184 REPORTS ON INVESTIGATIONS AND PROJECTS. 



The result of this examination of the 21 standards indicates that there are 

 no errors exceeding 0.001 A., except in the case of one line (a 5434), where 

 the error is 0.002 A. 



Thus far 100 lines have been studied in the region X 5371 to 6494, and of 

 this number 21 are international standards of the second order. Seventeen 

 of these were found entirely suitable for such standards ; 25 of the 100 lines 

 are suitable for tertiary standards ; 44 are unsuitable for standards because 

 of excessive displacement under pressure accompanied with dissymmetry; 

 and 14 are provisionally classified as of questionable utility. 



The investigation has shown that, in the selection of a series of standards, 

 attention should be given to pressure displacement, and only such lines in- 

 cluded as do not differ greatly in this respect, if the standards are to be 

 equally available for stations at low and high altitudes. This becomes in- 

 creasingly important as the series is extended to the red, when the rapid 

 increase of pressure displacement with wave-length is considered. 



In his study of solar and metallic spectra, Rowland always found dis- 

 placements between solar and metallic lines of 0.01 to 0.02 A., which he 

 attributed to instrumental errors. Similar capricious results were encount- 

 ered in an investigation begun by Mr. St. John some time ago with the spec- 

 trograph used in connection with the 60- foot tower telescope on Mount Wil- 

 son, but with the new spectrograph of the 150- foot tower telescope a series 

 of plates of the solar and iron arc spectra have been obtained that are free 

 from the relative displacements arising from instrumental imperfections and 

 adjustments. Three sources of error have been associated with the insta- 

 bility of the apparatus, the illumination of the grating, and the centering of 

 the solar image. The first is overcome by the massive construction of the 

 spectrograph, the head of which has a weight of approximately 4,500 pounds 

 (2,000 kg.), and when clamped in position can not be appreciably disturbed 

 by the necessary manipulations. The second is eliminated by the large diam- 

 eter, 15 inches (38 cm.), of the cone of sunlight incident upon the grating 

 which the great focal length, 75 feet (22.9 m.), of the spectrograph permits 

 when the 12-inch objective of 60 feet focal length is used to form the solar 

 image on the slit. The third source from which misleading displacements 

 may arise is avoided by fixing the slit rigorously on the axis of the centering 

 circles. The error introduced from noncentering of the solar image may be 

 large; with the image now used (diameter 162 mm.) the solar lines are 

 shifted 0.001 A. by solar rotation when the slit is 2 mm. from the center of 

 the image, and with small images the error may be much greater. 



With the present equipment an investigation involving the comparison of 

 the arc and the solar spectra of iron has been resumed. The same plates are 

 being used for an examination of the relative accuracy of the international 

 standards of the second order and the determination of the wave-lengths of 

 the tertiary standards in connection with a series of plates of the arc spec- 

 trum of iron taken in the Pasadena laboratory. 



