TOTAL SOLAE ECLIPSE OF JULY 18, 1860. 
377 
in. 
The first reading of photograph No. 20 be 3-907 
Deduct the position of the centre on the scale «... 2-0025 
Eadius of the sun= 1-9045 
in. 
0- 094 
3-907 
2)3-813 sun’s diameter. 
1- 9065 radius. 
These measurements, in the example cited, differ from each other 0-002 inch, which, as 
will be hereafter seen, is equal to about 1" of arc. In Table I., columns 7 and 8, are 
given two series of measurements of the sun in these ways : in column 7 the numbers 
were obtained by the fir-st, in column 8 by the second method. 
The next operation was the rectification of the position of the photograph by means 
of the -wires depicted on it. I must here call attention to the circumstance, that in 
the various phases of the eclipse the -wires cannot be traced beyond the crescent of the 
sun; for the time occupied in taking the photograph is so extremely short, that the 
backgi-ound of the sky in the immediate neighbourhood of the sun does not in the 
slightest degree depict itself on the collodion plate. The wires are visible on the sun’s 
crescent, because they intercept his action, and produce a blank space corresponding to 
their shadows, but evidently the dark body of the moon and the adjacent sky could have 
no such effect. In photographs of the partial phases, therefore, the length of the wires 
depicted corresponds with the extent of the crescent unobscured, and is shorter the nearer 
the picture is to the totality. A wire which is visible in some cases is afterwards covered 
by the moon, so that the correct apposition of the picture could not be effected, in every 
instance, by means of the same wire. In the totality-pictures the whole four wires are 
\-isible, from the fact of one or other of them having intercepted either the light of the 
prominences or that of the corona, the latter even having depicted itself sufficiently to 
render the shadow of the wires quite perceptible. In the other phases, either wire IV. 
or wire II. was used for rectifying the picture ; if wire II. were used, the circle was 
clamped to read 134° 54' 10", because this had been found, by measurement, to be the 
position which con-esponded to 315° for wire IV. 
Supposing wire IV. to have been visible, and the circle clamped so as to read exactly 
315° after centering, it would generally happen that the picture of wire IV. was neither 
under the centre of the microscope, nor parallel with the wire corresponding to the 
slide A, because, in the first place, the picture may not have been taken exactly in the 
centre of the heliograph ; and in the second place, in centering, the image of the wire 
may not have been brought exactly parallel with a normal diameter from 315° to 135° 
of the circle C. If such were the case, by means of the upper circle D the image of 
the wire was first made approximately parallel with a wire of the microscope, and then, 
by means of the screw h" of the slide B, was brought exactly under this wire of the 
The second reading of same photograph . . . . 
x\nd the first reading having been 
Difference between first and second reading gives . 
