_Test for Lenses._--In applying the methods recommended in your last Number
for the purpose of testing lenses, there is one precaution absolutely
necessary to be taken, but which all your correspondents have omitted to
point out. The operator must take care that his _focussing-glass_ is placed
at precisely the same distance from the lens as the _collodionised_ glass
is. To insure this, my practice is to place a piece of ground glass in the
dark frame, which is afterwards to receive the collodionised glass, and to
obtain the focus of the lens on that; then to put in the proposed plate,
and obtain an impression as described by MR. SHADBOLT. In this way I secure
myself from what I believe is often a source of fallacy in these
experiments, and am sure that I give the lens a fair trial.