TOTAL SOLAH eclipse OE JULY 18, 1860. 
379 
of the imaginary chord joining the cusps. Thus, in the photograph under consideration, 
The first reading of vernier V was 
The second reading 
Length of the chord = difierence . 
Length of the semichord or sine . 
3-841 
0- 124 
3-717 
1- 8585 
Slide B was now brought to its normal central position, 1-981 inch, and clamped, and 
vernier a! of the slide A being read off, gave the position of the imaginary chord. The 
shde A was now moved sufiiciently to bring, first, the moon’s periphery and then the 
sun’s periphery into coincidence with the centre of the microscope-cross, and the read- 
ings were recorded in each case. These readings gave the means of finding the versed 
sine of the moon, the versed sine of the sun, and the distance of the moon’s periphery 
from the sun’s periphery. 
Thus the first reading of the vernier a! gave the position of the chord . . 2-407 
The second reading the position of the moon’s limb 1-154 
Their difierence the versed sine of the moon 1-253 
Again, the first reading was 2-407 
The third reading giving the position of the sun’s limb 0-094 
And their difierence the versed sine of the sun 2-313 
Position of the moon’s periphery as above 1-154 
Position of the sun’s periphery as above 0-094 
Their difierence gave the distance of the sun and moon’s peripheries . . 1-060 
Table 11. , columns 2 to 10 inclusive, gives a series of readings of the verniers and 
5', and the resulting determinations of the chords and the versed sines of the cusps of 
the sun and the moon. 
By making the vernier of the circle C read 360°, plus the correction for the error of 
the wires of the heliograph, the position of the cusps could be read ofi" in difierences of 
right ascension and declination from the sun’s (or the moon’s) centre. Thus, in the 
photograph quoted. No. 20, the circle was set at 0° 9' 30", and the northern cusp made 
to coincide with the centre of the cross of the microscope by moving both the slides ; 
the same thing was done with respect to the southern cusp. The readings were as 
follows ; — 
Northern Cusp. 
Slide A 3-853 Slide B 2-429 
Centre 2-0025 Centre 1-981 
Difierence of declination . . -1-1-8505 Difierence of R.A. . . 4-0-448 
Southern Cusp. 
Slide A 0-505 Slide B 0-803 
Centre 2-0025 Centre 1-981 
Difierence of declination . . —1-4975 Difierence of R.A. . . —1-178 
