[We have submitted this Query to DR. DIAMOND, who informs us that he never adopts the practice of washing off the developing fluid, and considers it not only needless, but sometimes prejudicial, as when such washing has not been resorted to, the hyposulphite solution flows more readily over the picture, and causes none of the unpleasant stains which frequently occur in pictures which have been previously washed, especially if hard water has been used. But besides this, and the saving of time, the doing away with this unnecessary washing economises water, which in out-door practice is often a great consideration. DR. DIAMOND would again impress upon our readers the advantage of using 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.