustment screws on both ends of the mirror bar. The thumbnail below is a hyperlink to a 1913 advertisement for a model 1905 heliograph like the one in the Library of Congress photo:

The Library of Congress also holds a movie of a heliograph like that above in use by US Marines in 1915. The movie shows the heliograph being transported, set up, and operated. A copy is on YouTube, here:
www.youtube.com/watch?v=tdv11gZAUI8

The thumbnail below is a hyperlink to this still from that movie:

A modern video of a heliograph of the type in this LOC photo (operated by a complete beginner) is on YouTube here:
youtu.be/pBgH0Rz7kXY