116 
MR. W. GEOFFREY DUFFIELD ON THE 
upon which rests either the grating holder or the camera box. The holes to receive 
the ends of the cross-beam are at opposite extremities of a diameter of the lower 
circular plate, and this provides that the distribution of the weight of the cross-beam 
upon the carriage is not altered when the cross beam is moved. 
Tlie Grating Holder. (Fig. 4.) 
The grating itself is held in a metal box, the front of which has been cut away, and 
against this frame it is gently pressed by light springs, SS. The whole is mounted 
upon a circular brass plate 7|- inches diameter, with a f-inch hole drilled at its 
centre, through which passes a short pivot fixed to a cast-iron plate, shaded in the 
Fig. 4. The grating holder and grating 
table. 
Fig. 5. The camera and its mounting. 
diagram, to which the base plate can then be clamped—a fine adjustment, G, con¬ 
sisting of a flange attached to the upper plate held between two screws attached to 
the lower plate, allowing the correct angular position of the grating about its vertical 
axis to be accurately determined. 
