some concession to my ideas in _my_
paragraph 3, above.

I am, my dear Sir,
Yours very truly,
G.B. AIRY.

_Rev. Dr Lightfoot._

* * * * *

1869

From the Report to the Board of Visitors it appears that application
had been made for an extension of the grounds of the Observatory to a
distance of 100 feet south of the Magnetic Ground, and that a Warrant
for the annexation of this space was signed on 1868, Dec. 8. The new
Depot for the Printed Productions of the Observatory had been
transferred to its position in the new ground, and the foundations for
the Great Shed were completed.--"The courses of our wires for the
registration of spontaneous terrestrial galvanic currents have been
entirely changed. The lines to Croydon and Deptford are abandoned; and
for these are substituted, a line from Angerstein Wharf to Lady Well
Station, and a line from North Kent Junction to Morden College
Tunnel. At each of these points the communication with Earth is made
by a copper plate 2 feet square. The straight line connecting the
extreme points of the first station intersects that connecting the two
points of the second station, nearly at right angles, and at little
distance from the Observatory.--The question of dependence of the
measurable amount of sidereal aberration upon the thickness of glass
or other transparent material in the telescope (a question which
involves, theoretically, one of the most delicate points in the
Undulatory Theory of Light) has lately been agitated on the Continent
with much earnestness. I have calculated the curvatures of the lenses
of crown and flint glass (the flint being exterior) for correcting
spherical and chromatic aberration in a telescope whose tube is filled
with water, and have instructed Mr Simms to proceed with the
preparation of an instrument carrying such a telescope. I have not
finally decided whether to rely on Zenith-distances of gamma Draconis
or on right-ascensions of Polaris. In any form the experiment will
probably be troublesome.--The transit of Mercury on 1868, Nov. 4th,
was observed by six observers. The atmospheric conditions were
favourable; and the singular appearances usually presented in a
planetary transit were well seen.--Mr Stone has attached to the
South-East Equatoreal a thermo-multiplier, with the view of examining
whether heat radiating from the principal stars can be