His studies reveal a fascinating story, not least that of Cohen himself. He grew up in the army town of Aldershot, which got his juices going whenever military matters came into consideration. He thought of the idea of asking "the handful of Jewish families" living in Aldershot to invite Jewish soldiers to spend Shabbat with them and perhaps on Rosh Hashanah, Yom Kippur and Pesach, too.
Here, Lord Rothschild, the undoubted lay head of the community was persuaded by the young man - he was in his 30s - to take the suggestion to the War Office. "Cohen, as a result, gets an invitation to meet the GOC, (General Officer Commanding) of the garrison and permission is given for Jewish soldiers to have a short service on Sundays and a small hut is provided." The idea took off.
Soon, he was asking students of his old alma mater, Jews College, to conduct services. "It was difficult to get a minyan but, before long," says Judge Lewis, "there are Shabbat and yomtov services, too."
There was no suggestion that Cohen should go into uniform or be given a rank. There would be a few shillings in expenses for the students and young ministers (none of them were called Rabbi - the chief rabbi of the day, Herman Adler, saw to that. He believed he should be the only rabbi in Britain, although there were the occasional holders of semichah who slipped by). "Cohen himself didn't get a penny," Lewis emphasises.
As Britain entered the Boer War, Cohen and his civilian colleagues went into action. It was, Lewis says, a chaplaincy by correspondence. "Cohen finds all the Jewish soldiers he can." They were encouraged to give the names and addresses of relatives and or other people close to them to whom the men in dog collars could write. "They acted as conduits," he says.
Two questions arose from all this: why was the Anglo-Jewish community so keen on all this and why did these men in the first khaki uniforms ever worn in battle by British soldiers join the army in the first place? "They were mostly poor, some of them had even come from workhouses and it was a chance to get security of a kind. All were volunteers." A surprising statistic here: "100 of them were killed in the Boer War". The war ended in 1902.
But why was the Jewish establishment, working originally with the visitation committee of the Board of Deputies, so keen? "It was the loyalty thing. They wanted it known that British Jews were doing their bit. They were very keen to send other people's sons into uniform."
It was particularly useful at a time when the mass immigration from Eastern Europe was becoming controversial. It worked. The army - "there were, however, horrible stories of antisemitism in parts of the services" - showed its appreciation when Lord Roberts, head of the British forces in South Africa, attended a Jewish service after the war.
Cohen, who was also an expert on Jewish liturgical music, had plainly carved a niche for himself in the community. And, Judge Lewis emphasises again and again, he was a "self-starter" A self-starter who was about to drive away. He answered a call to serve as minister of the Jewish community in Sydney, Australia. Chief Rabbi Adler, who was beginning to regard him as a thorn in the flesh, granted him semichah "on condition that he didn't darken his doorstep again."
Quickly established Down Under, Cohen set up a similar unofficial chaplaincy in Australia.
Meanwhile, a replacement had to be found for Cohen in Britain. The minister of the fashionable Central Synagogue in London's Great Portland Street, the Rev. Michael Adler (no relation of the Chief Rabbi) reluctantly accepted the job after - along with Simeon Singer, of authorised prayer book fame - at first turning it down.
He had previously started a series of Chanucah services at his shul for servicemen that became so popular that congregations could be counted in the hundreds, always attended by the grandees of the Anglo-Jewish establishment.
Adler put the work on a different footing. His became an official job. His activities became particularly ground-breaking when the First World War broke out in 1914. He asked permission to go to the front in France. This was initially turned down. But it wasn't long before he was crossing the Channel and wearing a tin hat and a mud-splattered khaki uniform. There were 20 other chaplains who served wherever they could get close to where Jewish troops were fighting. He produced prayer books and other literature.
"He was a most unmilitary-looking man," Lewis says. "He was in his 40s, an author of Hebrew grammar books. Looked rather like Captain Mainwaring ". But there was no doubting the extraordinary efforts of Adler and his colleagues - each of whom wore a Magen David cap badge. Adler also had the Magen David at the front of his staff car - the only Jewish chaplain to have one. When the Americans joined the war in 1917, each of their chaplains did have a car - and helped their British colleagues .
But it wasn't really enough -- and that was the real problem: having to spread the resources extremely thinly, Adler prepared an English-language edition of the siddur and machzor to be used by Christian chaplains when they were the only ones near a group of Jewish soldiers at the time of the High Holy days. Conversely, there were times when a former United Synagogue minister was the only chaplain available to Christian troops in need of comfort. Miraculously, all the Jewish chaplains survived the war.
The army always regarded Jewish chaplaincy as important, says Lewis. They allowed Jewish troops to take time off for Rosh Hashanah and Yom Kippur when possible. The navy were less indulgent. They left it to ship's captains to decide if the "exigencies of the service" could allow it. "But not when ships were at sea."
All this set the pattern for the Second World War when there were 50 chaplains, sometimes spread out thousands of miles, through all the various theatres of war. It was also the first notable time when Anglo-Jewish politics entered the scene. The senior chaplain, Dayan M Gollop was forced to retire because of ill health. He nominated Rabbi Leslie Edgar as his successor. The United Synagogue complained - Edgar was a Reform rabbi and the establishment insisted it should be a job held by an Orthodox cleric.
Their view was accepted and a Royal Air Force chaplain, who, earlier when in the army, had been at Dunkirk, was appointed. His name: Israel Brodie - who immediately had to get himself transferred back to the army with the rank of Lieut.Colonel. He remained in office until 1948 when he was appointed Chief Rabbi, a post he held until 1967.
The most well-known of the war's Jewish chaplains was Leslie Hardman, who entered Bergen-Belsen with the liberating British army. He famously said Kaddish at the burial there of 20,000 victims. He also circumcised Jewish babies who had been born in the camp and conducted the marriage of a survivor to a British sergeant who had liberated her.
His experience of what he saw at Bergen-Belsen, he said, almost made him lose his faith.
Peacetime successors included the elegant Isaac Levy, minister at Hampstead, Cyril Harris, later to become Chief Rabbi of South Africa and the redoubtable Reverend Malcolm Weisman who, in addition to being senior chaplain in the RAF, was also minister to small communities and prisons.
Today, the senior chaplain Rabbi Reuben Livingstone, is a full-time army major who did a course at Sandhurst. Francis Lyon Cohen would have been pleased.
