ASTRONOMY — HAI,E. 65 



struction in our instrument shop and will soon be mounted in the 

 laboratory on Mount Wilson. It will also be suitable for the meas- 

 urement of the position angle, height, and heliographic latitude of 

 prominences, as well as for the monocular comparison of two photo- 

 graphs in a manner similar to that employed in the stereocompar- 

 ator. A measuring machine, recently completed for us by William 

 Gaertner & Co., after the designs of Dr. Frank Schlesinger, fur- 

 nishes the means of determining rectangular coordinates on solar 

 or stellar photographs, when it is desired to employ the ordinary 

 methods of measurement and reduction. A special micrometer for 

 the stereocomparator, designed for use with a reseau, is also under 

 construction by the Zeiss Company. 



SPECTRA OF SUN-SPOTS AND FIvOCCULI (Messrs. Hale and Adams). 



The plan of research prepared for the Solar Observatory lays 

 special stress on the simultaneous study of solar phenomena from 

 several points of view. To obtain adequate knowledge of the nature 

 of sun-spots, for example, it is not enough to make direct or mono- 

 chromatic photographs of the spots, or to devote all our attention 

 to a study of their spectra. Direct photographs are useful in giving 

 the heliographic position and general character of the spots and the 

 details of their .structure. They tell nothing, however, of the forms 

 and motions of the invisible vapors surrounding them, or of those 

 peculiarly characteristic vapors within the spots that are represented 

 by the widened lines. To study these forms we must employ spec- 

 troheliographs, of moderate dispersion for high and low level photo- 

 graphs with the calcium lines, of great dispersion when it is desired to 

 study the distribution of the rarest and most lofty vapors of calcium 

 or the luminous clouds revealed with the aid of the narrow lines 

 of other substances. In the interpretation of these photographs, 

 however, and for other purposes as well, we must have the assistance 

 of high-dispersion photographs of spectra. The spectroheliograph 

 plates themselves furnish the means of studying the motions of the 

 luminous vapors parallel to the solar surface, but the distortions and 

 displacements of the spectral lines, as determined by the most precise 

 measurements, supply the only available means of measuring the 

 velocities of ascent or descent. For the interpretation of the phe- 

 nomena of widened lines and for an explanation of the causes which 

 bring about the disappearance or complete reversal of Fraunhofer 

 lines over spots, recourse must be had to such laboratory experi- 

 ments as are referred to elsewhere in this report. 



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