1866 in science

Astronomy

May – William Huggins studies the emission spectrum of a nova and discovers that it is surrounded by a cloud of hydrogen.^[1]
June 4 – Pluto (not known at this time) reaches its only aphelion between 1618 and 2113.
Giovanni Schiaparelli realizes that meteor streams occur when the Earth passes through the orbit of a comet that has left debris along its path.

Biology

Gregor Mendel publishes his laws of inheritance.^[2]
Ernst Haeckel challenges the plant/animal division of life, observing that single celled organisms, the protists, do not fit into either category.
Élie Metchnikoff describes the early separation of "polecells" (progenital cells) in parthenogenetic Diptera.^[3]
Robert John Lechmere Guppy discovers the guppy (fish) in Trinidad.
Frederick Smith first discovers Formica candida in the Bournemouth district of England, describing it as Formica gagates.
Nikolai Kaufman publishes his Moscow Flora.^[4]

Medicine

February 21 – Lucy Hobbs Taylor becomes the world's first woman to receive a doctorate from a dental college (Ohio College of Dental Surgery).^[8]
July – Elizabeth Garrett Anderson opens the St Mary's Dispensary in London where women can seek medical advice from exclusively female practitioners.^[9]
Max Schultze discovers two sorts of 'receptors' in the retina.^[10]
Dr John Langdon Down publishes his theory that different types of mental condition can be classified by ethnic characteristics, notably "Mongolism", the genetic developmental disability now known as Down syndrome.^[11]^[9]
Invention of a clinical thermometer by Thomas Clifford Allbutt.^[9]
A cholera epidemic in London causes over 5,000 deaths.^[9]
Patrick Manson starts a school of tropical medicine in Hong Kong.^{[citation needed]}

Technology

January 12 – Royal Aeronautical Society is formed as 'The Aeronautical Society of Great Britain' in London, the world's oldest such society.
July 27 – The SS Great Eastern successfully completes laying the transatlantic telegraph cable between Valentia Island, Ireland and Heart's Content, Newfoundland, permanently restoring a communications link.^[13]
August 23 – Ralph H. Twedell patents the hydraulic riveter in the United Kingdom.^[14]

February 1 – Agda Meyerson (died 1924), Swedish nurse and healthcare profession activist
February 8 – Moses Gomberg (died 1947), Russian-born chemist
February 14 – Victor Despeignes (died 1937), French pioneer of radiation oncology
February 26 – Herbert Henry Dow (died 1930), Canadian-born chemist
April 17 – Ernest Starling (died 1927), English physiologist
July 13 – Emily Winifred Dickson (died 1944), Irish-born gynaecologist
July 25 – Frederick Blackman (died 1947), English plant physiologist
September 13 – Arthur Pollen (died 1937), English inventor
September 21 – H. G. Wells (died 1946), English scientific populariser
September 25 – Thomas Hunt Morgan (died 1945), American biologist, Nobel laureate in Physiology
October 8 – Reginald Fessenden (died 1932), Canadian pioneer of radio broadcasting
November 11 – Martha Annie Whiteley (died 1956), English chemist and mathematician
November 30 – Robert Broom (died 1951), Scottish-born paleontologist
December 7 – Maude Delap (died 1953), Irish marine biologist

Deaths

March 6 – William Whewell (born 1794), English scientist, philosopher and historian of science
March 14 – Alexander Morison (born 1779), Scottish physician and psychiatrist
April 4 – William Dick (born 1793), Scottish veterinarian
April 5 – Thomas Hodgkin (born 1798), English physician
July 20 – Bernhard Riemann (born 1826), German-born mathematician
September 16 – François Mêlier (born 1798), French physician ^[16]
October 18 – Philipp Franz von Siebold (born 1796), German physician, botanist and traveler in Japan
December 1 – George Everest (born 1790), British surveyor and geographer

Astronomy