Appendix V. 



EEPOKT ON THE WORK OF THE ASTROPHYSICAL OBSERVATORY FOR 

 THE YEAR ENDING JUNE 30, 1897. 



Sik: The work of the observatory during the past year has consisted largely in 

 preparing for publication an account of the research on the positions of absorption 

 bands in the infra-red solar spectrum. 



In this report, which was completed in May, 1897, but which, owing to unavoid- 

 able delay, has not yet appeared in type, the positions of about 225 absorption lines 

 and bands are determined in deviation and refractive index for a 60° rock-salt prism 

 at the temperature of 20° centigrade. These lines are distributed in the salt spec- 

 trum between deviations of 40° 25' and 38° 45', corresponding to wave lengths 0.76 /< 

 and 5.20 jx, respectively. The average error probable in the absolute angular devia- 

 tions of these lines is about 4 seconds of arc, but in their relative deviations meas- 

 ured from the A line in the visible spectrum the probable error averages only about 

 0.5 seconds of arc, a degree of accuracy even exceeding the anticipations held forth 

 in last year's report. The holographs from which these results were obtained were 

 taken on exceptionally favorable occasions between October 26, 1896, and January 9, 

 1897. These holographs, 13 in number, were selected from among many more, of 

 all degrees of excellence, taken during the same interval. 



The forthcoming report contains, in addition to the results above mentioned, an 

 account of the investigations leading up to this research on the infra-red absorption 

 lines ; a description of the successive improvements in the instrumental conditions 

 which have rendered possible the present results ; a description of the present appa- 

 ratus and its adjustment; and a discussion of present- sources of error and the pos- 

 sibility of future progress. The results of several subsidiary investigations are 

 given, among others being the change of deviation of rock salt with change of tem- 

 perature, the heat conductivity of rock salt, the effect of diftiaction in decreasing 

 the intensity of energy in the spectrum with narrowing slit widths. The report is 

 illustrated with reproductions of photographs and drawings of apparatus, platted 

 curves explanatory of various matters, holographic curves, and line spectra similar to 

 that here given. With the illustrations, the report is expected to cover about 175 

 quarto pages. 



DETAILS OP THE WORK OF THE YEAR. 



There have been taken during the year 83 holographs (none were taken after Jan- 

 uary 20, 1897), of which about one-third are notably superior to any of previous 

 years in freedom from ''drift" and accidental iniiections, in genuine detail, and in 

 accuracy of position. On 6 holographs covering the region 0.76 /< to 2 /<, and 7 con- 

 tinuing from 1.8 n to 5.5 //, all the lines determined to be "real," after the manner 

 described in the report of last year, have been measured in position upon a compara- 

 tor, both to determine the deviation in the spectrum and the relative intensity of 

 energy at the various points. Each curve was twice independently measured. 



These measurements were reduced and tables prepared of deviations, indices of 

 refraction, approximate wave lengths, and giving the relative importance of the 

 lines discovered (about 225 in number), and the relative intensities of energy at the 

 corresponding points in the solar spectrum. A composite linear photograph has 

 been prepared from three curves, for purposes of illustration after the manner here- 

 tofore described. 

 66 



