RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 49 
the hour-angle 0 of the index-hand, must be cut a very narrow longitudinal slit, com- 
mencing at the closed end, and extending down somewhat less than two thirds the 
length of the cylinder, to admit the sunbeam. And it is evident that if this slit be 
so placed at any moment of the day that the sunbeam admitted through it shall strike 
the (imaginary) axis of the cylinder, it will do so all day ; and that this will take 
place in the arrangement proposed, if the watch be set to show the correct hour- 
angle. 
123. But in place of freely traversing the hollow of the cylinder, the ray is to be 
received, immediately on entering the slit, on a band of sensitive paper rolled round 
an interior fixed wooden cylinder, concentric with the other, and barely leaving room 
for the exterior revolving cylinder to escape contact in every part. This is easily ac- 
complished by forming this interior cylinder of solid wood, bored along its axis, and 
causing it to travel smoothly along a cylindrical mandril of sufficient strength and 
solidity of support at its lower end to keep it always well in its true polar direction. 
For convenience of taking off the old and adapting new papers, this mandril may be 
furnished with a hinge at its lower end, which, however, must be very strong and of 
good workmanship ; so that being turned back to a proper angle, the wooden cylinder 
it carries may be enabled to slide entirely off it at its upper end, and in so doing clear 
the lower rim of the hollow cylinder. Supposing the mandril in this position, the 
paper fixed round the upper end of the wooden cylinder, the latter replaced on the 
mandril and thrust down far enough to clear as above mentioned; the mandril must 
be then restored to its polar direction by turning on its hinge till it abuts against a 
fixed obstacle, and the wooden cylinder with its paper must then be slided up it till 
its upper end shall be very near to, but not in contact with the close upper end of 
the hollow revolving cylinder : and in this situation it is to be pinned or clamped fast. 
To prevent admission of light between the hollow exterior and wooden interior cy- 
linders, the latter should be formed with a projecting shoulder and metal cylindrical 
guard-rim, which shall in its turn, when slided up as aforesaid, externally surround 
the lower part of the hollow cylinder, coming up, all round it, almost to the lower 
end of the slit which admits the sunbeam. A glance at Plate I. will explain what- 
ever may appear obscure in the description of this mechanism. The others require no 
illustration. I ought to state that it is the second only of these contrivances that I 
have actually executed. 
MDCCCXL. 
H 
