102 Dr. Rosisson—On Places of One Thousand Stars. 
There seems very little flexure in the telescope ; fifteen of the angles between 
the collimators gave its horizontal value 0°11”; this is probably due to the short- 
ness, aud large diameter of the tube, and to its being clamped at each end to 
the circle, the framing of which is very strong. Another error gave us some 
trouble. In winter all was right, but as summer came on it was observed that if 
the object glass was lowered from the south to the Nadir, the index correction 
obtained was available through the entire southern semicircle, but if it was lowered 
from the north the correction was as much as 4” less, but availed for the northern 
semicircle. The cause was obvious; in the cold weather the brass cells fitted the 
lenses tightly, but their expansion gave these play to shift by their own weight. 
Mr. Grubb remedied this by supporting the lenses on three equidistant bearings; two 
fixed at 60° east and west of the meridian; the third moveable, and pressed inwards 
by a spring, whose tension is a little more than the weight of the glass. And now 
the cell is of cast-iron, whose expansion is little more than half that of brass. These 
changes have proved so effectual that the difference of the index corrections is 
reduced to 0:07”. 
Some precautions, which need not be described, were required to prevent the 
iron, which carried the Ys, from a slight rocking motion when the circle was turned, 
and the clamps from exerting any lateral force which might disturb the azimuth or 
level. 
The mode of observing Polar distances is fully explained in the preface to the 
Armagh Star Places; so I will only describe that of Right Ascension. The tele- 
scope has a system of seven wires, designed for the usual mode of observing transits ; 
but, as I proposed, to use a chronograph these would have occupied too much time, 
and, therefore, two supplemental wires were added on each side of the central one, 
the equatorial intervals of this system being about 3 seconds. The illumination 
of the field is made by a small inclined central mirror, 0:4” in diameter, carried 
on a thin arm supported by the cover of one of the collimating apertures; and its 
intensity 1s controlled by a regulator containing three slips of orange glass, which 
can be combined by the observer. 
The chronograph was made by Knoblech of Altona, and is similar to that 
described by Dr. Peters in the Astron. Nach, XLIX., 1, except that as I was 
dissatisfied with the action of the conical pendulum by which it was regulated, 
Mr. Howard Grubb substituted a governor such as he applies to the driving clocks 
of his equatorials. On its records } of an inch represents a second of time. In this 
mode of observing the probable error of an equatorial star's transit is + 0°. 080, 
and of a single PD 0’. 816. For Zenith stars these are + 0°. 0995 and + 0”. 723, 
each deduced from 100 observations. 
On each night five standard stars were observed, both for clock correction and 
to check the Azimuth of the instrument. 
Lalande’s places were brought up to the time of observation by the preccssions 
