gh the quantity at each operation may be small), because
it causes a decomposition of the salt, setting free _sulphurous_ acid, and
also sulphur; which last is slightly soluble in the hyposulphite of soda,
and thus the sulphur is brought in contact with the reduced silver, and
forms a sulphuret of that metal. But the change does not stop here: for, by
the lapse of time, oxygen is absorbed, and thus a _sulphate of silver_ is
formed, and the colour changed from black to white. That sulphur is set
free by the addition of an acid to the solution of hyposulphite of soda, is
fact so easily demonstrable both to the eyes and nose of the operator, that
no one need remain long in doubt who is desirous of trying the experiment.

A correspondent desires to know how to test the coincidence or otherwise of
the visual and actinic foci of a combination: this is very readily
accomplished by the aid of a _focimeter_, which can be easily made thus:

Procure a piece of stout card-board, or thin wood covered with white paper,
on which draw a considerable number of fine black lines, or cover it with
some fine black net (what I believe the ladies call _blond_), which may be
pasted on. Cut up the whole into a dozen good-sized pieces of any
convenient form, so that about four square inches of surface at least be
allowed to each piece. Paste over the _net_ a circular or square label
about the size of a shilling, bearing a distinctly printed number one on
each piece, from 1 upwards; and arrange the pieces in any convenient manner
by means of wires inserted into a slip of wood; but they must be so placed
that the _whole_ can be seen from one point of view, although each piece
must be placed so that it is _one inch_ farther from the operator than the
next lowest number. Having placed the camera eight or ten feet from the
cards, carefully focus to any one of the numbers, 4 or 5 for instance and
observe, not that the _number_ is distinct, but that the minute lines or
threads of the net are visible: then take a picture, exposing it a very
short time, and the threads of the card bearing the number that was most
perfectly in focus visually _ought_ to be most distinct; but, if otherwise,
that which is most distinct will not only show whether the lens is over or
under corrected, but will indicate the _amount_ of error. If under
corrected, a lower number will be most distinct; if over corrected, a
higher.

GEO. SHADBOLT.

_Test for Lenses._--I beg to sub