Indian soldiers of the 58th Vaughan's Rifles (Frontier Force) in a trench in Fauquissart, France wearing gas masks in anticipation of a gas attack, 9 August 1915. [colorized]

 The hoods they are wearing are gas masks; the First World War was the first war in which manufactured poison gas was used as a weapon on a large scale. 

This series of several hundred photographs recording the contribution of Indian soldiers to the Allied war effort was produced in 1915 by the Canadian-born photographer Charles Hilton DeWitt Girdwood (1878-1964).

As a professional photographer Girdwood had an early connection with India, photographing the Delhi Durbar of 1903, the royal tour of 1905-06 and the Delhi Durbar of 1911. In 1908 he set up a photo agency called Realistic Travels, specialising in stereoscopic photographym

With the outbreak of war in 1914, Girdwood returned from India and in April 1915 was given permission by the India Office to photograph the work of the Indian military hospitals in Bournemouth and Brighton.

From July to September 1915 he worked in France as an official photographer to record Indian and later British troops in the field. In the later part of his time in France he also made ciné film of the campaign (which later appeared under the title With the Empire’s Fighters.