364 
ME. WAEEEN DE LA EIJE ON THE 
Previous to taking the picture, the sliding plate is drawn up just so high that the 
unperforated part of it completely shuts off the sun’s image ; it is held in this position 
by means of a small thread attached to it at one end and looped at the other, the loop 
being passed over a hook on the top of the tube ; and the slide is pulled downwards, in 
opposition to the thread, by means of a spring of vulcanized caoutchouc attached 
to the inferior side of the tube. When the picture is about to be taken, the retaining 
thread is set on fire*, and the rectangular aperture, as soon as the sliding plate be- 
comes released, flashes across the axis of the secondary object-glass — thus allowing the 
different parts of the sun’s image to pass through it in succession, and to depict them- 
selves one after another, after enlargement, on the collodion-plate. Although the time 
of exposure is so short as to be scarcely appreciable, yet it is necessary to regulate its 
duration ; and it is therefore controlled by adjusting, 1st, the strength of the walcan- 
ized caoutchouc spring ; 2ndly, the width of the aperture. In practice, the opening is 
usually varied between one-twentieth and one-fortieth of an inch. 
A number of plates, with ground rims and edges, were cleaned in London, so as to 
permit of their examination, and all defective ones were rejected; forty-eight selected 
plates were then numbered consecutively, and arranged in boxes marked very distinctly 
A, B, C, D, so as to ensure their being taken out in the proper order during the eclipse. 
The heliograph was furnished with three plate-holders, in order that no interruption 
might occur in the succession of the photographs ; and as these were filled, they were 
placed in such a way that each plate was sure to be exposed in its numerical order. A 
few spare plates were also, cleaned, and marked A, B, C, D, E, F, G, H, I, &c. 
On the day previous to the eclipse the plates were again carefully cleaned, and 
replaced in their proper order in their respective boxes. 
On the 18th the following plates were placed in the heliograph, and the time of 
taking each photograph noted by Mr. Becklet, with any requisite remarks. The time 
was observed with Frodsham No. 3094, whose error at Greenwich mean noon was, as 
already stated, fast of Greenwich mean time 4-6 seconds, and whose daily rate was losing 
2T seconds. The exact time of depiction was ascertained by listening to the click which 
the instantaneous slide made in striking home upon a stop, when it had flashed across, 
in front of the secondary magnifier. 
* Mr. Claek, -wlio undertook this task, is represented in the engraving with a lighted taper in his hand. 
