274 PROCEEDINGS OF THE AMERICAN ACADEMY 



brightness as givcMi in the photometric catalogue is 3.6. The agree- 

 ment of the observation No. 34 is probably accidental, since the large 

 interval — , cannot be estimated with accuracy. As far as it goes, 

 however, it indicates that the star was at its full brightness. The 

 other observation, No. 33, indicates a diminution of liglit, or that the 

 star was near a minimum. The law of variation of light is not known, 

 but probably the change in magnitude amounts to about 0.8. The 

 star retains its full brightness except for about two hours before and 

 after each minimum. We may accordingly assume that a minimum pre- 

 ceded or followed the observation No. 33 by about one hour. On this 

 day the sun set at about 3'' 47'" and X Tauri set at IC' 1()"\ Allow- 

 ing for twilight, we may accordingly assume 179G, Jan. l'^ 10'' i 5'' 

 for the time of observation. For the other date we obtain, in like 

 manner, 1796, Nov. 30'^ iT'itG**. The ephemeris of Schonfeld, 

 applying the equation of light, gives 1795, Dec. 31*^ 22''.6, for Ep. 

 — 6500. A correction to the ephemeris of — 11''. 5 is thus indicated. 

 This exceeds the possible error in the time, added to the probable 

 error in magnitude. In other words, if the star was really below its 

 full briafhtness, the minimum must have occurred several hours after 

 the computed time. In like manner, we obtain 179G, Dec. 1*^ 22''.3 

 for Ep. — 6485, or the nearest minimum does not occur until 35 

 hours after the observation No. 34. Accordingly, as the observation 

 hidicated, the star should have had its full brightness. The first 

 minimum previously known of this star occurred on Dec. 6, 1848. 

 If it were possible to determine the hour of Ilerschel's observation, 

 the mean period of this star would be determined with great precision. 

 An uncertainty of one hour would correspond to about half a second 

 in a single period. 



X Sagittarii. — This star varies in light from about 4.5 to 5.3 in a 

 period of a little over seven days. The only comparison made by Mer- 

 schel places this star a little fainter than 2 Sagittarii. The latter star 

 is commonly placed in Ophiuchus, — in f^ct, it is nearly in line with 

 52 and 58 Ophiuchi, and between them. Its magnitude, according to 

 the Uranometria Argentina is 6.8, which corresponds to 6.6 on the 

 photometric scale. This would make the variable much too faint, 

 even if at its minimum. It is also strange that Ilerschel should have 

 employed a star at so great a distance (about 8°), when he might 

 have taken others about equally faint and nearer. The hypothesis 

 that 2 Sagittarii was much brighter then than now, is negatived by the 

 fact that Ilerschel compared it with 52 Ophiuchi, and found it only 

 slightly brighter. The magnitude of this star in the Uranometria 



