PHOTOGRAPHING THE HEAVENS. 73 



the history of the universe, a work which astronomy teaches us to read. 

 On one of these pages, that has already been in part deciphered, is re- 

 corded the destiny of our planet. 



It is, then, not surprising that astronomers seek to gain possession 

 of as many reliable copies of such leaves from this history as possible ; 

 in other words, seek to own as exact and extensive star-maps as will 

 include the very smallest luminous points in the heavens. What un- 

 told work the compiling of such charts entails may well be imagined ; 

 indeed, this is a task which is almost beyond human power. The chart 

 from which the above picture is a copy was compiled at the observa- 

 tory at Paris, and work at the same has already been continued for 

 many decades. For years past, the two brothers, Paul and Prosper 

 Henry, have been engaged in this exacting undertaking ; but, notwith- 

 standing the great experience which they in the course of time had 

 gathered, their task almost came to a sudden end in the year 1884. 

 At that time, while pursuing their observations, they came to that 

 region of the heavens traversed by the milky-way. As is well known, 

 the mild, lambent light of the milky- way is caused by a conglomera- 

 tion of countless millions of stars placed behind one another to endless 

 depths. To reproduce these millions of stars on charts proved to be 

 utterly impossible. 



The two observers then summoned the art of photography, re- 

 cently so much improved, to their aid. Naturally they could not make 

 use of the ordinary apparatus of the photographer ; indeed, they 

 were obliged to build a special telescope for their purpose. By means 

 of clock-work, they succeeded in imparting to this a movement so 

 prescribed and so regulated that the stars, though continuing in their 

 unbroken course in the heavens, yet retain a stationary position with 

 reference to the photographic plate. After many painstaking experi- 

 ments, the enterprise was successful beyond expectation. Even the 

 faintest of stars were plainly discernible on the plate, and in this man- 

 ner more was accomplished in one hour than could be done by the old 

 method of inscribing each star in many months. 



These results incited to further progress. A new and very large 

 telescope was constructed and directed toward the starry heavens. The 

 plate now showed stars of the fifteenth magnitude, i. e., those whose 

 light is so faint that only very few telescopes in all Europe can render 

 them perceptible. In order to obtain this result, the plate, notwith- 

 standing its extreme sensitiveness, had to be exposed to the light of 

 these stars for fully an hour. If one were to carefully examine such a 

 plate, or rather a cliche made therefrom, doubts might perhaps arise as 

 to whether some of the little points thereon might not have been occa- 

 sioned by particles accidentally present on the original plate. Such 

 doubts might well be entertained, but Messrs. Henry have succeeded 

 in meeting them in a most ingenious manner. After having exposed 

 the plate for an hour, they shifted its position a trace to the right, and 



