[PLASKETT-DELURY] DETERMINATION OF THE SOLAR ROTATION 111 
rim 3 in. X 11 in. X 14 in. over which the wooden box 0 is tightly 
screwed and clamped. The axle of B, which is of cast-iron works in 
the cylindrical brass bearing B’. The sides of B project 3 in. on the 
top and 16 in. on the sides and bottom, the latter being milled smooth 
to give easy bearings for the grating and lens mountings. 
The rim of A is toothed and into these teeth fits a gear attached to 
the support A’; by turning this gear the spectrograph may be rotated. 
The circular face of A is graduated in degrees and by means of a vernier 
attached to A’, the angle may be read to tenths of a degree. This is 
necessary in determining the “East and West” line by allowing the 
image of the sun to drift across the face A tangentially to some arbitrary 
line on A. From this angle read on the vernier, the position of the 
sun’s dise, which lies in the plane of the sun’s equator, is easily found 
since the inclination of these two lines to each other at any time is 
known, and hence the arbitrary line on A may be made parallel to any 
required diameter of the sun’s image. 
The wooden box 0, which is strengthened by truss rods, is painted 
black on the inside and is provided with diaphragms, M, to prevent 
as much as possible the diffused light reflected from the lens and grating 
from striking the photographic plate in the holder, C. There is a hinged 
door just above the grating and lens, so that these may be conveniently 
reached. 
The box, 0, is lined with felt to prevent sudden changes of tempera- 
ture, and the end B is doubly boxed in order to prevent convection 
currents from passing in front of the grating and destroying the defini- 
tion. Doors D’ and D” are placed in the outer box. 
The system of reflecting prisms used for reflecting the light from 
opposite ends of a diameter of the dise through to the slit (as mentioned 
above), is arranged as shown in Fig. 2. The prisms A. A’ and B, B’ 
are mounted on brass holders on a solid casting which holds at its centre 
the slit with its adjusting micrometer and closing shutter, the whole 
being mounted on the face of the spectrograph. Light from one limb 
of the sun enters the reflecting prism A and is reflected through A’, which 
in turn reflects the light through slit,S. In a similar manner, B reflects 
the light from the opposite limb through B’, and this prism directs it 
through the slit. A’ is notched as shown in the upper part of Fig. 2 
to receive the end of B.’ In this way two beams of light are reflected 
to the grating through B’ with a beam from A’ placed midway between 
them and, consequently 3 strips of spectra are diffracted back to C as 
shown in Fig. 3. By means of a small three-tongued mask the widths 
of these strips and the spacing are kept constant for purposes of measur- 
ing. The prisms are adjusted to give even illumination of the grating 
