680 
PROFESSOR J. FT. LOCKYER ON THE PHOTOGRAPHIC 
fitted with a finder inclined at an angle to correct for the deviation of the prism. 
The spectra taken with this instrument are only 0'3 inch long from F to K, and the 
time of exposure is reduced in proportion. Even a fifth magnitude star may be 
photographed in 30 minutes. 
Fig. 2. 
Objective pi’isms of 7-|° eacb attached to lO-iuch object-glass. 
The Clock Rate. 
The proper regulation of the clock error and consequent “trail” of the spectrum 
across the plate parallel to itself are essential to the success of photographs taken by 
the objective prisms. The spectrum of a bright star must obviously be made to 
trail more quickly than that of a fainter one, and a shorter exposure is sufiicient. 
Since for the same clock error, and in the same time, a star near the pole will give a 
shorter trail than one nearer the equator, declination must also be taken into account. 
Keeping a constant clock error, equal widths of spectrum for stars of different 
declinations may be obtained by lengthening the time of exposure for stars away 
from the equator, but in that case, the stars near the pole would be over-exposed in 
relation to those nearer the equator. 
The exposure given to stars of equal magnitudes should evidently be the same, no 
matter in what parr of the sky they may he situated, and the clock error should, 
therefore, be increased in proportion to the secant of the angle of declination. 
The light-ratio of stars being 2'512't where n expresses the difference in magnitude, 
the time of exposure must vary in the same proportion, and the clock error in inverse 
