REPORT OF THE KEW COMMITTEE. XXXV 
The object-glass of this instrument is 3;4; inches aperture and 50 inches 
focal length ; it is not corrected for achromatism in the ordinary manner, but 
so as to produce a coincidence of the visual and photogenic foci. The 
secondary objectives for magnifying the image produced by the principal 
object-glass are of the Huyghenian form. They are three in number, pro- 
ducing respectively images of the sun 3, 4, and 8 inches in diameter. Between 
the two lenses of each of these secondary object-glasses is inserted a dia- 
phragm-plate carrying the fixed micrometer wires, which are of platinum; 
these wires are four in number, two at right angles to the other two. One 
of the wires of each pair is in such a position that they may both be made 
tangential to the sun’s image, while the other two cross at a point situated 
near the sun’s centre. By means of these wires, the distance in are between 
each pair having been once for all ascertained astronomically for each 
secondary object-glass, it will be easy to determine all the data necessary for 
aseertaining the relative magnitudes and positions of the sun’s spots. These 
micrometer wires are under the influence of springs, so as to preserve a tension 
upon them when expanded by the sun’s heat, and thus to keep them straight. 
The principal and secondary object-glasses are not mounted in an ordinary 
-eylindrical tube, but in a pyramidal trunk square in section, 5 inches in the 
side at the upper end, which carries the principal object-glass, and 12 inches 
in the side at the lower end, which carries the photographic plate-holder and 
the usual ground glass screen for focusing. 
This trunk is firmly supported by a declination axis of hard gun-metal 24 
inches in diameter; it is furnished with a declination circle 10 inches in dia- 
meter, reading to one minute of arc, and has a clamp and screw motion for 
fine adjustment in declination. 
The declination axis works in Y-bearings at the top of the polar axis, which 
is 12 inches long; it is 4 inches diameter at its upper end and 14 inch at 
its lower end. The lower end fits with a slight taper into a brass collar up 
to a shoulder, the friction being reduced by a steel spring plate pressing 
against a hardened steel hemisphere at the end of the axis. 
It will be seen by the above description, that every precaution has been 
taken to secure stiffness in the telescope combined with freedom in the 
motion of the polar axis. The polar axis is driven by a clock driver, which 
answers perfectly, and is easy of regulation to the greatest nicety, so that the 
sun’s limb remains for a long period in contact with the tangential wires. 
Near the lower end of the polar axis is fixed the hour-circle, which, like the 
declination circle, is 10 inches indiameter; it is graduated to read to 2 seconds of 
time. An endless screw, making about two revolutions in one minute, geers 
into the hour-circle and connects it with the clock. As it is generally ne- 
cessary to make small corrections in right ascension after the tangent screw 
has been geered with the driving clock, in order to bring the sun’s image in 
position with respect to the micrometer wires, a sliding plate is provided which 
carries the bearings of the tangent screw; this is acted upon by a second fine 
serew parallel with the tangent screw; so that by rotating the second screw, 
the sliding plate and the tangent screw are moved through a small space, 
and the hour-circle thus caused to rotate te the extent necessary for bringing 
the sun’s image in position. 
The clock is driven by two weights, one pulling upwards over a pulley, the 
other downwards, thus suspending the barrel and equalizing the pull and 
avoiding friction on its bearings. By causing the click of the winding lever 
to abut on the ratchet-wheel of the going part of the clock during the period 
of winding, the clock goes at its normal speed while it is being wound. 
The mode of regulating the clock is extremely simple and efficacious ; it is 
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