In the 7-in. heliometer, completed in 1887 for the Royal Observatory
at the Cape of Good Hope, Repsolds, on Gill's suggestion, introduced
the following improvements: (a) Four different speeds of motion in
position angle were provided. The quickest movement is given by the
hand-ring, 73 (fig. 15). This ring runs between friction wheels and is
provided with teeth on its inner periphery, and these teeth transmit
motion to a pinion on a spindle having at its other end another pinion
which, through an intermediate wheel, rotates the heliometer tube. The
transmission spindle, just mentioned, carries at its end a head, 74,
which, if turned directly, gives the second speed. The slowest speed
is given by means of a tangent screw which is carried by a
ball-bearing on the flange of the telescope-sleeve, whilst its nut is
double-jointed to a ring that encircles the flange of the
heliometer-tube. This ring is provided with a clamping screw, which,
through the intervention of bevel-gear and rods, is operated by means
of the hand-wheel 78. With similar bevel-gear and rods the tangent
screw is connected to the hand-wheel, 79, by which the observer
communicates the fourth or slowest motion in position angle. Finally
the hand-wheel 80 is connected by gearing to the rod carrying the
hand-wheel 79, and it can thus be used to give the latter a more rapid
motion than if used direct; this constitutes the third speed of
movement.
(b) In lieu of oil-lamps, small, conveniently placed incandescent
electric 6-volt lamps are employed; and these are fitted with suitable
switches and variable resistances. Thus the scales, the position- and
declination-circles, the field of view, the heads of all the
micrometer-microscopes, the focusing scale, &c., are read without the
aid of a hand-lamp and with an amount of illumination that can be
regulated at the observer's pleasure.
(c) A button in the centre of the position-angle handle (74) connects
with a chronograph which enables the observer to record the instant of
observation. Little card-holders (81) (also illuminated) enable the
astronomer to enter beforehand the R.A. and Dec. of the object to be
observed, the scale divisions to be pointed upon, and thus, in
measures of distance, with the aid of the chronograph and printing
micrometer, enable the observer to adjust the instrument for
observation and obtain a record of his observations without the aid of
a hand-lamp or the necessity to make any records in his notebook. In
observations of position angle one of the two tablets 81 can be used
to record the readings.
(d) The scales are made of iridio-platinum instead of silver, and the
magnifying power of the reading microscope is increased fourfold (viz.
to 100 diameters). A special microscope is introduced for determining
the division errors of the scales. It enables the observer to compare
any division-interval on one half of either scale with any
corresponding interval on the other scale. With this apparatus Gill
was enabled (_Annals Cape Obs._ vii. 29-42, and _Monthly Notices,
R.A.S._, xlix. 105-115) to determine the division error of every line
on both scales with a probable error corresponding to [+-]0".0092 arc.
(e) A position-micrometer is attached to the finder to enable the
observer to select comparison stars for observation with some
unexpected object. Thus a comet may be encountered in the morning dawn
or evening twilight, and without such an adjunct the astronomer may
lose the whole available opportunity for observation in the vain
endeavour to find a suitable comparison-star. But with such a
position-micrometer of large field he has no difficulty. Directing the
finder to the comet, he has at once in the field of view all available
comparison stars. Having selected the most suitable one he directs the
axis of the finder to the estimated middle point between the comet and
the star, turns the finder-micrometer in position angle until the
images of comet and star lie symmetrically between the parallel
position wires, and then turns the micrometer screw (which moves the
distance-wires symmetrically from the centre in opposite directions)
till one wire bisects the comet and the other the star. The reading of
the position-circle of the finder is then the reading to which the
position-circle of the heliometer should be set, and from the readings
of the micrometer-screw he finds, by a convenient table, the proper
settings of the heliometer scales in distance. When the scales and
position-circle of the heliometer have been set to these readings, the
comet and the selected comparison-star appear together in the field of
view.
Fig. 15 shows the very convenient arrangement of the eye-end of the
instrument. The disk, 30 with its small projecting handle enables the
2 segments of the divided object to be moved rapidly or with any
required delicacy relative to each other. The disk 32 operates the
wire gauze screens for equalizing the brightness of the two stars
under observation. The dial between 30 and 32 indicates the screen in
use. 18 clamps and 19 gives slow motion in declination; 20 clamps and
21 gives slow motion in right ascension. The two handles 82 serve for
manipulating the instrument. The microscopes adjoining 82 read the
position and declination circles; for, by an ingenious arrangement of
prisms and screens, the images of both circles can be read by each
single microscope as shown in fig. 16, thus avoiding the necessity for
the employment of two additional micrometers.
Experience has shown that there is little that can be advantageously
changed to improve this instrument either in convenience or precision
of working. A series of observations can be easily and more accurately
accomplished with the Cape heliometer in half an hour; with the Oxford
heliometer it would occupy 2 hours, and with the 4 in. Repsold
heliometer (fig. 9) 1 hour. Heliometers of 6 to 8 in. aperture have
subsequently been constructed by Repsolds on these plans for
Gottingen, Bamberg, Leipzig and the Kuffner Observatory (near Vienna),
and all of them have made important contributions to astronomy of
precision.
Heliometer observations of distance in their most refined sense cannot
be considered absolute measures of angles. Essentially the scale-value
of the instrument depends on the relation of the focal length of the
object-glass to the length of the unit of the scale. But _the eye is
tolerant of small changes in the focal adjustment which sensibly
affect the scale-value_. These changes may and do arise from the
following causes: (i.) The focal length of the object-glass and the
length of the tube are affected by temperature. (ii.) The focal length
is sensibly different for objects of different colour. (iii.) The
length of the scale is affected by temperature. (iv.) The state of
adaptation of the observer's eye is dependent on his state of health,
on a condition of greater or less fatigue, or on the inclination of
the head in consequence of the altitude of the object observed. (v.)
The temperature of the object-glass, of the scale and of the tube,
cannot be assumed to be identical.
[Illustration: From _Engineering_, vol. xlix.