G. Airy, J. Baden Powell, G. Green 121 



of astronomy with the problems of navigation has always 

 been maintained at the Royal Observatory, and the intro- 

 duction of iron ships presented new problems, because the 

 ship became magnetic under the influence of the earth's 

 forces, and the compass needles were very seriously deflected 

 from the normal direction. An iron ship, The Rainbow, 

 having been placed at his disposal, Airy was able to deter- 

 mine the amount of the deviation experimentally, and 

 following up the observations by a mathematical investi- 

 gation, he showed how the effects could be compensated 

 by placing small permanent magnets near the compass. 



In the work of spreading the new ideas on the nature of 

 light, useful help was given by J. Baden Powell (1796-1860), 

 the son of a gentleman who at one time was High Sheriff 

 of Kent. He graduated at Oxford, took holy orders, and 

 devoted himself to mathematical studies while holding 

 a living in Kent. In 1827 he was appointed Savilian Pro- 

 fessor of Geometry at Oxford, where he took an active part 

 in advocating University reform. Powell wrote a treatise 

 on experimental and mathematical Optics, investigated the 

 reflexion of light from metallic surfaces, and showed that 

 highly absorbing bodies in the crystalline state resembled 

 metals in some of their optical peculiarities. He also estab- 

 lished the commonly used empirical law connecting the 

 refractive 'ndices of rays of light with their wave-length. 



Important as these results may be, they only dealt with 

 isolated problems, but did not touch fundamental principles. 

 The work of George Green (1793-1841) stands on a higher 

 level; indeed, had it become more generally known and 

 appreciated, it might rank as one of the landmarks of 

 science. Green, the son of a miller in Nottinghamshire, 

 entered the University of Cambridge when he was forty 

 years old, and had already written a most important 

 mathematical investigation, which was published by private 

 subscription. This paper dealt with electricity and mag- 

 netism, and it was only during the last few years of his life 

 that he published his investigations bearing on Optics. 

 This part of his work was introduced by a paper on Sound, 

 in which the subject is treated by powerful methods, now 

 familiar to every student of mathematical physics, but 



