iv Proceedings of Royal Society of Edinhurgli. 
told, was spared to secure accuracy in the results, the errors of the 
instrument being determined, and proper numerical corrections 
applied to the observations. Although fully ten weeks of the 
summer of 1828 were devoted to visiting the observatories of 
France and the north of Italy, and although for the first year Airy 
had no assistant, the volume containing the earlier Cambridge 
observations appeared in April of the year after they were made. 
An assistant was then appointed, without whose aid in the routine 
work of the observatory it would have been physically impossible 
for Airy to devote due attention to the investigation of the motion 
of Venus, which now engaged his attention. In 1831 he presented 
to the Koyal Society his celebrated memoir “ On an Inequality of 
Long Period in the Motions of the Earth and Venus,” which, as Airy 
himself said, “ contains the first specific improvement in the solar 
tables made in this country since the establishment of the theory 
of gravitation.” About this time also he deduced the mass of 
Jupiter from elongations of the fourth satellite observed with a 
small equatorial telescope. These important investigations failed, 
however, to divert Airy altogether from the study of the Undulatory 
Theory of Light, which he materially helped to develop. Perhaps 
the most generally known of his discoveries in this field is that of 
the exquisitely beautiful phenomenon caused by the passage of 
polarised light through two thick plates of quartz cut from right 
and left-handed crystals, which will ever bear the name of Airy’s 
Spirals. 
For the first meeting of the British Association at York in 1831, 
Airy wrote a “Beport on the Progress of Astronomy during the 
Present Century.” It is in fact a masterly history of the science 
from the beginning of this century to the date of the meeting, and, 
as such, can never lose its value. Towards the close of 1832 the 
mural circle, 8 feet in diameter, was erected, and at the same time 
an additional assistant was appointed in the person of James 
Glaisher, afterwards so well known in the scientific world. About 
1833 the Duke of Northumberland presented to the Cambridge 
Observatory a refracting telescope with an object-glass of the then 
unrivalled aperture of nearly 12 inches. The mounting for this 
instrument was designed by Professor Airy, who adopted the Eng- 
lish arrangement of a long polar-axis, which when built in the solid 
