260 Mr. E. C. Pickering. [May 24; 



which forms the insoluble portion of the meteorite of Ensisheim. 

 What was supposed to be alumina was further examined, and was 

 found to be almost entirely chromium oxide, doubtless present in 

 combination with some iron protoxide, alumina, and magnesia as 

 chromite. And it appears not improbable that this part of the 

 meteorite contains some tridymite, a few per cent,, in fact. 



V. " Circular concerning Astronomical Photography^ From E. 

 C. Pickering, Director of Harvard College Observatory, 

 Cambridge,. Mass., U.S.A. 



Astronomical Photography. 



The important part that photography is likely to play in the future 

 of astronomy renders it desirable that an opportunity should be 

 afforded to astronomers to acquaint themselves with the « improve- 

 ments continually made in this branch of their science. This could 

 best be done by the establishment at convenient places of collections 

 designed to exhibit the progress of j photography as applied -to astro- 

 nomical observations. 



The Harvard College Observatory has some special advantages for 

 forming such a -collection, since it already possesses many of the 

 early and historically important specimens which would naturally 

 form part of the series. . Among these may be mentioned four series 

 of daguerreotypes and photographs of various celestial objects taken 

 at this Observatory. These series were - respectively undertaken in 

 1850, 1857 ; 1869, and 1882. 



At present, the-astronomers of the United States have no ready 

 means of comparing their photographic work with that done in 

 Europe, or even. with that of their own countrymen. The proposed 

 collection of photographs, - so far as it could be rendered , complete, 

 would greatly reduce the difficulty. 



It is therefore desired to form, at the Harvard College Observa- 

 tory, a collection of all photographs of the heavenly bodies and of 

 their spectra which can be obtained for the purpose; and it. is hoped 

 that both European and American astronomers will contribute 

 specimens to this collection. Original negatives would be particularly 

 valuable. It may ' happen that ^ some such negatives, having slight 

 imperfections which would limit their value for purposes of engrav- 

 ing, could be spared for a collection, and would be as important 

 (considered as astronomical observations) as others photographically 

 more perfect. In some cases,- astronomers may be willing to. deposit 

 negatives taken for. -ia.- special purpose, and no longer required for 

 study, in a collection where they would retain a permanent value as 



