624 PROCEEDINGS OF THE AMERICAN ACADEMY 



I have engaged for this purpose the most skilful instrument maker we 

 ever had at the Observatory, and feel certain that he will build a most 

 satisfactory instrument. The large 60° prisms (4 in. on an edge and G^V in. 

 high) have already been obtained, and as I am now engaged in making 

 the drawings of the spectro-heliograph, we may reasonably hope that it 

 will be ready before many months have elapsed." 



Professor Hale adds, " I feel very grateful to the Rumford Commit- 

 tee for rendering possible the construction of an instrument which 

 should certainly give valuable results when employed with the 40-inch 

 telescope." 



Professor Webster writes as follows regarding (2). The " appropria- 

 tion of $250 for a revolving mirror has not been drawn upon. I have 

 made various experiments to determine the power required, and the fi'ic- 

 tion of bearings of various sorts, and expect to have the instrument 

 constructed during the summer vacation." 



Regarding (3) he writes : " The intention was to apply Michelson's 

 method of the interferometer, using however an instrument to measure 

 the visibility, instead of depending, as has hitherto been done, upon eye 

 estimates. On considering the relative advantages of the bolometer, the 

 radiomicrometer, and the radiometer, the last named instrument appeared 

 to be the most convenient, and was therefore chosen. Most of the time 

 has been spent in constructing and experimenting with the radiometer, 

 with the result that, if the instrument was made sensitive enough to 

 measure the quantities involved, its zero was constantly changing, and the 

 instrument was altogether too slow for the purpose. The glass plates of 

 the interferometer also absorbed such a large fraction of the radiation 

 that the method would have been impracticable, without fluorite plates, 

 which would have entailed a large expense. We have therefore 

 reluctantly been obliged to give up work on the Zeeman effect, and the 

 money will be returned." 



Professor Michelson wn-ites that his paper read at the April meeting of 

 the Academy may be considered as a report of progress of his investigation, 

 the " echelon spectroscope " there shown being the first made instru- 

 ment of its kind. The amount of the appropriation still unexpended, 

 about $300, will be applied to the construction of a larger instrument 

 of the same character. 



Professor Sabine states that the camera made for his investigation by 

 Clark and Sons has barely been completed wMthin the past few days, under 

 which circumstances it has not yet been possible to begin experimental 

 work. 



