vi 



examine the discrepancies between the computed right ascensions of 

 the Sun and those observed at Greenwich. On making a comparison 

 between the discrepancies in the position of the Sun's perigee as 

 given by late observations with those given by the observations of 

 the last century, he concludes there must be some yet undiscovered 

 inequalit}^ which has been omitted from the calculations. He soon 

 discovered that this originated in the fact that thirteen times the 

 periodic time of Venus is so nearly equal to eight years that the term 

 depending on this phase received a multiplier of more than two 

 millions in integrating the differential equations. On the other 

 hand, the coefficient is of the fifth order with regard to the eccen- 

 tricities and inclinations of the orbits. In the report on this paper 

 drawn up by "Whewell and Lubbock for the Royal Society, it is 

 pointed out that no numerical calculation of a perturbation of the 

 fifth order had been executed, except in the case of Jupiter and 

 Saturn, where, as Laplace states, this labour, " penible par son exces- 

 sive longueur," had been performed by Burckhardt ; and no calcula- 

 tions of a new inequality of a high order, requiring to be placed in 

 the planetary tables, with a new argument, had been published since 

 that of the great inequality by Laplace in 1784. They conclude by 

 remarking that this is the first specific improvement in the solar 

 tables made by an Englishman since the time of Halley. For this 

 brilliant investigation the Astronomical Society in 1833 awarded to 

 its author their gold medal. The whole of Professor Airy's process 

 was afterwards verified, first by Pontecoulant, and secondly by 

 Leverrier, and found to be correct. 



In the years 1831-32, Professor Airy, though so fully employed at 

 the Observatory, was yet able to make some important investigations 

 in the theory of light. Thus he communicates to the Cambridge 

 Philosophical Society a paper to show that the two rays produced by 

 the double refraction of quartz are elliptically polarised. This is soon 

 followed by two or three papers on some phenomena connected with 

 Newton's rings. Just as Sir W. Hamilton afterwards predicted 

 internal and external conical refraction after studying the analytical 

 properties of the wave surface, so Professor Airy discovered these 

 phenomena by using Fresnel's general formula for the intensity of 

 reflected light. When Newton's rings are formed by light polarised 

 in a plane perpendicular to that of incidence between two substances 

 of different refractive indices, and the angle of incidence lies between 

 the polarising angles, the rings should appear white centred, instead 

 of having a central dark spot. Here was a recondite phenomenon 

 which could only be seen when several special conditions were satisfied. 

 Would it be confirmed by experiment ? He describes the difficulties 

 of the experiment and its final success. As we read the paper, we 

 perceive how he is led on by slight unexpected discrepancies to 



