32 BEPORT— 1891. 



into the hands of the astronomer a photographic surface adapted fully to 

 his wants. 



The modern silver-bromide gelatine plate, except for its grained 

 texture, meets the needs of the astronomer at all points. It possesses 

 extreme sensitiveness ; it is always ready for use ; 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 diflBculties of con- 

 structing large telescopes, the astronomer would have to depend in the 

 last resource upon his eye. Now we cannot 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. 

 Bach hour the plate gathers up 3,600 times the light-energy which 

 it received during the first second. It is by this power of accumu- 

 lation 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 which photography is 

 of great service to the astronomer. It enables him within the compara- 

 tively 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 nebulas or other objects, or the phenomena 

 of a passing eclipse, tasks which by means of the eye and hand could 

 only be accomplished, if 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 an heritage of labour 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 new regions of know- 

 ledge, 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 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 



