Friday, August 19, 2011

The Pressure of Light

Ever think that you can feel the sun's rays beating down? While that impression may be illusory, visible light and all forms of electromagnetic radiation actually do exert pressure on a surface.

First predicted by James Maxwell in 1871, experimental evidence for the phenomenon wasn't produced until the turn of the century. The Russian physicist Peter Lebedev was the first to measure this radiation pressure and announced his findings in 1900 at a conference in Paris. The measurement was independently confirmed by two Dartmouth researchers in 1901 - Gordon Ferrie Hull and Ernest Fox Nichols (later President of the College, 1909-1916). Pictured here is the apparatus used by Hull and Nichols in their experiment, including the Nichols radiometer, which is now in the Smithsonian.

Hull and Nichols were apparently unaware of Lebedev's findings until after their discovery which they first published in an article titled "A Preliminary Communication on the Pressure of Heat and Light Radiation" in the November 1901 issue of The Physical Review (conclusive evidence was published in The Physical Review in July of 1903).  In a later article in The Astrophysical Journal, they wrote that "Unfortunately the proceedings of the Paris Congress did not reach the writers, nor any intimation of the methods or results of Professor Lebedew's work, until after the publication of their own preliminary experiments."
Left to Right: E.F. Nichols, J.A. Brown, G.F. Hull, 1903
Rauner holds Gordon Ferrie Hull's papers which include correspondence, photographs and other material about his work with Nichols on radiation pressure, as well as Hull's later work in ballistics during World War I. In addition there is correspondence between Hull and various scientific notables, including Ernest Rutherford, George Hale, and Robert Milliken.

To read more about radiation pressure, ask for the Hull Papers (ML-47).  A guide to the collection is available.

Tuesday, August 16, 2011

Stefansson's Son

Alex Stefansson
In 1908, arctic explorer Vilhjalmur Stefansson set out on his second expedition. He hired an Inuit woman named Fanny Pannigabluk to travel with him. On March 10, 1910, Pannigabluk gave birth to a boy. Alex Stefansson was never acknowledged by Stefansson as his son, however, people of the arctic community who knew about the relationship between Stefansson and Pannigabluk, never doubted Alex's paternity.

Stefansson remained in the arctic until 1912 living with his Inuit family. He returned to them once more during his 1913-1918 expedition. During Stefansson's lifetime the paternity question remained a rumor. However, after his death in 1962, it garnered attention once again. Stefansson's wife Evelyn, a notable author of arctic books in her own right, who had married Stefansson in 1940s, felt it was "regrettable that only a few months after Stef's death a matter based on hearsay rather than evidence should be brought up for the first time." According to her "Stef did not believe that Alex Stefansson was his son."
Vilhjalmur Stefansson
Frank Stefansson
Based on photographic evidence and intensive research gathered since his death, Alex Stefansson's paternity is no longer questioned by Stefansson scholars. For Evelyn the revelation came in 1976, when in response to a picture of Alex Stefansson's son Frank she wrote: "The picture of Frank Stefansson is stunning evidence to me that Alex was Stef's son. For the first time a feeling of conviction arrived. It is the set of the mouth and the tip of the nose, the dimpled chin, and the shape of the head,--the family resemblance is inescapable and is so much stronger in the grandson than the son, strangely enough."

To read the correspondence or learn more about Evelyn Stefansson Nef's life and work as an author and philanthropist ask for Stef MSS-99.