120 Britain's Heritage of Science 



figure of the earth " ; " On the use of silvered glass for the 

 mirrors of astronomical telescopes " ; " On the figure assumed 

 by a fluid whose particles are acted on by their mutual 

 attraction and small extraneous forces " ; "On the principles 

 and construction of the achromatic eye-pieces of telescopes, 

 and on the achromatism of microscopes " ; "On a peculiar 

 defect in the eye and a mode of correcting it " ; "On the 

 forms of the teeth of wheels." All these papers mark an 

 advance in their subject matter, and they were written 

 before Airy had reached the age of twenty-four. 



His investigation on eye-pieces was considered to be of 

 sufficient importance for the Royal Society to vote him the 

 Copley Medal, their highest award, in 1831. The paper which 

 he wrote on a " peculiar defect of the eye " deals with 

 astigmatism. Airy, finding that he could not read with one 

 eye, investigated the cause, and observed that the defective 

 eye could not properly focus a point of light which was 

 drawn out into line. This suggested the method of correcting 

 the defect by employing a cylindrical lens. Airy was not 

 aware that Thomas Young had already previously described 

 the astigmatism of the eye. But Young had only met with 

 slight cases, and thought that an ordinary lens slightly 

 inclined was sufficient to correct the defect. 



Airy's principal contribution to Physical Optics is con- 

 tained in a paper in which the coloured curves observed in 

 crystalline plates are mathematically explained and the 

 results more particularly applied to the beautiful spiral forms 

 seen in quartz under certain conditions. Another paper 

 deals with the rainbow, the general explanation of which was 

 first given by Descartes. Most people will have observed that 

 the violet of the rainbow is frequently followed by a dark red 

 and a succession of colours, sometimes twice repeated. The 

 cause of these so-called supernumerary rainbows was given in 

 a general way by Young, who showedfthat their appearance 

 depends on the interference of light which manifests itself 

 when the sizes of the raindrops are nearly equal ; but Airy 

 gave the first mathematical treatment of the subject. 



Terrestrial Magnetism was another subject to which 

 Airy? devoted his attention, more especially after he had 

 gone to Greenwich as Astronomer Boyal. The connexion 



