oublesome double reflections. In fact, it answers very respectably,
especially when we consider that perfection of definition is thrown
away on composites. I thought of a mirror silvered on the front of
the glass, but this would soon tarnish in the gaslight, so I did not
try it. For safety against the admission of light unintentionally, I
have a cap to the focusing-screen in the roof, and a slide in the
fixed body of the instrument immediately behind the reflector and
before the dark slide. Neither of these would be wanted if the
reflector was replaced by a prism, set into one end of a sliding
block that had a large horizontal hole at the other end, and a
sufficient length of solid wood between the two to block out the
passage of light both upwards and downwards whenever the block is
passing through the half-way position.

As regards the fiducial lines, they might be drawn on the glass
screen; but black lines are not, I find, the best. It is far easier
to work with illuminated lines; and it is important to be able to
control their brightness. I produce these lines by means of a
vertical transparency, set in an adjustable frame, connected with A,
and having a gas-light behind it. Below the eye-hole _e_, through
which I view the glass-screen _g_, is a thin piece of glass set at
an angle of 45 deg., which reflects the fiducial lines and gives them
the appearance of lying on the screen, the frame being so adjusted
that the distance from the thin piece of glass to the transparency
and to the glass-screen _g_ is the same. I thus obtain beautiful
fiducial lines, which I can vary from extreme faintness to extreme
brilliancy, by turning the gas lower or higher, according to the
brightness of the image of the portrait, which itself depends on the
density of the transparency that I am engaged upon. This arrangement
seems as good as can be. It affords a gauge of the density of the
negative, and enables me to regulate the burners behind it, until
the image of the portrait on _g_ is adjusted to a standard degree of
brightness.

For convenience in enlarging or reducing, I take care that the
intersection of the vertical fiducial line with that which passes
through the pupils of the eyes shall correspond to the optical axis
of the camera. Then, as I enlarge or reduce, that point in the image
remains fixed. The uppermost horizontal fiducial line continues to
intersect the pupils, and the vertical one continues to divide the
face symme