_Washing Collodion Pictures--Test for Lens._--As I was indebted to the kindness of DR. DIAMOND, amongst other friends, for my original initiation into the mysteries of photography, it may appear somewhat presumptuous in me to differ from one who has had so much more experience in a point of practice. I allude to that of _washing_ the collodion negative after developing, previously to fixing with the hyposulphite of soda; but, probably, the reasons I urge may have some weight. As the hyposulphite solution is intended to be used repeatedly, it appears to me not advisable to introduce into it _any free acid_ (which must occur if the negative be not washed, although 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: