REPORTS OF AMERICAN OBSERVATORIES. 573 



A change also in the method of transmitting time-signals and of 

 dropping the Washington and New York time-balls is nearly completed, 

 and will probably be in operation by the end of October. 



PUOTO-HELIOGRAPHIC AND MISCELLANEOUS WORK. 



Prof. William Harkness reports as follows : 



The photographs of the late Transit of Mercury were examined, and 

 out of the whole number it was found that twenty -five of the Cambridge 

 pictures, twenty-three of the Washington pictures, and sixty -four of the 

 Ann Arbor pictures were sufficiently well defined for measurement. 

 Accordingly, these one hundred and twelve plates have been read off, 

 all but twelve being done in duplicate; and the computations, also in 

 duplicate, have been nearly completed. About three months' work is 

 yet required to determine from the photographs the final corrections to 

 the right ascension and declination of Mercury. 



The drawings of Mars, made by Professor Harkness during the op- 

 position of 1877, have been transformed from the orthographic to Mer- 

 cator'e projection, and a map of the planet has been constructed. Gen- 

 eral tables have also been computed, which give directly the areographic 

 latitude and longitude of the center of the disc of Mars, and the posi- 

 tion angle of its axis as seen from the earth; the arguments being the 

 geocentric right-asceusion and north polar distance of the planet. 



Mr. Jos. A. Rogers has been employed under a special appropriation 

 for experiments in astronomical photography, and has spent most of his 

 time in endeavoring to overcome the uncertainties of the emulsion pro- 

 cess. Here it may be well to remark that our success in photograph- 

 ing the total eclipse of the Sun of July, 1878, was largely due to the 

 excellence of the emulsion which he furnished; and the future of astro- 

 nomical photography seems to a great extent dependent upon the 

 emulsion process. Hence the importance of the experiments in which 

 Mr. Rogers is engaged. He has also photographed the Sun on every 

 clear day, and has made numerous copies of the negatives of the corona 

 taken during the eclipse mentioned above. 



tiie library. 



The library is in charge of Prof. Edward S. Holden, in addition to 

 his other duties. It is now in a satisfactory condition. 



A card catalogue has been begun, and over 0,000 cards made. Part 

 I of the catalogue of the Library ''Astronomical Bibliography" will be 

 printed in 1870. 



A complete index to the publications of the observatory, from 1845 

 to 1875, has been made. It will be printed as Appendix I to the Ob- 

 servations for 1870. At intervals often years similar indexes should 

 be made. 



The usual annual appropriation of 81,000 for the purchase and care 

 of astronomical works should be continued. 



