These instrumental defects may be too small to be detected in ordinary
micrometric or photographic observations, and still sufficient to affect
the photographs just described.

A method of enlargement has been tried which gives very satisfactory
results, and removes the lines above mentioned as defects in the
negatives. A cylindrical lens is placed close to the enlarging lens,
with its axis parallel to the length of the spectrum. In the apparatus
actually employed, the length of the spectrum, and with it the
dispersion, is increased five times, while the breadth is made in all
cases about four inches. The advantage of this arrangement is that it
greatly reduces the difficulty arising from the feeble light of the
star. Until very lately, the spectra in the original negatives were made
very narrow, since otherwise the intensity of the starlight would have
been insufficient to produce the proper decomposition of the silver
particles. The enlargement being made by daylight, the vast amount of
energy then available is controlled by the original negative, the action
of which may be compared to that of a telegraphic relay. The copies
therefore represent many hundred times the original energy received from
the stars. If care is not taken, the dust and irregularities of the film
will give trouble, each foreign particle appearing as a fine spectral
line.

Our methods of enlargement have been considered, and some of them tried,
with the object of removing the irregularities of the original spectra
without introducing new defects. For instance, the sensitive plate may
be moved during the enlargement in the direction of the spectral lines;
a slit parallel to the lines may be used as the source of light, and the
original negative separated by a small interval from the plate used for
the copy; or two cylindrical lenses may be used, with their axes