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MR. W. GEOFFREY DUFFIELD ON THE 
of -g-inch brass tubing, W, into which fits a second tube of slightly smaller diameter, 
fixed to one end of the shutter, whose height is then capable of adjustment by sliding 
the tube up or down. It is then Held in position by means of a set-screw. 
The shutter itself is of the ordinary pattern, consisting in this case of' two steel 
rules •§- inch apart, soldered into end-pieces which are shaped to form the pivots about 
which it may be turned. It is placed as close as possible to the photographic plate; 
in fig. 6 for clearness it is shown some distance away. To screen the plate from 
diffused light, a light wooden box is supported on the cross-beam, and dark cloths 
attached to its sides are drawn over the camera box when the dark slide is in its 
place. 
The Slit. (Fig. 7.) 
This was designed by Sir Howard Grubb and made at the works of the Cambridge 
Scientific Instrument Company. 
The requirements of a slit for use with a concave grating are that it be movable 
both along, and perpendicular to, the camera girder, that its height be adjustable, 
and that it be capable of rotation about its centre in its own plane. 
The base of the mounting is grooved to rest evenly across the rails of the camera 
girder along which it may be moved, a scale and pointer indicating its position. 
Upon this base a plate carrying the slit slides in grooves, and the motion is controlled 
by a rack and pinion. The height of the slit is regulated by the screw Y, which 
passes through the top of the frame in which the slit itself slides. The jaws are held 
