In the closing decades of the 19th century, it became evident that atmospheric ozone was responsible for the sharp "cut-off" at the ultraviolet end of the solar spectrum.
High in the atmosphere a tiny quantity of elusive substance absorbs these very short and potentially harmful wavelengths, preventing them penetrating to the Earth below. But until Gordon Dobson came along, there was no easy way of measuring the small amount of ozone in the rarefied air above our heads.
Dobson was born in 1880, worked at Oxford, and devoted most of his life to the study of the subject. His first attempt at ozone measurement was the ozone spectrograph - a sort of camera which measured the intensity of solar radiation at a very specific "ozone-sensitive" wavelength.
In the mid-1920s he organised a network of these instruments around the continent to assess ozone distribution over Europe, but they were troublesome and not always accurate.
The development of the photoelectric cell in 1927, however, allowed Dobson to design the ozone spectro-photometer, a major improvement on its predecessor. It compared the intensity of solar radiation at two different wavelengths - one of which was known to be absorbed by ozone, and the other known to be unaffected.
The relative strength of the two was an indication of how efficiently the ozone was doing its job, and this in turn was a measure of how much of it was present in the atmosphere directly above the point of observation.
Today there are other ways of obtaining accurate data on stratospheric ozone distribution. Very large balloons, for instance, carry ozone-sondes to the very highest levels of the atmosphere and send information back to ground by radio.
High-flying aeroplanes are also used, and even more importantly, satellites carrying total ozone mapping spectrometers, known as TOMS, provide a picture of ozone concentration over a large segment of the globe.
But for very many years Dobson's ground-based spectrophoto meter was the standard instrument for measuring ozone concentrations in the atmosphere for many years.
Gordon Dobson died 25 years ago tomorrow, on March 11th, 1976. He lived to receive the ultimate scientific accolade - to have a unit of measurement named in his honour. He had calculated that if all the ozone in the atmosphere were brought down to the Earth's surface, it would form a layer only about 3 mm thick.
Becoming rather fond of this analogy, he converted all his readings into "thickness" units. The idea caught on, and ozone concentration came to be expressed in Dobson Units - the hypothetical thickness in hundredths of a millimetre of a "concentrated", hypothetically ground-based ozone layer.