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camera revolving be made to carry with it a strait rail at the 
back, the plateholder has only to traverse this in a right line 
to give the due motion required. 
By forming a groove upon the edge of the circular base 
plate upon which the camera rotates, and turning this to the 
proper depth, the due rectilinear motion of the plate behind 
the lens may be given by means of two strings or wires, one 
end of each of which is attached to the disc or base plate 
and the other to the carriage containing the plateholder, so 
that if we remove the clockwork by releasing the spring which 
holds it up to the inner edge of the circular base, we can 
obtain all the necessary motions for working the camera with- 
out either wheel or pinion or any mechanism whatever unless 
the wires and disc and the five friction rollers upon which the 
apparatus moves can be called such. Views may be taken 
with the instrument in this state, but the accuracy necessary 
to produce an equal tint, particularly in the skies, is unattain- 
able except by clockwork. The latter is extremely simple. 
It is a small clock movement moved by a spring, chain, and 
fusee, and is regulated by a fly or series of flys, the blades of 
which may be set at any angle from the horizontal to the 
vertical. It drives the instrument by means of a small steel 
pulley gearing into the milled edge of the circular base by a 
process which may be called a compromise between frictional 
and toothed gearing, and which has advantages that neither 
of these possesses singly. 
In my rude attempts to illustrate the principle of the rotating 
camera I said that the ray of light passing through the lens 
must be limited. The reason of this is obvious. The true 
surface for receiving the image is a cylinder, but that substi- 
tuted in the flat plate camera is a plane surface. 
Now, the smaller the angular aperture of the narrow strip 
of light employed, the nearer the tangent approaches the 
curve. 
