170 CARNEGIE INSTITUTION 



pattern were made in two parts, back and front, of cast iron and 

 carefully finished, it is quite possible that a uniform thickness of 

 Y% inch would be quite sufficient ; this would reduce the weight of 

 the finished speculum to 900 pounds. Probably the chief practical 

 difficulty in making such a casting will be so to arrange the mold 

 that it will readily yield to the contracting speculum- metal. Very 

 much will depend on the nature and condition of the sand or loam 

 used in forming the mold, but doubtless valuable advice on the 

 whole process could be obtained from an experienced molder who 

 has been accustomed to the production of complicated and fragile 

 castings. 



In conclusion, if we regard the rapid progress in spectroscopy of 

 late years associated with the improvement of the diffraction grating, 

 it seems that this is probably no less due to the happy choice of 

 speculum metal for the material of gratings than to the improved 

 accuracy of the ruling. It is therefore reasonable that further ad- 

 vances in many other branches of astrophysics may be expected 

 from a return to the use of the solid metallic reflector in place of the 

 mirrors of silvered glass now so much in favor. 



Acknowledgments. 



The Committee desires to acknowledge the important advice and 

 suggestions received from Dr. Blihu Thomson, Professor Joseph N. 

 Le Conte, Major George W. Stewart, Professor E. C. Pickering, 

 Professor H. Rubens, Professor F. Paschen, Dr. S. W. Stratton, 

 Mr. T. P. Lukens, Mr. James Gamble Rogers, Mr. C. A. Phillips, 

 Mr. Wm. R. Staats, Miss A. M. Clerke, Professor H. H. Turner, 

 Mr. John Broder, Mr. James layman, Dr. G. K. Gilbert, Dr. C. Hart 

 Merriam, and others, particularly from Mr. Charles G. Abbot, 

 Assistant in Charge of the Smithsonian Astrophysical Observatory, 

 who, with the approval of Secretary L,angley, furnished a very large 

 amount of detailed information. 



