TOTAL SOLAE ECLIPSE OF JULY 18, 1860. 
371 
the plate. It was then put on the straight-line engine, and a scale of minutes and 
seconds of arc set off from the moon or the sun’s periphery, in accordance with the 
previously calculated value in arc of subdivisions of an inch ; both sets of division were 
then etched in the same way as the outline. 
Sometimes, according to the position in which the photograph was taken*, the etching 
was performed at the back of the plate, to correspond with the previous tracing through 
the collodion on its face. In this case the collodion picture might be allowed to remain 
as “a witness” (as workmen caU it) of the correctness of the etching. In other cases, 
if the original negative had been purposely turned over, so as to present the opposite face 
to the camera, then the etching was made through the collodion, which had to be 
removed before the subsequent operations about to be described were performed. 
An etched glass plate, if filled with printing ink, could be made to give a print by 
placing india proof-paper over it, and, after superposing a sheet of glazed paper upon 
this, rubbing the latter carefully with a burnisher ; but it would not be advisable to 
attempt to take many impressions in this way. However, by the well-known processes 
of electrotype, copper duplicates of the glass plate can be procured, which can be printed 
from in the ordinary copper-plate press ; and as the glass plate is only used for furnish- 
ing the matrices, and is not injured thereby, the printing-plates may be procured with- 
out practical limit as to number. In this way Plates XIII., XIV., XV., XVI. and XVII. 
were obtained. The original glass plate of Plate XV. was, however, made in a some- 
what different manner from the others. Originally, it was a photograph of the sun ; 
after the outline of the sun and his spots had been etched, and the normal line marked 
thereon, the collodion was entirely removed, to permit of the plate being superposed, 
accurately, first over Plate XIII., and then over Plate XIV. Previously, however, 
Plate XV. was coated with the transparent etching-ground, and the luminous promi- 
nences depicted on Plate XIII. traced off, care being taken to ensure the parallelism 
of the normal line of one plate with that of the other, and internal contact between 
the peripheries of the sun and moon respectively. The same thing was done with 
Plate XIV., the prominences visible in the two pictures being placed in coincidence. 
In this way the pictures of the prominences could be made to assume their proper 
position around the sun’s picture. In order to facilitate this operation, a positive 
picture had been previously taken with the enlarging-camera, from both the original 
totality negatives laid one over the other, and combined suitably together, so as to form 
in one picture a correct representation of the whole of the prominences. When the two 
totality pictures had been traced off on Plate XV., a line was drawn to join the two 
positions of the moon’s centre, which had been set off from Plate XIII. and Plate XIV. 
respectively ; this line was then prolonged to show the path of the moon’s centre during 
the period of totality; lines were also drawn to join these positions of the moon’s centre, 
and the sun’s centre, and prolonged to the periphery of the sun, to indicate the points 
of disappearance and reappearance of the sun’s limb. When etched, this plate was 
* By placing the original negative in the copying-camera with the collodion film either turned towards 
the lens or away from it, the picture produced was either in its natural position or reversed right for left. 
