TRANSACTIONS OF SECTION A. 335 
4. On Lunar Radiation. By the Eart or Rosse, K.P., F.R.S. 
In this paper the author, after recapitulating briefly the leading points and results 
of determinations of lunar radiation made at different ages of the moon, more fully 
described in ‘ Proc. Roy. Soc.’ 1869 and 1870, and ‘ Phil. Trans.’ 1873, also of 
measurements of heat during lunar eclipses (see ‘Trans. Roy. Dublin Soe.’ 1885 
and 1891), made some suggestions for future work with specially made, but simple 
and cheap, apparatus, also for prosecuting the inquiry in better climates than that 
of the United Kingdom. 
He further suggested co-operation between workers at widely different lati- 
tudes and longitudes, so that every eclipse, whether occurring during the northern 
winter or southern winter season, may be equally well observed, during the whole 
of its progress, under equally favourable conditions, at one or other of the 
stations. 
He also discussed different modifications both of the heat-collecting and heat- 
detecting appliances. 
5. A New Instrument for Measuring Stellar Photographs. 
By Artaur A. Rampavt, /.A., D.Sc. YRS. 
The instrument recently constructed by Sir Howard Grubb, F.R.S., for the 
Radcliffe Observatory, Oxford, which is the subject of this note, is designed for the 
purpose of measuring, with all available precision, the position of a star-image 
with regard to the réseau square within which it falls, the réseau being impressed 
photographically on the plate in the usual way before development. In what 
follows this réseau is referred to as the ‘ black’ réseau. 
Externally the instrument ‘somewhat resembles that constructed by Messrs, 
Repsold for the Cape Observatory, and described by Sir David Gill in the 
‘Monthly Notices’ R.A.S., vol, lix., No. 2. It differs, however, from the latter in 
many essential details. 
The base of the instrument is a massive iron casting, the plane top of which is 
inclined at an angle of 45° to the horizon. The photographic plate is carried, as in 
the Cape instrument, on two slides running on two mutually pérpendicular steel 
cylinders. The movement of the slide is in each case effected by means of a rack 
and pinion, and each cylinder has attached to it a scale divided to 5 mm., and 
numbered to correspond with the réseau lines on the plate. An adjustment is 
provided by which the réseau lines may he placed with great delicacy exactly 
parallel to the axes of the cylinders. The instrument will measure plates up to 
12 inches square. 
The microscope, which is of a novel construction, is carried by a strong cast- 
iron tribrach, supported on pillars, as in the Cape instrument. It consists of a 
central cube, D, into three sides of which are screwed brass tubes carrying three 
objectives, A, B, andC. At the centre is fixed a cube of glass composed of two 
parts, each of which is a right-angled prism. The face opposite the right angle of 
one of these prisms is coated by a special process with a thin film of galena, and 
the two prisms are then cemented together so as to form a cube. The two objec- 
tives, A and B, together form a doublet lens, the light from any point of the plate 
passing from one to the other in a parallel beam, Any colour effect due to 
refraction at the surfaces of the cube is thus avoided. 
In the focal plane of the objective O is a small réseau ruled ona silvered glass 
plate, which is movable on slides at right angles to each other by means of 
micrometer screws. The objectives B and C together also form a doublet lens, 
the light being reflected as a parallel beam from the galena surface. The small 
réseau is thus seen projected on the plate, and, as the lines appear bright, it is 
convenient to refer to it as the ‘white’ réseau. There are adjustments for setting 
the slides parallel to the lines of the réseau and for setting the micrometer as a 
whole, so that, as seen in the eyepiece, either system of lines in the white réseau 
may appear exactly parallel to the corresponding system in the black réseau. 
