94 DR J. HALM ‘ON 
paper on the ‘“ Law of Diurnal Rotation of the Optical Field of the Siderostat and 
Heliostat” (Astrophysical Journal, vol. xi., 1900, pp. 148-162). It appears from his 
calculations that in our case, where a siderostat oriented in the meridian has been used, 
the angle Y at the centre of the field between the reflected image of the meridian and 
that of the hour circle of the sun is expressed by the formula, 
tan 4Y = K tan 3¢, 
where ¢ is the hour angle, reckoned positive towards the west, and 
K—Sin2(¢-?P) 
sin }(p+p)’ 
# being the latitude of the place and p being the polar distance of the sun. These 
equations determine the amount and the direction of the rotation. We are thereby 
enabled to fix the position of the north point of the solar disc as it appears in the field 
of the siderostat for any time of the day. ‘The values Y can be tabulated with the 
arguments ¢ and declination of the sun. The position circle of the heliometer is so 
oriented that the diameter 0°-180° falls in the plane of the meridian. The angular 
distance of the pomt under observation from the north point of the solar disc, that is, 
the position angle P of the point, is then found by the expression 
P=90°— heey. 
where II is the reading of the heliometer position circle. The value of P being found, 
the heliographic latitude of the point observed can be directly computed from the 
formule given by Professor DuNER in his treatise, where he also exhibits extensive 
tables, greatly facilitating these computations. I need not, therefore, enter more fully 
upon this part of the reductions. 
I shall now briefly describe the way in which observations were performed with this 
instrument. The sunlight reflected from the siderostat mounted on the main platform 
of the Observatory is thrown into a meridional direction upon a window of 6 inches 
aperture in the north wall of the great optical room, and falls upon the object-glass of 
the heliometer placed immediately behind this window. The halves of the object-glass 
are screwed apart until the two solar images are nearly in contact, but still separated by 
a narrow space, which in the viewing telescope appears as a dark horizontal band between 
the spectra of the two limbs. The right-angled prism mentioned above is then turned 
and the heliometer adjusted so that the solar images fall upon the slit in the position 
indicated in fig. 2. After reading the position circle of the heliometer, the observer 
commences the measurements by pointing the first pair of the micrometer wires on the 
four lines of the upper of the two spectra seen in the viewing telescope. He begins, 
say, with the left-hand line of the group, and proceeds towards the right. He then 
measures with the same pair the lines of the lower of the two spectra in the direction 
from right to left. The observations are afterwards repeated with the second pair of wires ; 
this time, however, they are begun on the lower spectrum from left to right, and finally 
the upper spectrum is measured from right to left. This arrangement, though it may 
