Early example of Kiwi ingenuity

Press, 12 May 1984, Page 19

Early example of Kiwi ingenuity

By

DUNCAN STEEL,

physics

department, University of Canterbury

Physicists have known for many years that if you rotate a liquid, then that liquid’s surface adopts a specific shape, known as a paraboloid. For example, when you stir a cup of tea, after the spoon is removed and the wavelets die down, the rotating fluid takes a smooth parabolic curve until the rotation stops. To maintain a constant shape it would be necessary to put the cup on to something like a gramophone turntable. This phenomenon would be of little interest if it were not for the fact that this same parabolic shape is that required for the large primary mirror of a telescope. Over recent decades several telescopes have been constructed by rotating a large dish of liquid plastic or resin, and maintaining its rotation until set. Mirrors of up to two metres diameter, taking a number of days to solidify, have been made in this way. Although the precision of the surface is not as good as that obtainable by conventional means, they can be polished and aluminised to provide instruments usable at infra-red wavelengths. A similar technique is being explored by a team led by Dr Peter Gough in the department of electrical and electronic engineering at the University of Canterbury. A better surface can be made by using a non-setting liquid; then the focal length of the telescope can be altered by changing the spin-rate. The metal mercury is excellent in this respect, although its vapour is poisonous. A limitation, though, is that the liquid can only be spun about a vertical axis. Because of this, the telescope is always directed towards the zenith (the point directly overhead), and stars can only be observed as they cross this point. One way around this is to use a large plain mirror above the liquid paraboloid. By tilting the plane mirror to reflect light down on to the mercury dish below, most objects in the heavens can be

observed. The largest modern telescope, made of a solid mirror with a highreflection aluminium coating, has a diameter of about six metres. A group at a university in Montreal have recently suggested using a mercury reflector of diameter 30 metres, although it would be used only as a transit instrument (observing stars as they cross the zenith). Since this aperture is five times as big as any other, stars 25 times fainter would be observable. The Canadians have successfully built and tested a prototype of one metre diameter. But who invented this liquidmirror method, and who built the first one? The honour is usually given to an American, Professor R. W. Wood, who built a half-metre device in 1908. However, he himself pointed out that in the early 1870 s a Mr R. C. Carrington had constructed one. The subject receives scant attention in the standard reference books on the history of the telescope. Yet the earliest evidence I have been able to find for its invention is in 1850, by two members of the Belgian Royal Academy. An astronomer named Capocci suggested using rotating mercury, and another called Krecke thought that it would be better to use a molten metal which would solidify into the required form. Purely by accident I came across a letter published in “Nature” magazine of London in 1874 from a Mr Henry Skey, of Dunedin. He claimed to have built a telescope using rotating mercury in England before his emigration in 1860, and repeated this in Dunedin. On November 19, 1872, he exhibited a model to the Otago Institute, and an article on his instrument was published in the Transactions of the New Zealand Institute of that

year (now the Royal Society of New Zealand). Who was Henry Skey? Better known was his brother, William Skey (1835-1900), with whom Henry emigrated. While Henry concentrated on astronomical interests, William was a chemist and geologist. Before leaving Britain, William had erected a laboratory on a farm for testing manures, and also experimented in distilling spirits from beetroot. Henry and William Skey worked together in Otago before the latter took a job in Wellington with the Geological Survey, eventually becoming Government Analyst. He made many discoveries which were of immense use to the mining industry in New Zealand, although he fell out later with the government. He also fancied himself as a poet, and his discontent with officialdom led to a number of poetical broadsides. Clearly, Henry and William Skey still had much close contact with each other after their separation. William’s collection of poems published in 1889 (“The Pirate Chief and the Mummy’s Complaint with various Zealandian poems”) contained several poems on astronomical topics. These are particularly interesting since they contain information upon the astronomical knowledge of the day. In one poem the belief is expressed that the Sun is rapidly running out of energy so everyone on the Earth will freeze: it was not until after Rutherford’s pioneering work on the atom that it was realised the Sun’s energy output comes from nuclear fusion in its core, which will continue for several billion years. Whether Henry Skey was the first person to construct a liquid mirror telescope is not known. Nevertheless, his work is a classic example of early Kiwi ingenuity.