Google is celebrating the 115th anniversary of the X-ray with its latest doodle - and also nodding to its own past April Fools jokes, with the idea of "PigeonRank", where the ordering of pages would be determined by pigeon pecks.
The X-ray is usually credited to Wilhelm Röntgen, who was the first to study its effects in 1895, though he wasn't the first to observe them: that occurred in 1875, when scientists investigating cathode rays - high-energy electron beams - noticed that unexposed photographic tubes put near the tubes in which the rays were generated became fogged.
The tubes in which the rays were generated were known as Crookes tubes, which generated free electrons - then a topic of intense study - by applying a (DC) voltage of up to 100 kilovolts across a gap. The electrons in the elements of the air in the tube were accelerated as they travelled from the cathode to the anode; some were accelerated enough that the generated X-rays when they struck the glass of the tube.
Röntgen- who had an extremely difficult academic life, came across X-rays on 8 November 1895 while experimenting with Crookes tubes, and began systematic studies of them. He dubbed them "X-rays" to indicate that the radiation being observed was unlike any recorded previously. His discovery won him the first Nobel Prize in Physics, which was awarded in 1901.
It's not known exactly how he discovered X-rays, because he ordered all his papers, including his lab books, should be burned after his death. The best suggestion of how it came about it that he was investigating cathode rays with a fluorescent screen painted with barium platinocyanide and a Crookes tube wrapped in black cardboard so visible light from the tube wouldn't interfere. He noticed a faint green glow from the screen, about 1 metre away, and realised that some invisible rays coming from the tube were passing through the cardboard to make the screen glow. He found they could also pass through books and papers on his desk.
Röntgen threw himself into investigating these unknown rays systematically. Two months after his initial discovery, he published his paper.
X-rays have since become a part of daily life - especially for the luggage of people who are travelling, which is regularly subjected to X-rays. (However, travellers are not - the arches you walk through are metal detectors, which use electromagnetic fields.)
Marissa Mayer, one of its senior executives, explained the doodle's significance on Twitter earlier on Monday - and pointed out that the small 'g', with its pigeon feathers and bones, is a nod to the "PigeonRank" April Fool's joke of 2002:
"PigeonRank's success relies primarily on the superior trainability of the domestic pigeon (Columba livia) and its unique capacity to recognize objects regardless of spatial orientation. The common gray pigeon can easily distinguish among items displaying only the minutest differences, an ability that enables it to select relevant web sites from among thousands of similar pages. "