lengthening the string, more or less rapid exposures may be obtained.

[Illustration: AAA, lens; B, aperture of lens; C, metallic disk; D,
wheel on the axis; E, cord or string; E¹E¹E¹E¹, knots in string; G, steel
spring; H, catch; K, socket for catch.]

* * * * *




SULPHUROUS ACID.--EASY METHOD OF PREPARATION FOR PHOTOGRAPHIC PURPOSES.


Within a short period sulphurous acid has become an important element in
the preparation of an excellent pyro developer for gelatine plates; and as
it is more or less unstable in its keeping qualities, some easy method of
preparing a small quantity which shall have a uniform strength is
desirable. A method recently described in the _Photographic News_ will
afford the amateur photographer a ready way of preparing a small quantity
of the acid.

[Illustration]

In the illustration given above, A and B are two bottles, both of which can
be closed tightly with corks. A hole is made in the cork in the bottle, A,
a little smaller than the glass tube which connects A and B. It is filed
out with a rat-tail file until it is large enough to admit the tube very
tightly. The tube may be bent easily, by being heated over a common
fish-tail gas burner or over the top of the chimney of a kerosene lamp, so