Let there be light, said the council

There was nothing very extraordinary about the city at night, argues Joachim Schlor in his attractive study of Berlin, London and Paris, until it became lit up. That had already happened to a small degree in London by the 18th century: finding on his arrival street-lamps and shop-lighting all over the metropolis, a minor European princeling leapt to the conclusion that the town had been illuminated just for him. And who can blame his presumption, since most cities were routinely pitch-black after sundown? Berlin's old oil lamps, the saying went, did nothing but reveal just how dark it was.

It was gas that really made the difference. Introduced into London's West End in 1807, gas reached Berlin in 1826, courtesy of the Englische Gasgesellschaft. And, in its turn, gaslight finally began to be replaced by electricity from the 1870s - again London led the way: from then on it was bright enough to read newspapers in a public place.

Lighting spotlighted the paradox of the nocturnal city, thereby releasing floods of fear and fascination. God had made the night dark, insisted a German evangelical magazine, so as to make people properly afraid of all the terrible things that could be done under cover of darkness. If the streets were made brilliant, the city's temptations (it followed) would be precisely like candles to moths. And that is what endless social commentators and preachers then found to criticise about the new "cities of dreadful night".

For crowds now flocked out at night, seduced by the rows of theatres, music-halls, gin-palaces, pubs, bars, dance-halls - everything from the most sumptuous nightclub to the seediest dive. Once mainly the preserves of the privileged few who could afford carriages and link-boys, night-time entertainments became now democratised. Night-town, protested a German observer, was no longer just the scene of crime, begging and prostitution; it had become the theatre of the proletariat. Night thus turned into nightmare for those middle-class moralists who, as Richard Evans argued in his Tales from the German Underworld (1998), were filling their imaginations with phobias of the "underground".

Luckily for the moralists, if the night had grown irresistible to those addicted to gaslight - and a veritable paradise for voyeurs from Mr Gladstone to George Orwell - light also provided the searchlight enabling the authorities to survey the sordid and patrol criminals such as Paris's notorious gangs of apaches. A light, it was said, is as good as a policeman: a German advertisement for Osram shows a lightbulb decked out as a constable, arresting a burglar.

Schlor writes well on the replacement of the old bungling night-watch, calling the hours, by professional police forces, and also on the programmes pursued by moral rescue outfits - the Salvation Army in "darkest London", the Mitternachtsmission fighting "sexual Bolshevism" in "red Berlin", with their night-shelters and soup-kitchens. Their attempts to "protect" virtuous young women - many of them emancipated and enjoying a bit of night-life themselves - were not always welcome, however. One is reminded of the young lady in the cartoon protesting that she was not a social problem, she was just waiting for the omnibus.

Schlor also documents new attempts to regulate nightlife via legislation, as with licensing laws and heures de fermeture. The question of whether such street-sweeping did more harm than good surfaced plainly and perennially with prostitution. Paris largely continued with its regulated brothels. Condemning such institutions as moral cesspools, Berlin, by contrast, closed down its own versions of the maisons de tolerance in 1843, only to find that the women of the night, far from disappearing, simply oozed onto the thoroughfares themselves, with Friedrichstrasse and Unter den Linden rivalling Piccadilly and the Haymarket.

Schlor's study is not without its shortcomings. Though nominally covering developments up to 1930, the 20th century is rather shortchanged - there is surprisingly little on the cinema and the automobile, or even on Weimar Berlin. The dialogue between the night-town as reality and as literary image might have been explored more critically. While there are many excellent illustrations, including photographs from Brandt and Kertesz, text and image do not always match up - thus Gustav Dore is discussed but, oddly, none of his engravings is reproduced. And, finally, Schlor, who researches at the Moses Mendelssohn Center in Potsdam, does not seem totally sure-footed when treading on English ground. We are bizarrely told that "until the end of the 18th century", London can more or less be equated with the confines of the City of London proper. Despite such shortcomings, Nights in the Big City, elegantly translated from the 1991 German original, is an illuminating guide to the idioms of the night.

Roy Porter is professor in the social history of medicine, Wellcome Institute.