24 KEPORT OF THE SECRETARY. 



territoiy of the United States to civilization and settlement will have 

 resulted in the extermination of its large fauna, but these efforts have 

 hitherto been wholly unsuccessful. 



A secondary purpose of Congress in establishing- the park was the 

 recreation of the people, and this purpose has certainly proved of great 

 public benefit, while the advantage of the zoological park as a means of 

 education has become interesting!}^ manifest by the constantly increas- 

 ing number of school children who, with their teachers, visit the park 

 and there make a life study of the animals. Interesting illustrations 

 of this are given in the superintendent's report. 



ASTROPHYSICAL OBSERVATORY. 



The operations of the Astrophysical Observatory during the last 

 year have been of continued interest, although the work is done under 

 difficulties, owing to the excessively confined quarters, which were 

 originally intended only as temporary sheds for the instruments, and 

 which are shown in the accompanying illustration. 



In my last Report attention was called to the remarkable periodic 

 changes in the absorption of water vapor in the air, as recently dis- 

 covered })v inspection of the records of the Astrophysical Observatory. 

 This matter has been more carefully investigated this year, and it has 

 l)een found that these changes occur to a marked degree in the spring 

 and fall months each year, and occasionally at other times as well. It 

 would be a matter of great interest if these remarkable variations in 

 the atmospheric absorption could be traced simultaneousl}^ at different 

 latitudes and longitudes, and such a research might lead to results of 

 far-reaching importance. 



Attention is again called to the announcement in the report of the 

 Aid Acting in Charge of a technical result of interest to the effect that 

 prisms can now be easily constructed in which indices of refraction 

 and wave lengths are, so far as can be seen, in as "constant" a relation 

 as exists between deviation and wave lengths in the grating. 



The accuracy of the Observatory work, it will be remembered, 

 depends, strange as it may appear, on our increased knowledge of the 

 optical properties of rock salt. This 3'ear it has been shown that, so 

 far as the accuracy of the most painstaking measurements can go, these 

 indicate that rock-salt prisms, whether mined at one part or another of 

 the. earth's surface, have identical refractive indices, and thus the meas- 

 ures of 1897-98, which determined the exact positions of 700 Fraun- 

 hofer lines in the infra red spectrum of rock salt, may be regarded as 

 fixing "constants of nature." 



One of the pieces of work accomplished in this last year, however, 

 has been to go further and redetermine the dispersion of rock salt in 



