276 SECTIONAL TRANSACTIONS.—A. 
All the motions of the telescope, viz. quick setting, guiding and clamping 
in both R.A. and declination, also focusing of the Cassegrain mirror, are 
electrically operated. 
The main mirror, now being worked at Newcastle-on-Tyne, is of a 
special Pyrex glass, 76 in. diameter by 12 in. thick and focal length of 
30 ft. 
The Cassegrain mirror gives an equivalent focus of 108 ft. 
The dome, which has a diameter. of 61 ft. with an opening 15 ft. wide, 
is fitted with motor-driven shutters and wind screens, and carries an elec- 
trically operated observing carriage for use at the Newtonian focus. 
The dome is mounted on a circular steel building 24 ft. high. 
Mr. W. M. H. Greaves.—The new 36-inch reflector at the Royal 
Observatory, Greenwich. 
Mr. C. R. Burcu.—On null systems for testing concave telescope 
MuUrvrors. 
Zonal tests on concave specula have neither the accuracy nor the speed 
of null tests. The most delicate test—Prof. Zernike’s phase-contrast 
test—is essentially a null test. We need a method of null testing paraboloids 
without using a full-size flat, and methods of null testing mirror curves 
other than conic sections—e.g. the Ritchey-Chrétien curve. The asphericity 
of a 36-in. paraboloid of F/4 can be compensated with one 9-in. concave 
spherical mirror, one 13-in. convex mirror aspherical by only } wave-length, 
and one 3-in. flat. For an F/6 paraboloid, both compensating mirrors may 
be spherical. Asphericities up to a few wave-lengths may be compensated 
by a figured transmission plate, checked with an optical flat—in this check 
the transmission errors are seen multiplied by 4. ‘The figured plate may 
conveniently be 1 in. diameter: the star is decentred so that the light 
passes through it once only, and the consequent astigmatism is annulled 
by two plane-parallel plates placed with equal and opposite obliquities to 
the central ray. By placing the figured plate at different distances from the 
star, a range of paraboloids can be tested. 
Mr. N. R. CAMPBELL and Mr. C. C. Paterson, O.B.E.—Photoelectricity, 
art and politics : an historical study (11.30). 
(Ordered by the General Committee to be printed in extenso. See p. 445.) 
Wednesday, September 12. 
Dr. W. H. McCrea.—Observable relations in relativity (10.0). 
The formulation of an invariant which represents ‘ spatial distance,’ as 
measured by some prescribed experiment, in the space-time of general 
relativity has been studied by E. T. Whittaker and others. Also E. A. 
Milne has emphasised the importance of interpreting any given space-time 
in terms of the ‘ world-pictures ’ of an observer belonging to it. In this 
paper it is shown how the apparent size, brightness, etc., of nebule in certain 
models of the ‘ expanding universe ’ can be calculated by fairly elementary 
methods. Thence one obtains, for example, the number of nebule in a 
given range of apparent magnitude, and the relation between apparent 
