The happy marriage between gin and tonic took place in the halcyon days of British India. Long before that era, it had been discovered that a substance called quinine was by far the most effective treatment available for malaria.
It therefore became the custom for those charged with shouldering the white man's burden to take daily preventive doses of it as a "tonic", by dissolving in water a small amount of quinine sulphate. . Since the taste was not particularly pleasant, it became the norm to wash it down with gin, and thus arrived the invigorating recipe for G&T.
Now if take your G&T outside on to the terrace and inspect it closely, you may notice that it sometimes has a slightly bluish tinge. At first it was assumed that this blueness had something to so with scattering by tiny particles suspended in the liquid, and that it acquired its colour in much the way as the sky appears as blue.
It was noticed later, however, that if you move a liquid with quinine sulphate in suspension through light from the various sectors of the natural spectrum, the solution remains transparent everywhere except at the violet end: here the tinge of blue is seen - and if the solution is moved even further into a beam of ultraviolet light, it becomes a vivid duck-egg blue. It was obvious that, in significant quantities, quinine sulphate is fluorescent.
Fluorescence is a process whereby the absorption of radiation by a substance in one wavelength, usually the ultraviolet, excites the substance to emit radiation in a different wavelength - usually the visible. Butter, for example, can sometimes give off a yellowish glow when exposed to ultraviolet light, since the riboflavin it contains is an effective flourophore. In the case of quinine, the reemitted light is of only slightly longer wavelength than the incident ultraviolet, and hence is seen as blue.
This ghostly blue light in your gin and tonic is only really noticeable if the beverage is viewed by natural daylight. Electric light from a standard incandescent bulb does not generate sufficient ultraviolet for the phenomenon to occur, and fluorescent tubes are only slightly better. The most effective trigger is the tiny bit of ultraviolet light that remains after natural sunlight has been filtered by the ozone layer.
And, of course, it follows that as the ozone layer is steadily weakened by the catalytic effect of pollutants introduced by humans, and more ultraviolet light finds its way towards the surface of the Earth, we should have the consolation of seeing our G&Ts al fresco turn a prettier and steadily deeper shade of blue.