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used. In 1897 September central illuminations by the usual method of small reflecting prism (-inch square) cemented on the outside of the object-glass and collimating lens with a small electric lamp in its focus was arranged for, and was subsequently brought into use. There was difficulty, however, with the illumination, owing to the filaments of the lamp forming an enlarged image in the focus, and the wires were badly defined, owing, as afterwards appeared, to diffraction effects from the small aperture of the illuminating pencil. Further, with such a small pencil, dust on the field lens of the eyepiece would be sensibly in focus with the wires.

Attention being again called to the question, it seemed that too much importance had been given to simply getting enough light without sufficient consideration of the optical conditions required in the arrangement for central illumination of the field. These conditions are:

(1) That a uniform illumination shall be secured over the
field, without such defects as are due to the source of
light coming to a focus near the plane of the wires.
(2) That the illuminating pencil should have a sufficient
aperture to minimise diffraction effects on the wires,
giving rise to shadow-bands.

The condition (1) can be secured by substituting a matt surface, such as opal glass (finely ground) or plaster of Paris, for the specular reflector, the condensing lens with electric lamp in its focus being retained. The parallel rays which fall on the matt surface from each point of the source of light in the focus of the condensing lens are scattered uniformly in directions slightly inclined to the axis of the telescope, and after passing through the object-glass, converge respectively to each point of the field, so that each point is illuminated by a pencil uniformly distributed over the whole of the opal reflecting surface.

As regards condition (2), it is to be borne in mind that in transit observations sharp definition of the wires is as important as sharp definition of stars; and that, as the wires are seen as shadows thrown by the illuminated field, the aperture of the illuminating pencil determines the sharpness of the wires. With the small pencils which have commonly been used even on large instruments, there are necessarily large diffraction effects which are serious with the relatively very high powers employed. At the same time it is to be noted that comparatively little loss of light, and practically no loss of definition for a star, is entailed by cutting out a circle of, say, one-fourth of the aperture at the centre of the object-glass.*

After some preliminary experiments with central illumination for the portable transit, the following arrangement was adopted on 1907 May 8 to secure the two conditions stated above. An elliptical finely ground opal glass was used as the reflector, and mounted in a brass tube which was attached to the outer surface of the object-glass by shellac. The illuminating source was a small * The loss of light would in this case only be one-sixteenth.

electric lamp in the focus of a condensing lens, throwing a parallel beam of light on the opal reflector. The brightness of the electric lamp was controlled by a rheostat, and a very satisfactory illumination of the field, with the wires sharply defined, was obtained. The diameter of the pencil of rays from the reflector was 3-inch, which was adopted as suitable for the 3-inch object-glass. The results given for Transit C, in Table IX.-central illumination -were all obtained with this arrangement. It was found, however, that the shellac was not a secure fastening for such an instrument as a portable transit, as any jar was apt to dislodge the arrangement, so a more secure form of support was adopted later on, and applied first to transit B in July 1907. This consisted of an opal reflector as before, fixed to the dew cap by three pieces of watch-spring, and is shown in Plate 7. This arrangement was found satisfactory, and so was attached to the other instruments (transits B and D) as well. No trouble has been caused by diffraction effects. The observations obtained are, on the whole, satisfactory, although two nights (September 10 and 11) with transit B give discordances W-E of over os 1. There was probably some other cause at work on these nights, as the collimation determined from the nadir gave a sudden jump of about 2" in the opposite direction to the error shown by the stars (Tables VI., IX., and X.).

As the application of the improved central illumination to the small transits proved so successful, it was applied to the New Altazimuth on 1907 June 5, the mode of attaching the reflector to the object-glass being slightly modified from the original plan with shellac cement. In order to secure a firmer attachment, a plain glass plate, worked slightly concave on the outside to fit the curvature of the outer surface of the object-glass, was burnished into the end of the tube carrying the opal reflector, and cemented on to the object-glass with Canada balsam. In this way the obstruction of light due to the attachment is reduced to a minimum, viz. the thickness of the brass tube. The general arrangement is shown in Plate 8. The aperture of the illuminating pencil adopted for the 8-inch object-glass was 1 inches, which was found to give greatly improved definition of the wires, which had been very unsatisfactory with the 3-inch pencil formerly in use. At the same time the troublesome markings previously seen in the illuminated field were got rid of, and a satisfactory uniform illumination substituted.

It is proposed to apply a similar method of illumination to the Greenwich transit-circle as soon as the necessary electrical connections can be made.

The tables show the difference between the mean clock error determined with the instrument micrometer West and micrometer East respectively. They are corrected for clock rate and reduced with a collimation error determined from nadir observations only. The weights given depend on the number of reversals of the instrument, a weight of 1 corresponding to a single reversal, while a weight of 2 corresponds to two or more.

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PORTABLE TRANSIT-ILLUMINATION OF FIELD.

a, Matt surface reflector (Opal glass finely ground).

b, Brass tube to carry reflector.

c. Supports for tube, made of watch-spring.

d, Condensing lens.

e, Small electric lamp.

f. Object-glass of telescope.

g, Object-glass cell.

h, Dew cap.

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