i6o 



REPORTS ON INVESTIGATIONS AND PROJECTS. 



The computation in duplicate of reductions from apparent to mean places 

 has progressed well, having been completed for 485 out of the 654 series at 

 this date (September i). 



The computation in duplicate of refraction has been finished for the entire 

 program. 



The magnitude-equation for each of the five observers of transits has been 

 well determined. For four of the five observers these equations materially 

 differ for transits north and south of the zenith respectively. The following 

 summary exhibits the gist of these determinations and shows for the respect- 

 ive observers the effect upon the chronographic registry of transit due to a 

 diminution of the brightness of star by one magnitude. That is to say, in 

 the case of each observer the transit of a relatively faint star is retarded for 

 the respective observers at the rate per magnitude shown in the table, and its 

 registered transit requires a negative correction. 



Summary of magnitude-equations. 



* The equation for R. H. Tucker for transits north of the zenith is expressed by a curve in which 

 the difference at magnitude, 2 Ji 0, is — i^oog, and at 8 *• o is — S023. The tabular value, — '^0150, is the 

 mean from fourth to eighth magnitude, corresponding to telescopic aperture of 4 inches. When aper- 

 ture is 8 inches this rate corresponds to magnitude, 9 M5. 



Except for very faint stars, it is seldom necessary to employ an actual 

 correction of more than two to three times that due to a diminution of one 

 magnitude. Normally, the aperture of the object-glass of the telescope is 

 reduced to 4 inches. Ordinarily stars brighter than 3 m 5 are seen through a 

 wire screen that absorbs 3^8 (Harvard Scale). In order to produce a finer 

 division of grades in diminution of the apparent magnitude of the focal 

 image during transit, the screen for very bright stars was applied during the 

 entire transit for the brightest stars ; for two-thirds of the transit, for one- 

 half the transit, or for one-third the transit, the screen was applied for suc- 

 cessive diminishing grades of brightness. An arrangement was provided by 

 which the 4-inch diaphragm could be removed and the stars observed through 

 the full aperture of the telescope. This method was adopted in case of stars 

 fainter than J^O', and by this means the apparent brightness of these fainter 

 stars was virtually increased by 1^5 over what they would seem to have if 

 the 4-inch diaphragm had been used. Thus the entire range of magnitude 

 over which the equations would be applicable would be diminished in ex- 



