MAVtech has a broad collection of magic lantern slides- but none are more haunting than this one. We know very little about the person in this photograph. From his uniform we know he was a lieutenant in the First World War. He is wearing a gas mask to protect against enemy chemical warfare attacks (or 'friendly' gas blowing back towards his own lines). His uniform is clean- it is unlikely he was photographed anywhere near 'The Front'.
It may have been a snapshot. Private cameras on the Western Front were banned from 22nd of December, 1914. One soldier found with a camera was sentenced to three months imprisonment with hard labour. However, the rules were often disobeyed and were sometimes laxer with regards to 'Officers' like a lieutenant. Cameras were also allowed 'behind the lines' or off-duty, when this picture may have been taken.
But it is more likely that this was an official photograph taken to be displayed during magic lantern lectures at the home front. People were desperate for any news of the war and were eager for photographs. This soldier looks clean, well fed and comparatively relaxed for someone preparing for a chemical attack. He would have been a reassuring image for those back home.