V4 The Total Solar Eclipse of 1900. 
The mechanism by which these motions are effected can best be understood 
by reference to figs. 8, 4, 5, and the accompanying description; and the 
chart, fig. 2, will show very clearly the effect of the arrangement. This chart 
is drawn out on Babbage’s system of notation, representing horizontally a single 
operation corresponding to two turns of the handle, which handle is turned 
uniformly. 
It will be observed that during the exposure of the plate the drum on which 
it is fixed is stationary, but the other drum is in the act of being turned round, 
so as to have a fresh plate ready for the next exposure on the other side. It 
Exposure of right-hand ) Exposed, 
plate, Shut off, 
Exposure of left-hand ) Shut off, 
plate, Exposed, 
Right drum advances 4th of a turn, 
Left drum advances 4th of a turn, 
Pawl of right-hand drum YAN 
to take up next tooth, 
Pawl of left-hand drum retreats 
to take up next tooth, ——— 
Fig. 2. 
will also be observed that there is a lapping between the exposures of the two 
plates, that is to say, the plate at the right-hand side has a portion of the spec- 
trum thrown upon it before the image is cut off from the left. 
The results obtained appear to show that an instrument of this sort would 
be valuable on future occasions, but that it would be necessary that it 
should be made stronger, heavier, and more stable than that used at Plasencia. 
This latter instrument was made up hastily, just before starting for Spain, and 
made mostly of wood, the result being that, although it was loaded with heavy 
stones during the operation, some of the photographs suffered. considerably from 
want of perfect steadiness. 
