G84 
PROFESSOR J. K LOCKYER ON THE PHOTOGRAPHIC 
show irregularities resembling the lines along the spectrum observed when the sht of 
a spectroscope is partly clogged with dust. It has been noticed that the period of the 
irregularities is equal to the time of revolution of the main driving screw of the tele¬ 
scope, and hence they may be accounted for by supposing the driving gear to be 
mechanically imperfect. In that case some of the parallel lines which, by their juxta¬ 
position form the broadened spectrum, are supeiqDosed, while others are drawn apart, 
thus giving rise to dark and bright lines parallel to the length of the spectrum. 
These lines are more apparent in the case of bright stars than fainter ones. If the 
telescope were driven with perfect regularity and the atmosphere were quite steady, 
we should obtain a spectrum of uniform intensity along its width. This condition 
has very nearly been obtained in some cases. 
The irregularities above described are eliminated in the enlarged negatives by 
giving them an up-and-down motion during exposure in a direction parallel to the 
lines of the spectrum. This was originally done by hand, but a negative holder has 
been constructed in which the necessary motion is given to the negative by a small 
driving clock. 
Fig. 4. 
Negative Folder used in eulargiug. 
A diagram of the arrangement is given above. The only drawback to this method 
is that defects of the film are apt to produce, by a succession of their images on the 
enlarging plate, lines (generally very faint) which have a semblance of the tiaie 
spectrum lines. 
To distinguish the real lines from the artificial ones, a direct enlargement of the 
spectrum is made on the same plate alongside the other, the to-and-fro motion being 
dispensed with. By a comparison of the two enlarged strips, one can see at a glance 
vBich are the true lines of the spectrum, and which are those produced by small 
irregularities on the film. It may be stated that Dr. Scheiner has also used a some¬ 
what similar method to the one described, the only difterence being that he caused the 
plate on which the enlargement was to be taken to have the oscillating motion, instead 
