Horace Dall

Horace Edward Stafford Dall (5 January 1901 – 9 May 1986) was an English amateur astronomer, optician, telescope and microscope maker and inventor.

Biography Horace Dall was born on 5 January 1901, in Chelmsford, England. His father worked for Guglielmo Marconi; his mother died when he was three. After that, Dall lived at multiple addresses, moving twelve times before the age of six, until his father remarried and moved to Luton. Dall excelled at science and maths at school, and started to work for the aircraft manufacturer Hewlett & Blondeau at 14. He worked there for three years at a drawing office. Dall became interested in astronomy and got his first telescope, which he soon upgraded by himself. At 16, he bought a used microscope, and a year later a camera obscura. Dall enrolled at the night-school of the Luton Technical College, and in 1918 he joined George Kent Ltd, a water meter manufacturer. He spent his whole career there working in the Research and Development department, retiring in 1965. E. J. Hysom wrote later that in the field of water engineering the name "Dall" is "as famous as 'Hoover' in vacuum cleaning".

In 1925 Dall "started making optics" and joined the British Astronomical Association (BAA). In the early 1930s, he made a Cassegrain telescope with spherical rather than hyperboloidal lenses. It became the basis of a portable telescope that Dall was developing; now it is known as the Dall-Kirkham reflecting telescope. When Dall learned about "an Englishman" who was able to engrave "letters so small the content of 5 bibles could be fitted to the square inch", he developed a technique to engrave even smaller letters; after several improvements he was able to reach "280 bibles/square inch".

According to Hysom, Dall set several more world records: inventing "the most accurate spherometer (a mechanical device capable of detecting a change in shape as small as 1/40 the wavelength of visible light)", a barometer "capable of showing a change when raised or lowered the height of a thick book", creating lenses from diamonds and using them as microscope objectives, "achieving a numerical aperture of 1.92".

Biologist Brian J. Ford wrote that Dall gave him a microscope with "a magnification of 295x, similar to the best of Leeuwenhoek's home-made microscopes"; he used it for Giardia research. Dall also introduced a "visibility index" in microscopy, "to quantify refractive index differences".

Personal life Dall built a 390 mm Cassegrain telescope at his home, and used it for lunar and planetary photography.

Dall was fond of hiking and cycling; he spent many winters in Switzerland. He once made a "19-day trip cycling across the North Cape and down into Lappland". He also travelled to the Middle East, crossed the High Atlas Mountains on a bicycle in 1930, where he was arrested for spying and rescued by the French Foreign Legion,

Selected publications

Multiple articles are reprinted in the I&I News 2011.