TOTAL SOLAE ECLIPSE OF JULY 18, 1860. 
401 
single plate, although only a part had been visible at one time, and the plate would have 
shown veiy nearly the position of the moon at the conclusion of the operation. Unless 
an instantaneous picture of the phenomena of totality could be procured *, as in the case 
of the other phases, no photograph would show the precise state of matters at any one 
moment; consequently, if it be desired to know what was the condition of things at 
any one instant of the period during which the plate was in the heliograph, for example, 
at the commencement of the totality, and a minute afterwards, recourse must be had to 
the expedient of completing the circle of the lunar disk for the position she occupied 
at these two epochs respectively. The photograph itself affords the necessary data for 
effecting this ; for it will be found that the disk of the moon, as depicted on the photo- 
graph 25, represented in Plate IX. fig. 1, is not quite a complete circle, and that the 
longest diameter is in a direction at right angles to prominence A (see Plate XV.). 
By measuring the diameter in the direction of prominence A with a divided beam 
compass, and taking the half of the quantity as a radius, it was a matter of no great 
difficulty to find the centre of the picture for the two epochs in question, namely, near 
the commencement and near the end of the first mmute, as indicated by the photograph, 
and to draw in the lunar disk from either centre. In this way two photographs, a and /3, 
7 inches in diameter, were corrected, and served as originals for Plates X. and XI., which 
show the state of the phenomena as accurately as if two instantaneous pictures had been 
taken. 
Plate X., which is the copy of a, represents the appearance of the phenomena of 
totality at the commencement, and Plate XI. the copy of j3 nearly at the end of the first 
minute. A line drawn to the centres, laid down for the two epochs, was found to corre- 
spond absolutely with the direction ascertained independently to be that of the motion 
of the moon’s centre, and measured 23". Allowing a period of five seconds for the pro- 
duction of a picture sufficiently intense to show itself clearly on the plate, both at the 
commencement and at the end of the exposure, the period traceable would be fifty seconds, 
23" X 60 w 
and — =27"‘6 would be the motion of the moon’s centre during a minute, a result 
not differing by more than a few tenths of a second of arc from the mean derived from 
the measures of the other phases of the eclipse. 
In Plate IX. fig. I, representing the untouched photograph No. 25, a portion of the 
prominence II is visible ; but it came into view after the commencement of totality, and 
was therefore painted out in completing the lunar disk in the touched photograph os-, 
represented in Plate X.; at the epoch shown in photograph /3, represented in Plate XI., 
however, the lunar disk had revealed so much of prominence E as is seen in the original 
picture. 
The dark lines in the original photograph, represented in Plate IX. fig. 1, situated 
respectively above the floating cloud C, and across the broad part of G, are the images 
in shadow of the position-wires. In the original negatives and positive copies taken 
* The possibility of doiog this during future total eclipses will be presently pointed out. 
