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CAPTAIN ABNEY AND MAJOR-GENERAL FESTING 
was placed slightly in rear of the bottom carbon, so that the crater, which was used as 
the source of light, might as nearly as possible face the apparatus. We used the cored 
carbons, as introduced by Siemens Brothers for use in their lamps. A. Gramme M 
dynamo which one of us has had in constant use in his laboratory for the last five years, 
and which is driven by a three-cylinder Brotherhood engine coupled to the axle, 
supplied the current. This machine has worked without a hitch since it was first set up, 
and for such work cannot be too highly commended. The current is remarkably constant, 
its strength being 11 amperes, with an electromotive force of 42^ volts at the terminals 
of the lamp, when the armature revolves at 1400 turns a minute. 
Fig. 2. 
§ V. The Spectroscopic Apparatus. 
The spectroscopic apparatus has already been described. It may, however, be 
remarked, that the collimating lens was filled with the light by means of a white 
flint-glass condenser, which formed the image of the centre of the crater on the slit. 
Of the parallel rays from the collimator a certain proportion are necessarily reflected 
from the first surface of the prism. These being received on a screen about three feet 
distant, and visible to the operator of the lamp, enabled him to see at a glance if the 
collimator lens was properly filled with light from the crater, and to readjust the lamp 
when necessary. After having passed through the second prism and the camera lens, 
the spectrum was accurately focussed on the focussing screen, and a card, as before 
stated, inserted in the slide grooves. The slit in this card was ‘05 inch wide, and on 
the edge of the card was a scale of 25 divisions to an inch, and read to quarters of a 
division. [It may be noted that the scale shown in the subsequent pages is converted 
