674 
DR. W. HUGGINS ON THE PHOTOGRAPHIC SPECTRA OE STARS. 
negatives were enlarged, but it was found that a more satisfactory determination of 
doubtful points could be made from the original small negative or the positive taken 
from it when viewed under a microscope of low power than from an enlarged copy. 
In the negatives the dark lines are represented by transparent spaces where the 
light has not acted. When these spaces are rather broad there may be some 
uncertainty in placing the wire of the micrometer exactly upon the middle of the 
transparent space. In the positives these spaces become dark lines, in which the 
middle part is usually the darkest. In nearly all cases, therefore, it was found 
desirable to confirm the measures of the lines made on the negatives by measures 
of the same lines in the positives taken from them. 
In some cases the negatives were intensified by the usual methods, but they were 
not varnished. 
§ V. Spectra for Comparison. 
It has been stated that one of the reasons for using a slit was that spectra for com¬ 
parison might be taken on the same plate. 
The slit is furnished with two sliding shutters (i and h, fig. 1), each of which closes 
one-half of the length of the slit. When a star was photographed, one-half only of 
the slit was in use, the other half being closed by its shutter. 
If the moon or one of the brighter planets was situated conveniently for the purpose, 
the shutter which had remained closed was withdrawn, and the other shutter pushed 
in over the half of the slit which had been in use for the star. The telescope was 
then directed to the moon or planet. In this way the star’s spectrum was obtained, 
together with that of solar light from the moon or planet. 
If this method was not available, after exposure to the star’s light the shutter was 
closed, and the apparatus with both shutters pushed in was allowed to remain until 
the next day, when by means of a small hand mirror, direct sun light or light from 
the sky, was reflected through the hole in the large speculum, so as to fall upon the slit 
in the direction of the axis of the collimator. 
More recently advantage was taken of those stars, the spectra of which had been 
compared with solar light. A spectrum of one of these stars was taken through the 
second half of the slit to serve as a fiducial spectrum of comparison. This method has 
the advantage of permitting the development of the plate to be performed the same 
evening. 
The spectra of the planets were obtained on the same plate with the lunar or solar 
spectrum. When, however, circumstances permitted, the plan employed by Dr. 
Miller and myself in our earlier eye observations was preferentially adopted. We 
wrote in 1864 :— 
“ The length of the opening of the slit is much greater than the diameter of the 
telescopic image of the planet (Jupiter) even after elongation by the cylindrical lens. 
If therefore at the time of observation the light from the sky is sufficiently intense to 
