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SCIENCE 



[Vol. XVIII. No. 456 



relative positions of hundreds or even of thousands of stars, 

 or the minutest features of nebulae or other objects, or the 

 phenomena of a passing eclipse, a task which by means of 

 the eye and hand could only be accomplished, if done at all, 

 after a very great expenditure of time and labor. Photogra- 

 phy puts it in the power of the 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 contrary, 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 his- 

 tory for the practical beginning of the photographic 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. I'Amiral Mouchez, director of the Paris 

 Observatory. 



The richness in stars down to the ninth magnitude of the 

 photographs of the comet of 1883 taken at the Cape Obser- 

 vatory under the superintendence of Dr. Gill, and the re- 

 markable star charts of the Brothers Henry which followed 

 two years later, astonished the astronomical 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 diiBcult questions which have una- 

 voidably presented themselves in an undertaking which 

 depends 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 organization of this work, 

 which has called for several elaborate preliminary investiga- 

 tions on special points in which our knowledge was insuffi- 

 cient, and which have been ably carried out by Professors 

 Vogel and BakhUyzen, Dr. Trepied, Dr. Scheiner, Dr. 

 Gill, the astronomer royal, and otliers. Time also was re- 

 quired for the construction of the new and special instru- 

 ments. 



The decisions of the conference in their final form provide 

 for the construction of a great photographic chart of the 

 heavens with exposures corresponding to forty minutes' ex- 

 posure at Paris, which it is expected will reach down to stars 

 of about the fourteenth magnitude. As each plate is to be 

 limited to four square degrees, and as each star, to avoid 

 possible errors, is to appear on two plates, over twenty-two 

 thousand photographs will be required. For the more ac- 

 curate determination of the positions of the stars, a reseau 

 7;ith lines at distances of iive millimetres apart is to be pre- 

 viously impressed by a faint light upon the plate, so that the 

 image of the reseau will appear together with the images of 

 the stars when the plate is developed. This great work will 

 be divided, according to their latitudes, among eighteen ob- 

 servatories provided with similar instruments, though not 

 necessarily constructed by the same maker. Those in the 



British dominions and at Tacubaya have been constructed by 

 Sir Howard Grubb. 



Besides the plates to form the great chart, a second set of 

 plates for a catalogue is to be taken, with a shorter exposure, 

 which will give stars to the eleventh magnitude only. These 

 plates, by a recent decision of the permanent committee, are 

 to be pushed on as actively as possible, though as far as may 

 be practicable plates for the chart are to be taken concur- 

 rently. Photographing the plates for the catalogue is but 

 the first step in this work, and only supplies the data for the 

 elaborate measurements which have to be made, which are, 

 however, less laborious than would be required for a similar 

 catalogue without the aid of photography. 



Already Dr. Gill has nearly brought to conclusion, with the 

 assistance of Professor Kapteyn, a preliminary photographic 

 survey of the southern heavens. 



With an exposure sufficiently long for the faintest stars to 

 impress themselves upon the plate, the accumulating action 

 still goes on for the brighter stars, producing a great enlarge- 

 ment of their images from optical and photographic causes. 

 The question has occupied the attention of many astronomers, 

 whether it is possible to find a law connecting the diameters 

 of these more or less over-exposed images with the relative 

 brightness of the stars themselves. The answer will come 

 out undoubtedly in the affirmative, though at present the 

 empirical formulae which have been suggested for this pur- 

 pose differ from each other. Captain Abney proposes to 

 measure the total photographic action, including density as 

 well as size, by the obstruction which the stellar image offers 

 to light 



A further question follows as to the relation which the 

 photographic magnitudes of stars bear to those determined 

 by eye. "Visual magnitudes are the physiological expression 

 of the eye's integration of that part of the star's light which 

 extends from the red to the blue. Photographic magnitudes 

 represent the plate's integration of another part of the star's 

 light — namely, from a little below where the power of the 

 eye leaves off in the blue to where the light is cut off by 

 the glass, or is greatly reduced by want of proper corrections 

 when a refracting telescope is used. It is obvious that the 

 two records are taken by different methods in dissimilar units 

 of different parts of the star's light. In the case of certain 

 colored stars the photographic brightness is very different 

 from the visual brightness; but in all stars, changes, espe- 

 cially of a temporary character, may occur in the photo- 

 graphic or the visual region, unaccompanied by a similar 

 change in the other part of the spectrum. For these reasons 

 it would seem desirable that the two sets of magnitudes should 

 be tabulated independently, and be regarded as supplementary 

 of each other. 



The determination of the distances of the fixed stars from 

 the small apparent shift of their positions when viewed from 

 widely separated positions of the earth in its orbit is one of 

 the most refined operations of the observatory. The great 

 precision with which this minute angular quantity — a frac- 

 tion of a second only — has to be measured, is so delicate an 

 operation with the ordinary micrometer, though, indeed, it 

 was with this instrument that the classical observations of 

 Sir Robert Ball were made, that a special instrument, in 

 which the measui'es were made by moving the two halves of 

 a divided object-glass, known as a heliometer, has been pressed 

 into this service, and quite recently, in the skilful hands of 

 Dr. Gill and Dr. Elkin, has largely increased our knowledge 

 in this direction. 



It is ob-vious that photography might he hers of great ser- 



