August 20, 1891 



NATURE 



381 



it can be placed in any position ; it can be exposed for hours ; 

 lastly, it does not need immediate development, and for this 

 reason can be exposed again to the same object on succeeding 

 nights, so as to make up by several instalments, as the weather 

 may permit, the total time of exposure which is deemed 

 necessary. 



Without the assistance of photography, however greatly the 

 resources of genius might overcome the optical and mechanical 

 difRculties of constructing large telescopes, the astronomer would 

 have to depend in the last resource upon his eye. Now we can- 

 not by the force of continued looking bring into view an object 

 too feebly luminous to be seen at the first and keenest moment 

 of vision. But the feeblest light which falls upon the plate is 

 not lost, but is taken in and stored up continuously. Each hour 

 the plate gathers up 3600 times the light-energy which it received 

 during the first second. It is by this power of accumulation that 

 the photographic plate may be said to increase, almost without 

 limit, though not in separating power, the optical means at the 

 disposal of the astronomer for the discovery or the observation 

 of faint objects. 



Two principal directions may be pointed out in whichiphoto- 

 graphy is of great service to the astronomer. It enables him 

 within the comparatively short time of a single exposure to 

 secure permanently with great exactness the relative positions of 

 hundreds or even of thousands of stars, or the minute 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 ac- 

 complished, if done at all, after a very great expenditure of time 

 and labour. Photography 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 labour to succeeding genera- 

 tions. 



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 

 for ever unknown but for artificial help. 



The present year will be memorable in astronomical history 

 for the practical beginning of the Photographic Chart and 

 Catalogue of the Heavens, which took their origin in an Inter- 

 national Conference which met in Paris in 1887, by the invita- 

 tion 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 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 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 Inter- 

 national 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 pre- 

 sented 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 de- 

 tailed organization of this work, which has called for several 

 elaborate preliminary investi>?ations on special points in which 

 our knowledge was insufficient, and which have been ably 

 carried out by Profs. Vogel and Bakhuyzen, Dr. Trepied, Dr. 

 Scheiner, Dr. Gill, the Astronomer- Koyal, 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 photographic chart of the heavens 

 with exposures corresponding to forty minutes' exposure 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 22,000 photographs will be required. 

 For the more accurate determination of the positions of the stars, 

 a reseau with lines at distances of 5 mm. apart is to be 

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



NO. TT38, VOL. 44] 



image of the riseau 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 observatories 

 provided with similar instruments, though not necessarily con- 

 structed 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 concurrently. 

 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 Prof Kapteyn, a preliminary photographic survey 

 of the southern heavens. 



With an exposure sufficiently long for the faintest stars to im- 

 press themselves upon the plate, the accumulating action still 

 goes on for the brighter stars, producing a great enlargement 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 overexposed 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 purpose 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 photo- 

 graphic 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. 

 I 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 coloured stars the photographic brightness is very 

 different from the visual brightness ; but in all stars, changes, 

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

 graphic or the visual region, unaccompanied by a similar change 

 in the other parl^ of the spectrum. For these reasons it would 

 seem desirable that the two sets of magnitudes should be tabu- 

 lated 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 fraction 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 measures are 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 in- 

 creased our knowledge in this direction. 



It is obvious that photography might be here of great service, 

 if we could rely upon measurements of photographs of the 

 same stars taken at suitable intervals of time. Prof Pritchard, 

 to whom is due the honour of having opened this new path, 

 aided by his assistants, has proved by elaborate investigations 

 that measures for parallax may be safely made upon photo- 

 graphic plates, with, of course, the advantages of leisure and 

 repetition ; and he has already by this method determined the 

 parallax for twenty-one stars with an accuracy not inferior to 

 that of values previously obtained by purely astronomical 

 methods. 



The remarkable successes of astronomical photography, which 

 depend upon the plate's power of accumulatioa of a very feeble 

 light acting continuously through an exposure of several hours, 

 are worthy to be regarded as a new revelation. The first chapter 



