The spectra have been produced by placing in front of the telescope a large prism, thus returning to the method originally employed by Fraunhofer in the first study of stellar spectra.  Four 15 deg. prisms have been constructed, the three largest having clear apertures of nearly eleven inches, and the fourth being somewhat smaller.  The entire weight of these prisms exceeds a hundred pounds, and they fill a brass cubical box a foot on each side.  The spectrum of a star formed by this apparatus is extremely narrow when the telescope is driven by clockwork in the usual way.  A motion is accordingly given to the telescope slightly differing from that of the earth by means of a secondary clock controlling it electrically.  The spectrum is thus spread into a band, having a width proportional to the time of exposure and to the rate of the controlling clock.

This band is generally not uniformly dense.  It exhibits lines perpendicular to the refracting edge of the prism, such as are produced in the field of an ordinary spectroscope by particles of dust upon the slit.  In the present case, these lines may be due to variations in the transparency of the air during the time of exposure, or to instrumental causes, such as irregular running of the driving clock, or slight changes in the motion of the telescope, resulting from the manner in which its polar axis is supported.

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.