the
hyposulphite over and over again, merely keeping up its full strength
by the addition of fresh crystals of the salt from time to time, as
such practice produces pictures of whiter and softer tone than are ever
produced by the raw solution.]

_Colouring Collodion Pictures_ (Vol. vii., p. 388.)--A patent has just been
taken out (dated September 23, 1852) for this purpose, by Mons. J. L.
Tardieu, of Paris. He terms his process _tardiochromy_. It consists in
applying oil or other colours at the back of the pictures, so as to give
the requisite tints to the several parts of the photograph, without at all
interfering with its extreme delicacy. It may even, in some cases, be used
to remedy defects in the photographic picture. The claim is essentially for
the application of colours at the back, instead of on the surface of
photographs, whatever kind of colours may be used. It is therefore, of
course, applicable only to photographs taken on paper, glass, or some
transparent material.

A. C. WILSON.

_Wanted, a simple Test for a good Lens._--As all writers on Photography
agree that the first great essential for successful practice is a good
lens--that is to say, a lens of which the visual and chemical foci
coincide--can any of the scientific readers of "N. & Q." point out any
simple test by which unscientific parties desirous of practising
photography may be enabled to judge of the goodness of a lens? A country
gentleman, like myself, may purchase a lens from an eminent house, with an
assurance that it is everything that can be desired (and I am _not_ putting
an imaginary case), and may succeed in getting beautiful images upon his
focussing-glass, but very unsatisfactory pictures; and it may not be until
he has almost abandoned photography, in despair at his own want of skill,
that he has the opportunity of showing his apparatus, manipulation, &c. to
some more practised hand, who is enabled to prove that _the lens was not
capable_ of doing what the vendors stated it could do. Surely scientific
men must know of a simple test which would save the disappointment I have
described; and I hope some one will take pity upon me, and send it to "N. &
Q.," for the benefit of myself and every other

COUNTRY PRACTITIONER.

_Photographic Tent--Restoration of Faded Negatives._--In Vol. vii., p.
462., I find M. F. M. inquiring for a cheap and portable tent, effective
for photographic operations out of doors. I have for t