378 
ME. WAEEEN DE LA EUE ON THE 
microscope, so that the latter bisected longitudinally the broad image of the wire in 
the photograph ; the slide A was now undamped, and drawn along : if the microscope- 
cross continued to bisect, in the direction of its length, the wire of the photograph, the 
operation had been successful ; if not, by again turning the upper circle D through a 
small arc, and moving the slide B suthciently to cause bisection, this coincidence was 
finally brought about. Slide B was then screwed back to its original central position, 
namely 1‘981 inch, and slide A to that position in which the periphery of the sun 
came exactly under the centre of the cross of the microscope, which in the example 
cited would be 3‘907 inches. 
The instrument was then m a position for measuring the position-angles of the cusps, 
which was effected by rotating the circle C so as to bring first one and then the other 
cusp under the microscope. Table III., columns 8 and 9, contains a series of such 
measurements, corrected, however, for the ascertained error of the position-wires of the 
heliograph, which is given in column 7, for the epoch of each photograph : the original 
numbers, before the correction was applied, were the means of three measurements for 
each cusp. 
The position-angles of the cusps gave the means of finding the position-angle of the line 
joining the centres of the sun and moon, which is at right angles to the chord joining 
the cusps. For example, in photograph No. 20, the measurement of which has been 
quoted by way of illustration. 
o i a 
The position-angle of the northern cusp was found to be 
13 
49 
30^ 
That of the southern cusp 
218 
19 
Adding 
360 
And dividing by 2 
2)592 
8 
30 
We obtain for the position-angle of the hue joining the 
sun and moon’s centres 
j 296 
4 
15 
The circle C was now fixed, so as to read the angle thus found to be that of the line 
joining the centres of the sun and moon, and the slides A and B were both imclamped, 
and so placed as to bring one of the cusps exactly to coincide with the centre of the 
microscope-cross. Slide A was then clamped, and slide B drama along so as to bring 
the other cusp under the microscope. If it coincided with the centre of the cross, the 
operation was so far completed ; but if not, by causing the circle C to move through a 
small arc, and adjusting A a little, the coincidence of both cusps was brought about. 
In the photograph cited, the circle had to be brought to read 296° 0' 30"; and this, and 
other numbers obtained in a like manner, are the lines of centres given in Table III., 
column 11, corrected for the errors of the wires of the heliograph. 
The coincidence of the cusps with the microscope-cross having been effected, slide B 
was moved so as to bring one cusp exactly under the centre of the microscope ; and its 
position having been read off on the vernier b', the other cusp was made central, and 
the vernier b' again read off, the difference between the two readings giving the length 
* These niimbers are not corrected for the errors of the position-wires as in Table III. columns 8 and 9. 
