336 REPORT—1905. 
The white réseau contains sixteen lines in each direction (fifteen spaces), the 
outer lines forming a square which just fills one of the squares of the black réseau. 
Since a black réseau interval (5 mm.) corresponds to an angle of 150” in the 
Radcliffe telescope, it follows that a white réseau interval is equal to 10”, The 
pitch of the micrometer screws is equal to one white réseau interval, z.e. 10”, and 
the heads being divided into 100 parts, which are read by estimation to tenths, a 
unit in the last figure is equivalent to 0’”01. 
The eyepiece is carried on two slides at right angles to each other, so that any 
object can be brought rapidly into the centre of the field of view. 
When a plate is inserted in the machine the preliminary adjustments of 
focussing, bringing the lines of the black réseau parallel to the slides, and those of 
the two réseaux into sensible parallelism, can be eflected in two or three minutes 
by means of the adjusting screws provided for that purpose. These adjustments 
having been satisfactorily completed, the observations for measuring the position 
of a star-image with regard to the réseau square in which it lies are as follow : 
By means of the racks the black réseau is set so as to coincide approximately with 
the white. Using the micrometer screws three corners of the white réseau are 
then brought in succession to coincide exactly with the corresponding corners of 
the black réseau. As the white lines are considerably thinner than the black, and 
appear neat and sharp when projected on the somewhat softer black lines, this is 
an observation which can be made with very great delicacy. Froma large number 
of observations the probable error of a single setting of this sort was found to be 
+ 0":002 = + 07:02. 
The corners of the square being numbered in rotation 0, 1, 2, 3, beginning at 
the left-hand bottom corner and going round in an anti-clockwise direction, three 
pairs of readings of the horizontal and vertical micrometer screws are taken 
corresponding to the corners 0, 1, and 8, which may be denoted by hy, vp; Ay, v, 5 
ha, vs, respectively. If the star-image lies between the mth and (m+ 1)th vertical 
lines, and between the nth and (7 + 1)th horizontal lines, the intersection of the mth 
and nth lines is made, by means of the micrometer screws, to coincide with the 
centre of the star-image, and the readings 2 and v of the two screws are taken. 
If, then, we take as unit tbe fifteenth part of a black réseau interval (10” 
approximately), and if the values of the micrometer screws are 1—p and 1—g 
respectively, when expressed in terms of a white réseau interval (so that p and q 
are very small quantities), and if we put 
E=mt+h—h, and n=n+v-v, 
