98 CELESTIAL SPECTROSCOPY. 
astronomer to accomplish in the short span of his own life, and so 
enter into their fruition, great works which otherwise must have been 
passed on by him as a heritage of labor to succeeding generations. 
The second great service which photography renders is not simply 
an aid to the powers the astronomer already possesses. On the con- 
trary, the plate, by recording light-waves which are both too small 
and too large to excite vision in the eye, brings him into a new region 
of knowledge, such as the infra-red and the ultra-violet parts of the 
spectrum, which must have remained forever unknown but for artificial 
help. 
The present year will be memorable in astronomical history for the 
practical beginning of the Photographie Chart and Catalogue of the 
Heavens, which took their origin in an international conference which 
met in Paris in 1887, by the invitation of M. ?Amiral Mouchez, director 
of the Paris Observatory. 
The richness in stars down to the ninth magnitude of the photo- 
graphs of the comet of 1882 taken at the Cape Observatory under the 
superintendence of Dr. Gill, and the remarkable star charts of the 
Brothers Henry which followed two years later, astonished the astro- 
nomical world. The great excellence of these photographs, which was 
due mainly to the superiority of the gelatine plate, suggested to these 
astronomers a complete map of the sky, and a little later gave birth 
in the minds of the Paris astronomers to the grand enterprise of an 
International Chart of the Heavens. The actual beginning of the 
work this year is in no small degree due to the great energy and tact 
with which the director of the Paris Observatory has conducted the 
initial steps, through the many delicate and difficult questions which 
have unavoidably presented themselves in an undertaking which de- 
pends upon the harmonious working in common of many nationalities, 
and of no fewer than eighteen observatories in all parts of the world. 
The three years since 1887 have not been too long for the detailed or- 
ganization of this work, which has called for several elaborate prelim- 
inary investigations on special points in which our knowledge was in- 
sufficient, and which have been ably carried out by Profs. Vogel and 
Bakhuyzen, Dr. Trépied, Dr. Scheiner, Dr. Gill, the astronomer-royal, 
and others. Time also was required for the construction of the new 
and special instruments. 
The decisions of the conference in their final form provide for the 
construction of a great photographie chart of the heavens with expo- 
sures corresponding to forty minutes’ exposure at Paris, which it is 
exp. cted will reach down to stars of about the fourteenth magnitude. 
As each plate is to be limited to 4 square degrees, and as each star, to 
avoid possible errors, is to appear on two plates, over 22,000 photo- 
graphs will be required. For the more accurate determination of the 
positions of the stars, a réseau with lines at distances of 5 millimeters 
apart is to be previously impressed by a faint light upon the plate, so 
