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MR. W. GEOFFREY DUFFIELD ON THE 
Without the lens K, the direction of the beam of light passing through the window 
would change as the arc shifted its position, and it would be necessary to alter the angles 
of the mirrors and the position of the lens L in order to keep the image in focus on the 
slit, but the lens K has been arranged to move up or down just as much as the arc 
moves up or down, by means of the system of pulleys shown in the diagram, which 
ensures the vertical motion of the lens being equal to the horizontal motion of the 
mirror M; when M moves towards the arc, L is pulled up, and vice versa. The ray 
passing horizontally through the window thus always proceeds through the centres of 
the second lens, the slit and the grating, and all danger of a fictitious shift due to 
want of centrality is obviated. When the arc fluctuates horizontally round the poles 
the image is brought back upon the slit by turning the mirror M about its pivots; 
the motion required is very small and the adjustment can be made rapidly enough for 
an experienced operator to keep the image of the arc almost continually in position. 
Since the arc and slit are in the focal planes of their respective lenses the image is 
always in good focus. 
The turning of the mirror about its long axis is effected by the screw F, which 
presses against a projection T from the back of the frame holding the mirror, a spring- 
keeping F and T in close contact. Universal joints were attached to the ends of E 
and F, and long handies provided, fig. 2, Plate 3, so that the assistant whose duty 
it was to keep the image on the slit was not too close to the window. 
This mirror system was designed by the writer and constructed by Mr. Griffiths 
in the workshop of the Physical Laboratory. To it the success in photographing the 
spectrum of the arc under high pressures is largely due. 
5. General Arrangement of the Apparatus . 
Plate 3, fig. 2, shows the cylinder in position in front of the Howland grating 
spectroscope, the slit of which is to be seen at the right of this photograph standing- 
on the cast-iron girder along which the camera carriage runs. The cylinder is mounted 
on a Swinging jib turning on the vertical axis at the left of the picture, so that it can 
be moved out of position when the grating is required for other work. A screw-jack 
at the base of the jib allows the cylinder a vertical range of adjustment of about 
6 inches. 
On a level with the window is the lens which slides in vertical grooves, and beyond 
it the arrangement of mirrors is to be seen. These were operated by an assistant 
who stood at some distance from the apparatus and worked them by means of the 
two wooden handles. The second lens is held in the clamp of the retort stand and 
was accurately adjusted in line with the centres of the slit and the grating. 
The ropes for operating the mechanical feeding of the arc were not in position 
when the photograph was taken, nor is the auxiliary shutter shown; this latter was 
fixed beyond the slit, so that its operation caused no jarring to the mounting of the 
grating. 
