478 PHOTOGRAPHY IN TPIE SERVICE OF ASTRONOMY. 



without a special verification ; it is particularly necessary to look out 

 for deformations when the copies have been executed with a pencil of 

 • slightly convergent light. 



II. 



The truly invaluable advantage of this intervention of photochem- 

 istry in the processes of practical astronomy, is that iu transporting 

 (so to speak) an authentic image of the firmament into the study of the 

 astronomer, it frees him from obstacles without number which have so 

 long a time trammeled researches the most delicate ; the cost of crea- 

 tion and maintenance of an observatory, the difficult handling of great 

 instruments, fatiguing nightly vigils, fogs, and clouds which so often 

 put a stop to observations, the necessity of changing one's hemisphere in 

 order to study certain constellations, etc. Armed with a simple microm- 

 eter, he can henceforward explore collections of photographic i)lates, 

 taken with some years of interval, and make, in his chimney-corner, dis- 

 coveries which otherwise would demand long struggles, continued dur- 

 ing several generations, against the capricious inclemency of the sky. 



Indeed celebrated labors come to our mind which have cost in former 

 times long efforts which we shall have no more to renew. These are, 

 firstly, the gauges of the number of stars which William Herschel 

 undertook, a century ago, with the 20-foot telescope after a plan traced 

 by Wright. We know that, setting out with the hypothesis of a nearly 

 uniform distribution of the stars, he admitted for a long time that the 

 relative richness of a region indicated the depth of the heavens iu the 

 direction considered, which must conduct to attributing to the visible 

 universe a structure tolerably improbable. Later he changed his 

 method, and occupied himself with sounding the celestial spaces with 

 telescopes more and more powerful, in taking henceforth for a criterion 

 of distances the resolvability of clusters or groups of stars. The two 

 methods are incorrect, in confounding, with the effects of perspective, 

 the inequalities of constitution of different stellar regions, indications 

 of which indeed make us suspect the reality. But however in reserv- 

 ing the conclusions which may be drawn from the gauges or soundings 

 of the sidereal system, it will be necessary sooner or later to return to 

 this grand statistical work, and the photographic chart will singularly 

 tend to facilitate the task of astronomers who will be charged with it. 



Shall we speak of catalogues of stars? The most ancient, those of 

 Hipparchus, Ulugh Beigh, Tycho-Brahc, containing a thousand stars; 

 they were made without a telescope. The catalogue, so precious, which 

 Bessel has derived from the observations of Bradley (made at Green- 

 wich about the middle of the last century), and which has, so to speak, 

 inaugurated the astronomy of precision, contained only a little more 

 than 3,000. That which is founded upon the observations of Lalande, 

 executed towards the close of the century at the observatory of the mil- 

 itary school, and published, in 1801, in the French Histoire Celeste (the 



