120 
ME. W. GEOFFREY DUFFIELD ON THE 
such a manner that as the carriage was moved the wire remained over the centre of 
the hole in which the pivot of the grating table turned. 
III. and IV. Similar adjustments were made for the camera girder. 
V. The line of motion of the centre of the camera carriage was adjusted 
perpendicular to the line of motion of the centre of the grating carriage. 
Kayser employed a theodolite to effect this, Ames the 3, 4, 5 method. The latter 
was adopted in the present case, and lengths of 4‘5 m. and 6 m. were measured 
along the wires from their point of intersection, and a third wire, upon which a 
length of 7*5 m. was marked, was stretched across them, and the angle between the 
two girders altered by means of the cradle and side screws, fig. 1, G and IT, until 
these marks coincided with those on the first two wires. 
VI. The wires were removed and the slit placed at their point of intersection. 
VII. An examination was made of the grating to ascertain on which side it threw 
the brighter spectrum of the second order; it was then placed in the grating holder, 
so that this spectrum should be employed. 
VIII. The vertical bearing for the grating, ACF, fig. 4, was fitted into its carriage, 
and the grating in its holder placed in the hole-slot-plane system designed to receive 
it on the plate F. The base of the grating holder was made approximately horizontal 
by means of a spirit-level and the levelling screws L. 
IX. The slit was moved vertically by turning the screw Y, fig. 7, untd it was in 
the line of motion of the centre of the ruled space of the grating. The slit was 
illuminated by an arc and the light reflected from the grating received upon white 
paper surrounding the slit. The grating was moved to and fro along its girder, and 
the height of the slit altered until the centre of the reflected image remained upon 
the centre of the slit for all positions of the grating. 
X. The vertical bearing for the camera, ACE, fig. 5, was fitted into its carriage 
and the camera fixed on it by means of the pin P. The spectrum was then adjusted 
horizontal and at the right height in the camera. 
A piece of ground glass was placed in the dark slide in the camera, and by means 
of a scribing-block a mark was made on it at the height of the centre of the slit 
above the camera girder. The slit was illuminated and the spectrum observed on 
the plate. It was necessary to alter the screws 1/ If', fig. 4, to bring the spectrum 
to the required position. The camera was then moved along the girder and, as a 
change in the level of the spectrum was noticed, the screw Lwas adjusted until the 
spectrum remained at the same height in the camera for all positions of the camera 
and grating on their girders. 
The above operations insured the lines of the grating being vertical. The accuracy 
with which the face of the grating table had been turned was tested by rotating the 
grating about its axis and observing the position of the reflected image of the slit in 
the camera as it was moved along its girder. The centre of this image neither rose 
nor fell, so the mounting was considered satisfactory. 
