EFFECT OF PRESSURE UPON ARC SPECTRA. 
117 
Three levelling screws, L, on the lower j^late fit into a hole-slot-plane system on the 
top of the grating table F, and by their means the height of the grating, its rotation 
about its normal through its centre and about a horizontal axis in its plane can be 
regulated. A divided head attached to these screws allows a record to be kept of 
their correct positions. 
When not in use, the grating is protected by a glass plate which slides in vertical 
grooves in front of the ruled surface—this is replaced at other times by a black mask 
to cover the unruled portion of the mirror. 
The Camera. (Fig. 5.) 
The mounting of the camera box differs from that of the grating in having a 
rectangular metal plate, E, with grooves in which the base of the camera can slide, 
instead of the circular plate, F, to take the grating holder. The base of the camera is 
of brass and is capable of rotation about the pivot P in the sliding plate H, which is 
exactly below the centre of the photographic plate when that is in position in its 
dark slide.* The position of the pivot with reference to the camera carriage can be 
regulated by means of the screw K, which moves the sliding plate in its grooves. 
M is a fin© adjustment for turning the photographic plate about the pivot P, and 
when the correct angular position has been found the base plate may be fixed in 
position by means of the clamp N. M and N are not shown in elevation. 
The dark slide moves in vertical grooves on the camera, so that several photographs 
may be taken upon the same plate. The plate, which is usually 20 inches by 
2h inches, is pressed against rubber stops, which impart to it the necessary 
curvature. 
The Comjparison-sliutter. (Fig. 6.) 
This is now mounted apart from the camera box and is separately supported upon 
the grating girder to obviate any displacement of the spectra on the plate due to the 
operation of the shutter.* In the figure, Pi is a wooden block, heavily weighted with 
lead, to which is clamped the wooden grooved arm U, whose end carries a short length 
* The writer is responsible for these and a few other minor modifications of the original design. 
